Microbiome, Immunity, Digestive Health and Nutrition: Epidemiology, Pathophysiology, Prevention and Treatment

By Debasis Bagchi

Microbiome, Immunity, Digestive Health and Nutrition: Epidemiology, Pathophysiology, Prevention and Treatment addresses a wide range of topics related to the role of nutrition in achieving and maintaining a healthy gut microbiome. Written by leading experts in the field, the book outlines the various foods, minerals, vitamins, dietary fibers, prebiotics, probiotics, nutritional supplements, phytochemicals and drugs that improve gut health. It specifically addresses molecular and cellular mechanisms and pathways by which these nutritional components contribute to the physiology and functionality of a healthy gut microbiome and gut health.

Intended for nutrition researchers and practitioners, food experts, gastroenterologists, nurses, general practitioners, public health officials and health professionals, this book is sure to be a welcomed resource.

• Outlines the nutritional guidelines and healthy lifestyle that is important to boost gut health
• Demonstrates the effects of diverse environmental stressors in the disruption of the gastrointestinal ecology
• Discusses the molecular and immunological mechanisms associated with healthy gut microbiome functions
• Addresses how to boost healthy gut microflora and microbiome
• Suggests areas for future research of microbiome-based nutrition and therapies

• Language: English
• Publisher: Academic Press
• Release date: Jul 21, 2022
• ISBN: 9780128222393

Book preview

Preface

Debasis Bagchi and Bernard William Downs

This book explores a wide range of topics related to the role of nutrition in boosting a healthy microbiome in chapters written by leading experts in the field. To provide a comprehensive approach to the emerging field of nutrition, gut health, food recommendations, and the human microbiome, this book covers a wide spectrum of agents including food, minerals, vitamins, dietary fibers, prebiotics and probiotics, nutraceutical supplements, and phytonutrients that boost overall human health and performance by improving the health of the microbiome. This book specifically addresses the causes and pathological manifestations of microbiome dysbiosis and the molecular and cellular mechanisms and pathways involved by which nutritional components contribute to the physiology and functionality of a healthy microbiome and consequential human health. In addition to an extensive review of a variety of functional ingredients and dietary supplements, this book addresses information regarding nutritional guidelines and healthy lifestyle that are important to correct, boost, and maintain a healthy gut microbiome for optimal human health.

Digestion and metabolism are the most important steps in human physiology. It is important to note that 70%–80% of the lymphatic tissue in humans resides in the gastrointestinal tract. Moreover, there is more neurotransmitter activity in the gut than in the brain, prompting labels of the gastrointestinal tract as the second brain. Proper nutrition is essential for optimal human health, daily activities, and performance. The microbiota that resides in the gastrointestinal tract performs functions that are essential to overall health, including the synthesis and/or metabolization of certain vitamins, fatty acids, fibers, and amino acids. The health of the GI tract is ultimately foundational to maintaining good health and appropriately regulating immune function and maintaining metabolic homeostasis. Otherwise, unhealthy lifestyle choices and disruptions of health can cause drastic alterations of these gut microbial communities, which ultimately leads to dysbiosis and the potential for a range of disorders, including but not limited to immune dysregulation, mental dysfunction, and even autoimmune disorders. Therefore there is a growing need for a comprehensive appraisal of the proper nutritional benefits in boosting the human microbial ecosystem, which ultimately promotes optimal gastrointestinal health, immunity, and mental function.

This textbook is divided into 12 major section comprising of 33 chapters. The book starts with, the editorial preface, which was crafted by the editors to provide an overview of this book. Section I Microbiome and Human Health: An Introduction is comprised of five chapters. The first chapter, Oral Microbiome: A Gateway to Your Health, demonstrates the intricate aspects of the oral microbiome and its association with human health, while the second chapter highlights how a healthy microbiome is involved in shaping the newborn immune system. The third chapter demonstrates the impact of diverse gut microbiota on human health and prevention against diverse degenerative diseases, and the fourth and fifth chapters discusses how the individual microbiota participates in programming and reprogramming both systemic and local immune responses to modulate immune health.

Section II, Microbiome and Digestive Health, is composed of two chapters. The first of these chapters discusses the etiology of gut dysbiosis and its role in chronic diseases, and the second chapter explores the role of microbiome in the function and diseases of the digestive system. Section III, Microbiome and Metabolic Syndrome, is composed of seven independent chapters. These chapters discuss diverse aspects on the roles of a healthy microbiome in obesity, weight management, and cardiovascular health. The fourteenth chapter demonstrates the beneficial roles of resveratrol in remodeling the gut microbiota to prevent metabolic disorders. Section IV, Microbiome and Immune Health, is composed of two chapters. These two chapters emphasize the regulatory roles of the microbiome in immune competence, anaerobism, and inflammatory conditions.

Section V, Microbiome and Cognitive Health, is composed of four chapters. The first of these chapters demonstrates the importance of achieving dopamine homeostasis to combat brain-gut functional impairment and discusses the behavioral and neurogenetic correlation with reward deficiency syndrome. The second chapter discussed the influence of the gut microbial flora in the body’s serotonin turnover and its associated diseases. The third chapter explores the connection between diet, gut microbes, and cognitive decline, while the fourth chapter discusses the role of the gut microbiome in Rett syndrome, a rare genetic mutation that affects brain development in some girls. Section VI, Microbiome, Dermal Health, and Wound Healing, discusses the roles of the skin microbiota in dermal health, development of chronic wounds, and wound healing. Section VII, Microbiome and Cancer, is composed of one chapter discussing the roles of a healthy gut microbiome in the prevention of colorectal cancer.

Section VIII, Microbiome, Arthritis, and Multiple Sclerosis, explains the regulatory roles of the microbiome in arthritis, fibromyalgia, and multiple sclerosis in one independent chapter. Section IX, Environmental Pollutants and Gut Microbiome, explores the consequences of microplastic exposure on the gut microbiome.

Section X, Molecular and Immunological Mechanisms Associated With Healthy Gut Microbiome Functions, elaborates on the subject in seven independent chapters. The first three of these chapters discuss the beneficial roles of a healthy gut microbiome, prebiotics, and probiotics, while the second chapter highlights the roles of lactic acid bacteria in diverse foods and beverages in the promotion of gastrointestinal health. The fourth chapter explores demonstrates the novel application of probiotics in enhancing gut-brain communication. The following two chapters demonstrate the beneficial effects of flaxseeds, dietary flavonoids, and mushroom polysaccharides in improving the gastrointestinal microbiome and human health. The last chapter in this section demonstrates the roles and importance of healthy gut microbiome in sports nutrition.

Section XI, Microbiome and Immunomodulatory Peptides, details the beneficial roles of immunomodulatory peptides in human health. Section XII, Study Design and Statistical Interpretation, demonstrates the aspects of clinical intervention, study design, and different statistical methodologies for pretest–posttest studies.

The book concludes with a commentary entitled A Treatise on a Healthy Microbiome, which summarizes a vivid scenario of diverse aspects of a healthy microbiome.

We would like to express our sincere gratitude and thanks to all our eminent contributors as well as the helpful Elsevier/Academic Press editorial team members including Megan R. Ball, Lindsay Lawrence, and Praveen Sachidanandam for their continued support, cooperation, and assistance.

Section I

Microbiome and human health: an introduction

Outline

Chapter 1 Oral microbiome: a gateway to your health

Chapter 2 Influence of microbiome in shaping the newborn immune system: an overview

Chapter 3 Impact of the gut microbiome on human health and diseases

Chapter 4 The gut microbiome, human nutrition, and immunity: visualizing the future

Chapter 5 Individual microbiota correction and human health: programming and reprogramming of systemic and local immune response

Chapter 1

Oral microbiome: a gateway to your health

Na-Young Song¹,²,³, Se-Young Park¹,²,³, Won-Yoon Chung¹,²,³, Young-Joon Surh⁴,⁵,⁶, Kyung-Soo Chun⁷ and Kwang-Kyun Park¹,    ¹Department of Oral Biology, Yonsei University College of Dentistry, Seoul, South Korea,    ²Department of Applied Life Science, The Graduate School, Yonsei University, Seoul, South Korea,    ³BK21 Four Project, Yonsei University College of Dentistry, Seoul, South Korea,    ⁴College of Pharmacy, Seoul National University, Seoul, South Korea,    ⁵Department of Molecular Medicine and Biopharmaceutical Sciences, Graduate School of Convergence Science and Technology, Seoul National University, Seoul, South Korea,    ⁶Cancer Research Institute, Seoul National University, Seoul, South Korea,    ⁷College of Pharmacy, Keimyung University, Daegu, South Korea

Abstract

The oral microbiome is the second largest microbial ecosystem after the gut microbiome, containing more than 700 species of microorganisms. The oral cavity provides a humid and warm environment in which microbes\thrive. Although the oral cavity is one of the major microbial habitats, the role of oral microbiota in the human health has been underemphasized. The oral microbiome is closely associated with oral diseases, including dental caries and periodontitis. Furthermore, there is a large body of evidence to show that the oral microbiota can regulate systemic pathogenesis in a similar way to the gut microbiome. Here, we will review the role of the oral microbiome in the human health and disease, particularly in two aspects: oral and systemic implications.

Keywords

Oral microbiome; oral dysbiosis; oral disease; systemic disease

1.1 Introduction

It is now well recognized that the microbiome plays a crucial role in the human health and pathogenesis (Shreiner et al., 2015). The microbes in and on the human body outnumber the human cells by about ten to one and contribute to maintaining the immune tone by regulating the innate and adaptive immune responses in the body (Kamada and Nunez, 2014). Thus the microbiome is currently considered as a new organ system in the human body, and its dysbiosis is implicated in various types of diseases, from infectious disease to cancers. Conversely, the microbiota dysbiosis-driven disorders can be treated by transferring normal-like microorganisms into the patients, such as fecal microbiota transplantation to treat recurrent Clostridium difficile infection, which is already used in the clinics worldwide (Gupta et al., 2016).

The gastrointestinal (GI) tract is fully covered by mucosal biofilm, a thick layer of mucus, which creates environments for microorganisms to thrive (Donaldson et al., 2016). Because the GI tract is the largest microbial ecosystem, it is not surprising that most investigations to date have been focusing on the gut microbiome. The cumulative evidence from this intensive research strongly supports the conclusion that gut dysbiosis is associated with the pathogenesis of numerous diseases, from intestinal disorders, such as inflammatory bowel disease and irritable bowel syndrome, to extraintestinal diseases, such as cognitive disorders, cardiopulmonary disease, and obesity (Carding et al., 2015).

The oral cavity possesses the second largest and most complex microbiome after the gut (Zhang et al., 2018). According to the human oral microbiome database (HOMD), the oral cavity contains approximately 700 species of microorganisms, about 54% of which are named and cultivated, 14% of which are not named but cultivated, and 32% of which are known only as uncultivated phylotypes (from the HOMD website: http://www.homd.org). Like the GI tract, the oral cavity is lined by the mucous membrane. Owing to its warm and humid characteristics, the oral cavity provides a proper environment for microorganisms to reside and develop diverse biofilms, like a natural microbial growth medium in the human body (Deo and Deshmukh, 2019). In spite of its abundance, the potential effects of the oral microbiome on human health have been greatly underestimated to date. Here, we will highlight the role of the oral microbiome in human health, particularly in the context of its local effects in the oral cavity and its systemic implications.

1.2 Oral microbiome and oral disease

1.2.1 Caries

Dental caries, the most prevalent oral disease, is caused by polymicrobial infection (Bowen et al., 2018). Dental caries involves tooth destruction as well as pulp and periapical infection, which can reduce the quality of life. Intake of dietary sugars and carbohydrates is highly associated with dental caries. Sugars are degraded by the bacteria in the dental plaque, generating an acidic environment that is favorable to growth of acidophilic and cariogenic bacteria (Sharma et al., 2018). The acidogenic Streptococcus mutans and Streptococcus sobrinus are the most well-known species responsible for dental caries (Forssten et al., 2010). Salivary microbiota analysis revealed that individuals with high sugar intake showed the enrichment of acidogenic and acid-tolerant species in Actinomyces, Bifidobacterium, and Veillonella (Esberg et al., 2020). Overall, sucrose consumption promotes an imbalance between acid-producing and alkali-producing bacteria in oral biofilms, facilitating demineralization on enamel blocks in vitro (Du et al., 2020). Demineralization can be corrected by supplements of fluoride. In addition, modulation of oral microbiota could be a more proactive strategy to prevent or inhibit formation of dental caries. In this regard, it has been reported that the use of probiotics containing Lactobacillus rhamnosus GG, Streptococcus salivarius M18, or Lactobacillus paracasei SD1 effectively reduces the bacterial load of S. mutans and the risk of dental caries (Sivamaruthi et al., 2020).

1.2.2 Periodontal disease

Periodontal diseases cause destruction of the tooth-supporting periodontium and are divided into gingivitis and periodontitis (Gao et al., 2018). Gingivitis is a reversible inflammatory disease that is limited to the gingiva. However, if not controlled, gingivitis will progress into periodontitis, a chronic, irreversible inflammatory disease with further destruction of the periodontium, including the periodontal ligament and alveolar bone (Zhang et al., 2018). In conjunction with inflammatory responses, oral microbial dysbiosis is involved in the pathogenesis of the periodontal diseases (Van Dyke et al., 2020). Unlike dental caries, the periodontal diseases involve shifts in microbial communities rather than enrichment of single specific species identified as the etiology (Bartold and Van Dyke, 2019).

The subgingival region is one of the distinct microbial niches in the oral cavity. In healthy individuals the subgingival microbiota comprises mostly gram-positive bacteria, such as Actinomyces naeslundii, A. meyeri, Streptococcus sanguinis, S. oralis, and S. gordonii, and some gram-negative bacteria, including Capnocytophaga gingivalis and Fusobacterium nucleatum (Curtis et al., 2020). During the evolution of microbial communities from health to gingivitis, the relative abundances of Rothia dentocariosa, Propionibacterium, and Stenotrophomonas maltophilia are reduced (Van Dyke et al., 2020). In contrast, the gingivitis states showed five predominant phyla compared to healthy states: Actinobacteria, Firmicutes, Saccharibacteria (TM7), Bacteroidetes, and Fusobacteria (Huang et al., 2014). Thus the development of gingivitis is closely associated with an increased microbial load as well as a shift in the subgingival microbiota.

The subgingival microbiota transition is also observed during the progression from gingivitis to periodontitis. Both gingivitis and periodontitis involve a shift from gram-positives to gram-negative bacteria compared to healthy individuals, but the distinct gram-negative bacteria are enriched by each other. It is well known that the so-called red complex is enriched in periodontal microbial communities, consisting of Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola (Mohanty et al., 2019). Among them, P. gingivalis is considered a keystone pathogen in periodontitis, which can change the oral microbiota and provoke leukocyte recruitment (Darveau et al., 2012). In mice, oral inoculation of P. gingivalis induced periodontal bone loss, bacterial load, neutrophil recruitment, and serum proinflammatory cytokine levels (Hajishengallis et al., 2011; Maekawa et al., 2014). Concomitantly, the periodontitis-associated microbial community was much more diverse than the healthy microbial ecosystem (Curtis et al., 2020).

While it is obvious that oral microbiota dysbiosis is associated with periodontitis, the interplay between dysbiosis and inflammation has not yet been clarified. Recently, Dutzan et al. have shown that resident memory T helper 17 (Th17) cells expanded in human and murine experimental periodontitis models (Dutzan et al., 2018). In a murine ligature-induced periodontitis model, the oral microbiome composition was altered in comparison to the healthy control group, displaying shifts from Streptococcus-predominant to enrichment of Enterococcus, Lactobacillus, and Clostridium. However, systemic treatment with a broad-spectrum antibiotic cocktail suppressed not only the microbial shift but also Th17 expansion, implying a correlation between oral dysbiosis and inflammation in periodontitis. Whether through oral microbiota dysbiosis or not, periodontitis can induce systemic inflammation as well, contributing to extraoral pathogenesis (Hajishengallis, 2015), which will be discussed in later sections.

1.2.3 Oral cancer

Head and neck cancers occur in the oral cavity, salivary gland, pharynx, larynx, nasal cavity, thyroid, and bone, accounting for 5% of all cancers (Heroiu Cataloiu et al., 2013). Oral squamous cell carcinoma (OSCC) is the most prevalent type of the head and neck cancers and its 5-year overall survival rate remains low at approximately 50% (Kademani, 2007). Smoking and alcohol consumption are the best-known risk factors for developing OSCC (Rivera, 2015). In addition to these environmental factors, human papillomavirus (HPV) is believed to be a major microbial suspect for OSCC. On the basis of metaanalysis data, it is evident that HPV infection remarkably increases the incidence of OSCC (Rapado-Gonzalez et al., 2020; Saulle et al., 2015). In spite of its significance, however, the majority of OSCC cases are not related to HPV infection (Irfan et al., 2020).

Instead, recent investigations have been focusing on changes in the oral bacterial composition in OSCC patients. At the phylum level, OSCC patients showed enrichment of Bacteroidetes, Proteobacteria, Firmicutes, Fusobacteria, and Actinobacteria (Pushalkar et al., 2012; Schmidt et al., 2014; Zhang et al., 2019a; Zhao et al., 2017). On the basis of the Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analysis to predict functional profiles of microbial communities, the OSCC microbiome had a relatively higher abundance of genes associated with proinflammatory bacterial components, such as lipopolysaccharide (LPS) biosynthesis (Zhang et al., 2019a,b). These data suggest a correlation between oral microbiota dysbiosis and inflammation in OSCC.

Chronic inflammation is well known to contribute to cancer development and progression. In this regard, metaanalyses have revealed a strong correlation between periodontitis and OSCC (Javed and Warnakulasuriya, 2016; Ye et al., 2016). Of note, the OSCC microbiome signature is greatly overlapped with the periodontitis-associated communities (Pushalkar et al., 2012; Zhao et al., 2017). Chronic infection of periodontal pathogens promoted tumorigenesis of OSCC in mouse models (Binder Gallimidi et al., 2015; Kamarajan et al., 2020; Lafuente Ibanez de Mendoza et al., 2020). Thus oral microbiome dysbiosis promotes periodontitis, which can lead to OSCC development if not treated properly.

1.3 Oral microbiome and systemic disease

1.3.1 Alzheimer’s disease

Alzheimer’s disease (AD), the most common type of dementia in older adults, is a neurodegenerative disorder characterized by a progressive decline of cognitive functions. The hallmark lesions of AD are the extracellular deposition of amyloid β (Aβ) aggregates and the intracellular accumulation of neurofibrillary tangles (NFTs) derived from hyperphosphorylated tau protein (Kametani and Hasegawa, 2018). It is well known that inflammation plays a crucial role in AD pathogenesis by exacerbating the accumulation of Aβ and NFTs (Kinney et al., 2018). The blood levels of the proinflammatory markers, such as interleukin-1β (IL-1β), IL-6, and C-reactive protein (CRP), are elevated in patients with AD compared to healthy individuals, which correlates with increased risk for dementia and incidence of cognitive impairment (Alley et al., 2008; Engelhart et al., 2004; Holmes et al., 2003). Recently, the comparative proteomic profiling of plasma and brain tissues revealed elevation of the IL-6 level as well as enrichment of proteins associated with the IL-6 signaling pathway in AD patients compared to healthy subjects, further supporting a major role of systemic inflammation in AD pathogenesis (Chen and Xia, 2020).

Interestingly, a case-control study in Japan showed a correlation between AD risk and premature tooth loss that occurs largely as a result of periodontitis in adults, implying the relationship between dental health and AD (Kondo et al., 1994). In line with this, a longitudinal study has demonstrated that tooth loss is strongly associated with a higher prevalence and incidence of dementia (Stein et al., 2007). Chen et al. have further supported the significant correlation between chronic periodontitis and AD risk by conducting a retrospective matched-cohort study (Chen et al., 2017).

This association between periodontitis and AD might be explained by systemic inflammation as well as oral dysbiosis. Periodontitis is associated with elevated serum levels of the proinflammatory IL-1β, IL-6, and CRP, which are also increased in AD patients (Cheng et al., 2020; D’Aiuto et al., 2004; Noack et al., 2001). The murine ligature-induced periodontitis model showed increased serum IL-6 level and neuroinflammation due to permeability of the blood–brain barrier (BBB) (Furutama et al., 2020). However, the neutralization of IL-6 remarkably suppressed BBB disruption as well as proinflammatory cytokine expression levels in the hippocampus. Taken together, these data support the hypothesis that the periodontitis-induced systemic inflammation is a primary risk factor for AD.

In periodontitis-associated AD pathogenesis, oral microbiota dysbiosis, such as infection, is another crucial axis. As was mentioned in the previous section, P. gingivalis is the most distinguished species among the periodontitis signature microbial communities, the red complex. In a transgenic mouse model of AD, oral infection with P. gingivalis promoted alveolar bone loss in the oral cavity as well as proinflammatory cytokine release and Aβ accumulation in the brain, resulting in cognitive decline (Ishida et al., 2017). In an experimental chronic periodontitis model, P. gingivalis and its product gingipain were detected in the brain tissues along with an accumulation of Aβ and phosphorylated tau (Ilievski et al., 2018). Notably, synthetic small molecule inhibitors targeting gingipain reduced the bacterial load, neuroinflammation, and neurodegeneration in the brain of P. gingivalis–infected mice (Dominy et al., 2019). It is noteworthy that Poole et al. demonstrated the coexistence of P. gingivalis and its major virulence factor LPS in brain tissues from short-term postmortem AD patients (Poole et al., 2013). In middle-aged wild-type mice, chronic systemic exposure to P. gingivalis LPS induced neuronal Aβ accumulation, neuroinflammation, and learning and memory deficits (Wu et al., 2017). Similar to systemic exposure, topical application of P. gingivalis LPS induced periodontitis, neuroinflammation, and learning and memory impairment in Sprague-Dawley rats (Hu et al., 2020). These data all together suggest that oral pathogens, such as P. gingivalis, may translocate to the brain, and then either the pathogen itself, its products, or both can lead to neuroinflammation and neurodegeneration. Concomitantly, oral pathogen–induced systemic inflammation can further contribute to AD pathogenesis.

1.3.2 Cardiovascular disease

Cardiovascular disease (CVD) is the leading cause of death worldwide, including coronary heart disease (CHD), stroke, and myocardial infarction. The traditional risk factors of CVD have been well established, such as hyperlipidemia, obesity, hypertension, and smoking (Mahmood et al., 2014). Regardless of the type of risk factors, atherosclerosis is the most common pathological process that triggers CVDs (Ministrini et al., 2020). This can restrict the blood flow and oxygen supply due to the thickening and/or stiffening of the walls of arteries. Atherosclerotic lesions are characterized by the accumulation of lipids and infiltration of immune cells, mostly macrophages (Hansson and Hermansson, 2011). Thus atherosclerosis is thought to be a chronic inflammatory state (Galkina and Ley, 2009).

As was mentioned earlier, periodontitis involves systemic inflammation, not just local infection. Notably, periodontitis and atherosclerosis show similar patterns of systemic inflammatory mediators, including elevated serum levels of CRP, IL-6, and IL-18 (Schenkein et al., 2020). Based on the immunohistological analyses, the atherosclerotic lesions concomitantly had infiltration of inflammatory cells and periodontal pathogens, such as P. gingivalis (Ford et al., 2006). Furthermore, Spahr et al. demonstrated that the periodontal pathogen burden is correlated with the presence of CHD (Spahr et al., 2006). Metaanalysis data also showed that patients with periodontal diseases had an increased risk of CHD, myocardial infarction, and stroke (Blaizot et al., 2009; Janket et al., 2003; Khader et al., 2004). Given these investigations together, it is evident that periodontitis increases the incidence of CVDs through systemic inflammation and infection with periodontal pathogens.

In addition to periodontal pathogens, oral commensal microbiota can play an important role in CVD progression by regulating nitrogen oxide (NO), a potent vasodilator. NO controls vascular homeostasis and cardiac contractility, conferring preventive and therapeutic effects against CVDs (Blekkenhorst et al., 2018; Lundberg et al., 2015). Of note, the oral commensal bacteria, particularly facultative anaerobes, can reduce nitrate in the diet into nitrite, which is further reduced to NO (Duncan et al., 1995). In the oral microbiome a higher abundance of these nitrate-reducing bacteria, such as species of Firmicutes and Actinobacteria, was potentially associated with lower systolic blood pressure (Goh et al., 2019; Koch et al., 2017). In healthy adults, 10-day supplementation with nitrate increased plasma levels of nitrite and NO while reducing blood pressure compared to the placebo group, potentially through enrichment of nitrate-responsive species, such as F. nucleatum subsp. nucleatum, Prevotella melaninogenica, and Campylobacter concisus (Vanhatalo et al., 2018). Thus modulation of the oral microbiome can be a promising strategy to prevent and treat CVDs by regulating NO levels.

1.3.3 Diabetes

Diabetes mellitus (DM) is a chronic metabolic disease characterized by high levels of blood glucose. According to the pathogeny of diabetes, DM can be classified into type 1 (T1DM) and type 2 (T2DM). T1DM is also known as insulin-dependent diabetes, since it results from the inability of the pancreas to produce insulin. In contrast, peripheral insulin resistance is the key feature of T2DM, the more common type of diabetes.

Both types of DM are linked with periodontitis in a bidirectional way, which means that diabetes can aggravate periodontitis and vice versa (Mealey, 2006; Taylor, 2001). Patients with severe T1DM had increased gingival inflammation, dental plaque, and periodontal bone loss (Jindal et al., 2015; Tervonen et al., 2000). In case of T2DM, a population-based cross-sectional study demonstrated that patients with poorly controlled diabetes had a significantly higher prevalence of periodontitis compared to individuals without diabetes (Tsai et al., 2002). These results strongly support DM as a risk factor for periodontitis.

The underlying biological evidence for DM as a risk factor for periodontitis has not yet been fully clarified. However, it is noteworthy that diabetes can affect the oral microbiome. Compared to non-DM subjects, poorly controlled DM patients retained gingival enrichment of the periodontal pathogens, also known as the red complex, such as P. gingivalis, T. denticola, and T. forsythia (Aemaimanan et al., 2013). Furthermore, individuals with T1DM displayed alterations in the oral microbial composition, such as higher abundance of taxa belonging to the phyla Actinobacteria and Firmicutes, while Bacteroidetes and Proteobacteria phyla were enriched in controls (de Groot et al., 2017). The whole metagenomics shotgun sequencing of the subgingival microbiome showed that T2DM was associated with less diverse microbial composition and higher relative abundance of Anaerolineaceae bacterium oral taxon 439 (Farina et al., 2019). Still, these alteration of the oral microbiota cannot explain how DM increases the risk of periodontitis. In spite of the limitations of mouse model studies, Xiao et al. have provided more direct evidence to solve this question. Diabetic db/db mice had increased periodontal inflammation and bone loss as well as enrichment of periodontitis-associated bacteria in the oral cavity (Xiao et al., 2017). Moreover, local injection of anti-IL-17 into the gingiva antibody markedly reduced the oral microbiome–induced periodontal pathogenicity in diabetic db/db mice, implying that DM can increase severity of periodontal disease through periodontal inflammation and subsequent alteration of the oral microbiome.

Conversely, periodontitis can affect the pathogenesis of DM as well. Metaanalysis studies showed that periodontitis can exacerbate glycemic control and increase the risk of developing diabetes in healthy individuals (Borgnakke et al., 2013; Graziani et al., 2018). Even in subjects with diabetes, periodontitis exerted an adverse effect on glycemic control and diabetic complications (Borgnakke et al., 2013; Graziani et al., 2018). In a wider perspective, it has been reported that there is a correlation between the severity of periodontitis and metabolic syndrome, including hypertension, obesity, dyslipidemia, and DM (Gomes-Filho et al., 2020). Interestingly, the periodontal pathogens have been detected in the oral cavity of both T1DM and T2DM patients (Aoyama et al., 2018; Schara et al., 2013). In high-fat diet–induced diabetic mice, P. gingivalis infection exacerbated periodontitis, systemic inflammation, glucose intolerance, and insulin resistance, simultaneously displaying the oral microbiota alteration (Blasco-Baque et al., 2017). Taken all together, these data suggest a two-way relationship between the oral microbiota and diabetes.

1.4 Concluding remarks

There is a growing body of evidence to support the theory that the oral microbiome is closely associated with human health, not limited to the oral cavity, but also related to diverse systemic physiological functions. As depicted in Fig. 1.1, in certain pathogenic circumstances the oral microbiota can translocate into the other organs and directly facilitate pathogenesis. In rather indirect manners, the oral microbiota can regulate systemic inflammatory and immune responses, which is considered a main crossroad between oral dysbiosis and systemic diseases. Moreover, recent investigations suggest that oral microbiota dysbiosis can change the microbial ecosystem in distal organs, such as the gut microbiome, suggesting an oral-gut axis (Kitamoto et al., 2020; Olsen and Yamazaki, 2019). This means that it is possible to regulate the majority of the gut microbiota–associated human diseases by modulating the oral microbiota. Therefore it is not an overstatement to say that oral health is a gateway to maintaining whole-body health. In this regard, further research on the role of the oral microbiome in human health will enable the development of new strategies for diagnosis and treatment of oral diseases as well as systemic diseases.

Figure 1.1 Oral microbiota dysbiosis in oral and systemic diseases.

The oral microbiome can affect pathogenesis locally in the oral cavity, including dental caries, periodontal disease, and oral cancer. In addition to the local effects, oral dysbiosis can regulate systemic diseases, such as Alzheimer’s disease, cardiovascular disease, and diabetes, possibly through provoking direct translocation of certain types of oral pathogens to the distal organs, sustaining systemic inflammation, and/or changing the microbial ecosystem in other organs.

Acknowledgments

This research was supported by National Research Foundation of Korea (NRF) Grants funded by the Korean Government (grant numbers NRF-2020R1C1C1003338, NRF-2016R1A5A2008630, and NRF-2022M3A9F3016364 to N.Y.S.) and by the Yonsei University Research Fund of 2021 (Yonsei Signature Research Cluster Program 2021-22-0017).

References

Aemaimanan et al., 2013 Aemaimanan P, Amimanan P, Taweechaisupapong S. Quantification of key periodontal pathogens in insulin-dependent type 2 diabetic and non-diabetic patients with generalized chronic periodontitis. Anaerobe. 2013;22:64–68.

Alley et al., 2008 Alley DE, Crimmins EM, Karlamangla A, Hu P, Seeman TE. Inflammation and rate of cognitive change in high-functioning older adults. J Gerontol A Biol Sci Med Sci. 2008;63:50–55.

Aoyama et al., 2018 Aoyama N, Suzuki JI, Kobayashi N, et al. Increased oral Porphyromonas gingivalis prevalence in cardiovascular patients with uncontrolled diabetes mellitus. Int Heart J. 2018;59:802–807.

Bartold and Van Dyke, 2019 Bartold PM, Van Dyke TE. An appraisal of the role of specific bacteria in the initial pathogenesis of periodontitis. J Clin Periodontol. 2019;46:6–11.

Binder Gallimidi et al., 2015 Binder Gallimidi A, Fischman S, Revach B, et al. Periodontal pathogens Porphyromonas gingivalis and Fusobacterium nucleatum promote tumor progression in an oral-specific chemical carcinogenesis model. Oncotarget. 2015;6:22613–22623.

Blaizot et al., 2009 Blaizot A, Vergnes JN, Nuwwareh S, Amar J, Sixou M. Periodontal diseases and cardiovascular events: meta-analysis of observational studies. Int Dent J. 2009;59:197–209.

Blasco-Baque et al., 2017 Blasco-Baque V, Garidou L, Pomie C, et al. Periodontitis induced by Porphyromonas gingivalis drives periodontal microbiota dysbiosis and insulin resistance via an impaired adaptive immune response. Gut. 2017;66:872–885.

Blekkenhorst et al., 2018 Blekkenhorst LC, Bondonno NP, Liu AH, et al. Nitrate, the oral microbiome, and cardiovascular health: a systematic literature review of human and animal studies. Am J Clin Nutr. 2018;107:504–522.

Borgnakke et al., 2013 Borgnakke WS, Ylostalo PV, Taylor GW, Genco RJ. Effect of periodontal disease on diabetes: systematic review of epidemiologic observational evidence. J Clin Periodontol. 2013;40(Suppl 14):S135–S152.

Bowen et al., 2018 Bowen WH, Burne RA, Wu H, Koo H. Oral biofilms: pathogens, matrix, and polymicrobial interactions in microenvironments. Trends Microbiol. 2018;26:229–242.

Carding et al., 2015 Carding S, Verbeke K, Vipond DT, Corfe BM, Owen LJ. Dysbiosis of the gut microbiota in disease. Microb Ecol Health Dis. 2015;26:26191.

Chen et al., 2017 Chen CK, Wu YT, Chang YC. Association between chronic periodontitis and the risk of Alzheimer’s disease: a retrospective, population-based, matched-cohort study. Alzheimers Res Ther. 2017;9:56.

Chen and Xia, 2020 Chen M, Xia W. Proteomic profiling of plasma and brain tissue from Alzheimer’s disease patients reveals candidate network of plasma biomarkers. J Alzheimers Dis. 2020;76:349–368.

Cheng et al., 2020 Cheng R, Wu Z, Li M, Shao M, Hu T. Interleukin-1beta is a potential therapeutic target for periodontitis: a narrative review. Int J Oral Sci. 2020;12:2.

Curtis et al., 2020 Curtis MA, Diaz PI, Van Dyke TE. The role of the microbiota in periodontal disease. Periodontol 2000. 2020;83:14–25.

D’Aiuto et al., 2004 D’Aiuto F, Parkar M, Andreou G, et al. Periodontitis and systemic inflammation: control of the local infection is associated with a reduction in serum inflammatory markers. J Dent Res. 2004;83:156–160.

Darveau et al., 2012 Darveau RP, Hajishengallis G, Curtis MA. Porphyromonas gingivalis as a potential community activist for disease. J Dent Res. 2012;91:816–820.

de Groot et al., 2017 de Groot PF, Belzer C, Aydin O, et al. Distinct fecal and oral microbiota composition in human type 1 diabetes, an observational study. PLoS One. 2017;12:e0188475.

Deo and Deshmukh, 2019 Deo PN, Deshmukh R. Oral microbiome: unveiling the fundamentals. J Oral Maxillofac Pathol. 2019;23:122–128.

Dominy et al., 2019 Dominy SS, Lynch C, Ermini F, et al. Porphyromonas gingivalis in Alzheimer’s disease brains: evidence for disease causation and treatment with small-molecule inhibitors. Sci Adv. 2019;5:eaau3333.

Donaldson et al., 2016 Donaldson GP, Lee SM, Mazmanian SK. Gut biogeography of the bacterial microbiota. Nat Rev Microbiol. 2016;14:20–32.

Du et al., 2020 Du Q, Fu M, Zhou Y, et al. Sucrose promotes caries progression by disrupting the microecological balance in oral biofilms: an in vitro study. Sci Rep. 2020;10:2961.

Duncan et al., 1995 Duncan C, Dougall H, Johnston P, et al. Chemical generation of nitric oxide in the mouth from the enterosalivary circulation of dietary nitrate. Nat Med. 1995;1:546–551.

Dutzan et al., 2018 Dutzan N, Kajikawa T, Abusleme L, et al. A dysbiotic microbiome triggers TH17 cells to mediate oral mucosal immunopathology in mice and humans. Sci Transl Med. 2018;10.

Engelhart et al., 2004 Engelhart MJ, Geerlings MI, Meijer J, et al. Inflammatory proteins in plasma and the risk of dementia: the rotterdam study. Arch Neurol. 2004;61:668–672.

Esberg et al., 2020 Esberg A, Haworth S, Hasslof P, Lif Holgerson P, Johansson I. Oral microbiota profile associates with sugar intake and taste preference genes. Nutrients. 2020;12.

Farina et al., 2019 Farina R, Severi M, Carrieri A, et al. Whole metagenomic shotgun sequencing of the subgingival microbiome of diabetics and non-diabetics with different periodontal conditions. Arch Oral Biol. 2019;104:13–23.

Ford et al., 2006 Ford PJ, Gemmell E, Chan A, et al. Inflammation, heat shock proteins and periodontal pathogens in atherosclerosis: an immunohistologic study. Oral Microbiol Immunol. 2006;21:206–211.

Forssten et al., 2010 Forssten SD, Bjorklund M, Ouwehand AC. Streptococcus mutans, caries and simulation models. Nutrients. 2010;2:290–298.

Furutama et al., 2020 Furutama D, Matsuda S, Yamawaki Y, et al. IL-6 induced by periodontal inflammation causes neuroinflammation and disrupts the blood-brain barrier. Brain Sci. 2020;10.

Galkina and Ley, 2009 Galkina E, Ley K. Immune and inflammatory mechanisms of atherosclerosis. Annu Rev Immunol. 2009;27:165–197.

Gao et al., 2018 Gao L, Xu T, Huang G, Jiang S, Gu Y, Chen F. Oral microbiomes: more and more importance in oral cavity and whole body. Protein Cell. 2018;9:488–500.

Goh et al., 2019 Goh CE, Trinh P, Colombo PC, et al. Association between nitrate-reducing oral bacteria and cardiometabolic outcomes: results from ORIGINS. J Am Heart Assoc. 2019;8:e013324.

Gomes-Filho et al., 2020 Gomes-Filho IS, Balinha I, da Cruz SS, et al. Moderate and severe periodontitis are positively associated with metabolic syndrome. Clin Oral Investig. 2020.

Graziani et al., 2018 Graziani F, Gennai S, Solini A, Petrini M. A systematic review and meta-analysis of epidemiologic observational evidence on the effect of periodontitis on diabetes An update of the EFP-AAP review. J Clin Periodontol. 2018;45:167–187.

Gupta et al., 2016 Gupta S, Allen-Vercoe E, Petrof EO. Fecal microbiota transplantation: in perspective. Therap Adv Gastroenterol. 2016;9:229–239.

Hajishengallis, 2015 Hajishengallis G. Periodontitis: from microbial immune subversion to systemic inflammation. Nat Rev Immunol. 2015;15:30–44.

Hajishengallis et al., 2011 Hajishengallis G, Liang S, Payne MA, et al. Low-abundance biofilm species orchestrates inflammatory periodontal disease through the commensal microbiota and complement. Cell Host Microbe. 2011;10:497–506.

Hansson and Hermansson, 2011 Hansson GK, Hermansson A. The immune system in atherosclerosis. Nat Immunol. 2011;12:204–212.

Heroiu Cataloiu et al., 2013 Heroiu Cataloiu AD, Danciu CE, Popescu CR. Multiple cancers of the head and neck. Maedica (Bucur). 2013;8:80–85.

Holmes et al., 2003 Holmes C, El-Okl M, Williams AL, Cunningham C, Wilcockson D, Perry VH. Systemic infection, interleukin 1beta, and cognitive decline in Alzheimer’s disease. J Neurol Neurosurg Psychiatry. 2003;74:788–789.

Hu et al., 2020 Hu Y, Li H, Zhang J, et al. Periodontitis induced by P. gingivalis-LPS is associated with neuroinflammation and learning and memory impairment in Sprague-Dawley rats. Front Neurosci. 2020;14:658.

Huang et al., 2014 Huang S, Li R, Zeng X, et al. Predictive modeling of gingivitis severity and susceptibility via oral microbiota. ISME J. 2014;8:1768–1780.

Ilievski et al., 2018 Ilievski V, Zuchowska PK, Green SJ, et al. Chronic oral application of a periodontal pathogen results in brain inflammation, neurodegeneration and amyloid beta production in wild type mice. PLoS One. 2018;13:e0204941.

Irfan et al., 2020 Irfan M, Delgado RZR, Frias-Lopez J. The oral microbiome and cancer. Front Immunol. 2020;11:591088.

Ishida et al., 2017 Ishida N, Ishihara Y, Ishida K, et al. Periodontitis induced by bacterial infection exacerbates features of Alzheimer’s disease in transgenic mice. NPJ Aging Mech Dis. 2017;3:15.

Janket et al., 2003 Janket SJ, Baird AE, Chuang SK, Jones JA. Meta-analysis of periodontal disease and risk of coronary heart disease and stroke. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2003;95:559–569.

Javed and Warnakulasuriya, 2016 Javed F, Warnakulasuriya S. Is there a relationship between periodontal disease and oral cancer? A systematic review of currently available evidence. Crit Rev Oncol Hematol. 2016;97:197–205.

Jindal et al., 2015 Jindal A, Parihar AS, Sood M, Singh P, Singh N. Relationship between severity of periodontal disease and control of diabetes (glycated hemoglobin) in patients with type 1 diabetes mellitus. J Int Oral Health. 2015;7:17–20.

Kademani, 2007 Kademani D. Oral cancer. Mayo Clin Proc. 2007;82:878–887.

Kamada and Nunez, 2014 Kamada N, Nunez G. Regulation of the immune system by the resident intestinal bacteria. Gastroenterology. 2014;146:1477–1488.

Kamarajan et al., 2020 Kamarajan P, Ateia I, Shin JM, et al. Periodontal pathogens promote cancer aggressivity via TLR/MyD88 triggered activation of Integrin/FAK signaling that is therapeutically reversible by a probiotic bacteriocin. PLoS Pathog. 2020;16:e1008881.

Kametani and Hasegawa, 2018 Kametani F, Hasegawa M. Reconsideration of amyloid hypothesis and tau hypothesis in Alzheimer’s disease. Front Neurosci. 2018;12:25.

Khader et al., 2004 Khader YS, Albashaireh ZS, Alomari MA. Periodontal diseases and the risk of coronary heart and cerebrovascular diseases: a meta-analysis. J Periodontol. 2004;75:1046–1053.

Kinney et al., 2018 Kinney JW, Bemiller SM, Murtishaw AS, Leisgang AM, Salazar AM, Lamb BT. Inflammation as a central mechanism in Alzheimer’s disease. Alzheimers Dement (NY). 2018;4:575–590.

Kitamoto et al., 2020 Kitamoto S, Nagao-Kitamoto H, Jiao Y, et al. The intermucosal connection between the mouth and gut in commensal pathobiont-driven colitis. Cell. 2020;182:447–462 e414.

Koch et al., 2017 Koch CD, Gladwin MT, Freeman BA, Lundberg JO, Weitzberg E, Morris A. Enterosalivary nitrate metabolism and the microbiome: intersection of microbial metabolism, nitric oxide and diet in cardiac and pulmonary vascular health. Free Radic Biol Med. 2017;105:48–67.

Kondo et al., 1994 Kondo K, Niino M, Shido K. A case-control study of Alzheimer’s disease in Japan–significance of life-styles. Dementia. 1994;5:314–326.

Lafuente Ibanez de Mendoza et al., 2020 Lafuente Ibanez de Mendoza I, Maritxalar Mendia X, Garcia de la Fuente AM, Quindos Andres G, Aguirre Urizar JM. Role of Porphyromonas gingivalis in oral squamous cell carcinoma development: a systematic review. J Periodontal Res. 2020;55:13–22.

Lundberg et al., 2015 Lundberg JO, Gladwin MT, Weitzberg E. Strategies to increase nitric oxide signalling in cardiovascular disease. Nat Rev Drug Discov. 2015;14:623–641.

Maekawa et al., 2014 Maekawa T, Krauss JL, Abe T, et al. Porphyromonas gingivalis manipulates complement and TLR signaling to uncouple bacterial clearance from inflammation and promote dysbiosis. Cell Host Microbe. 2014;15:768–778.

Mahmood et al., 2014 Mahmood SS, Levy D, Vasan RS, Wang TJ. The Framingham Heart Study and the epidemiology of cardiovascular disease: a historical perspective. Lancet. 2014;383:999–1008.

Mealey, 2006 Mealey BL. Periodontal disease and diabetes A two-way street. J Am Dent Assoc. 2006;137(Suppl):26S–31S.

Ministrini et al., 2020 Ministrini S, Carbone F, Montecucco F. Updating concepts on atherosclerotic inflammation: from pathophysiology to treatment. Eur J Clin Invest. 2020;e13467.

Mohanty et al., 2019 Mohanty R, Asopa SJ, Joseph MD, et al. Red complex: polymicrobial conglomerate in oral flora: a review. J Family Med Prim Care. 2019;8:3480–3486.

Noack et al., 2001 Noack B, Genco RJ, Trevisan M, Grossi S, Zambon JJ, De Nardin E. Periodontal infections contribute to elevated systemic C-reactive protein level. J Periodontol. 2001;72:1221–1227.

Olsen and Yamazaki, 2019 Olsen I, Yamazaki K. Can oral bacteria affect the microbiome of the gut?. J Oral Microbiol. 2019;11:1586422.

Poole et al., 2013 Poole S, Singhrao SK, Kesavalu L, Curtis MA, Crean S. Determining the presence of periodontopathic virulence factors in short-term postmortem Alzheimer’s disease brain tissue. J Alzheimers Dis. 2013;36:665–677.

Pushalkar et al., 2012 Pushalkar S, Ji X, Li Y, et al. Comparison of oral microbiota in tumor and non-tumor tissues of patients with oral squamous cell carcinoma. BMC Microbiol. 2012;12:144.

Rapado-Gonzalez et al., 2020 Rapado-Gonzalez O, Martinez-Reglero C, Salgado-Barreira A, et al. Association of salivary human papillomavirus infection and oral and oropharyngeal cancer: a meta-analysis. J Clin Med. 2020;9.

Rivera, 2015 Rivera C. Essentials of oral cancer. Int J Clin Exp Pathol. 2015;8:11884–11894.

Saulle et al., 2015 Saulle R, Semyonov L, Mannocci A, et al. Human papillomavirus and cancerous diseases of the head and neck: a systematic review and meta-analysis. Oral Dis. 2015;21:417–431.

Schara et al., 2013 Schara R, Skaleric E, Seme K, Skaleric U. Prevalence of periodontal pathogens and metabolic control of type 1 diabetes patients. J Int Acad Periodontol. 2013;15:29–34.

Schenkein et al., 2020 Schenkein HA, Papapanou PN, Genco R, Sanz M. Mechanisms underlying the association between periodontitis and atherosclerotic disease. Periodontol 2000. 2020;83:90–106.

Schmidt et al., 2014 Schmidt BL, Kuczynski J, Bhattacharya A, et al. Changes in abundance of oral microbiota associated with oral cancer. PLoS One. 2014;9:e98741.

Sharma et al., 2018 Sharma N, Bhatia S, Sodhi AS, Batra N. Oral microbiome and health. AIMS Microbiol. 2018;4:42–66.

Shreiner et al., 2015 Shreiner AB, Kao JY, Young VB. The gut microbiome in health and in disease. Curr Opin Gastroenterol. 2015;31:69–75.

Sivamaruthi et al., 2020 Sivamaruthi BS, Kesika P, Chaiyasut C. A review of the role of probiotic supplementation in dental caries. Probiotics Antimicrob Proteins. 2020;12:1300–1309.

Spahr et al., 2006 Spahr A, Klein E, Khuseyinova N, et al. Periodontal infections and coronary heart disease—role of periodontal bacteria and importance of total pathogen burden in the coronary event and periodontal disease (CORODONT) study. Arch Intern Med. 2006;166:554–559.

Stein et al., 2007 Stein PS, Desrosiers M, Donegan SJ, Yepes JF, Kryscio RJ. Tooth loss, dementia and neuropathology in the Nun study. J Am Dent Assoc. 2007;138:1314–1322 quiz 1381–1312.

Taylor, 2001 Taylor GW. Bidirectional interrelationships between diabetes and periodontal diseases: an epidemiologic perspective. Ann Periodontol. 2001;6:99–112.

Tervonen et al., 2000 Tervonen T, Karjalainen K, Knuuttila M, Huumonen S. Alveolar bone loss in type 1 diabetic subjects. J Clin Periodontol. 2000;27:567–571.

Tsai et al., 2002 Tsai C, Hayes C, Taylor GW. Glycemic control of type 2 diabetes and severe periodontal disease in the US adult population. Community Dent Oral Epidemiol. 2002;30:182–192.

Van Dyke et al., 2020 Van Dyke TE, Bartold PM, Reynolds EC. The nexus between periodontal inflammation and dysbiosis. Front Immunol. 2020;11:511.

Vanhatalo et al., 2018 Vanhatalo A, Blackwell JR, L’Heureux JE, et al. Nitrate-responsive oral microbiome modulates nitric oxide homeostasis and blood pressure in humans. Free Radic Biol Med. 2018;124:21–30.

Wu et al., 2017 Wu Z, Ni J, Liu Y, et al. Cathepsin B plays a critical role in inducing Alzheimer’s disease-like phenotypes following chronic systemic exposure to lipopolysaccharide from Porphyromonas gingivalis in mice. Brain Behav Immun. 2017;65:350–361.

Xiao et al., 2017 Xiao E, Mattos M, Vieira GHA, et al. Diabetes enhances IL-17 expression and alters the oral microbiome to increase its pathogenicity. Cell Host Microbe. 2017;22:120–128 e124.

Ye et al., 2016 Ye L, Jiang Y, Liu W, Tao H. Correlation between periodontal disease and oral cancer risk: a meta-analysis. J Cancer Res Ther. 2016;12:C237–C240.

Zhang et al., 2019a Zhang L, Liu Y, Zheng HJ, Zhang CP. The oral microbiota may have influence on oral cancer. Front Cell Infect Microbiol. 2019a;9:476.

Zhang et al., 2018 Zhang Y, Wang X, Li H, Ni C, Du Z, Yan F. Human oral microbiota and its modulation for oral health. Biomed Pharmacother. 2018;99:883–893.

Zhang et al., 2019b Zhang Z, Yang J, Feng Q, et al. Compositional and functional analysis of the microbiome in tissue and saliva of oral squamous cell carcinoma. Front Microbiol. 2019b;10:1439.

Zhao et al., 2017 Zhao H, Chu M, Huang Z, et al. Variations in oral microbiota associated with oral cancer. Sci Rep. 2017;7:11773.

Chapter 2

Influence of microbiome in shaping the newborn immune system: an overview

Manoj Kumar Kingsley¹ and B. Vishnu Bhat²,    ¹Department of Neonatology, JIPMER, Puducherry, India,    ²Director-Medical Research, Professor of Pediatrics and Neonatology, Aarupadaiveedu Medical College and Hospital, Vinayaka Mission’s Research Foundation-DU, Puducherry, India

Abstract

Microbial colonization during the neonatal period and infancy can have long-term consequences for the health and immunity of the individual. Several factors influence the development of the infant microbiome, such as mode of delivery, type of feeding, dietary changes, and antibiotic use. The infant microbiome exhibits high plasticity and adaptability, thus providing a window of opportunity for inducing appropriate interventions and alterations to improve overall health. Maintenance of host-microbiome homeostasis is very important as when this homeostasis is disrupted, it leads to aberrant outcomes. The microbiome has a crucial role in the maturation of the immune system and providing immunity against infections in early life. Furthermore, in utero microbial stimulation, mostly through the maternal gut microbiome, has been found to influence prenatal and postnatal immune cell development. Studies of germ-free animals have revealed significant information on the importance of the microbiome in the development of the immune system in early life. Recent studies have revealed a crucial time window in early life during which appropriate microbial exposures need to take place for proper immune cell development. Appropriate microbial exposures during this time window are also associated with reduced risk of development of various diseases, such as allergic diseases, asthma, inflammatory bowel disease, and diabetes in later life. A better understanding of how the neonatal microbiome develops and its role in the development and maturation of the immune system in early life could help us in devising effective strategies to prevent various health disorders.

Keywords

Colonization; early life; homeostasis; immune cell; immune system; infant; microbiome; neonatal

2.1 Introduction

The microbiome is the community of microorganisms that dwell in and on the body. During birth, newborns are subject to colonization by foreign microorganisms that inhabit most exposed surfaces, such as the skin, mouth, vagina, and gut (Round and Mazmanian, 2009). Most of the human microbiome is present in the gastrointestinal tract, and a major portion of these microorganisms are present in the colon. The human intestinal microbiota comprises 10¹³ to 10¹⁴ microorganisms that have evolved alongside the host immune system (Gill et al., 2006). These mainly consist of 500 species of bacteria (Brown et al., 2013). Though the microbiome is predominantly bacterial it also contains other microbes such as fungi, yeast and viruses (Salzman, 2014). Recent studies have shown that the mammalian gut contains a rich fungal community that interacts strongly with the immune system (Iliev et al., 2012).

The commensal microbiota colonizes various locations in the host and strongly influences the development of the immune system in early life (Gensollen et al., 2016). Humans and other mammals maintain a symbiotic relationship with this microbial ecosystem. As long as this host-microbiota relationship is operating optimally, it results in protective immune responses to pathogens and maintenance of tolerance to innocuous antigens. When this symbiosis is affected, it leads to a rise in autoimmune and inflammatory disorders, as has been seen in several high-income countries (Belkaid and Hand, 2014). It is evident from a number of studies that early life colonization coincides with a limited time period during which the immune system is permissive to instructions from the microbiota. Furthermore, the immune influences that occur during this time may have long-lasting effects on one’s health and immunity, including proper (or improper) development of specific immune cell subsets and resistance (or susceptibility) to various diseases later in life (Gensollen et al., 2016). When appropriate colonization does not happen during this time window, aberrant outcomes may occur.

2.2 How the microbiota shapes the development of immunity in early life

The microbiome influences the development of the immune system. Studies have shown that 45%–50% of the genes that are induced by the microbiota are related to immune response pathways (Gaboriau-Routhiau et al., 2009). The microbiota promotes and shapes the development of both innate and adaptive immunity. The postnatal period is an important time when early life microbial exposures shape the morphological and functional development of the immune system (Gensollen et al., 2016). Moreover, epidemiological data reveal a critical time window in early life during which the microbiota shapes the development of the neonatal immune system (Holt and Jones, 2000). For instance, the exposure of mice to microbiota-induced T regulatory cells during a critical time window during the weaning reaction reduced the susceptibility to several inflammatory pathologies in adult life. When microbiota exposure happens outside this time window, these positive outcomes could not be replicated (Nabhani et al., 2019). Similarly, the microbial antigen encounter from certain gut bacteria has to take place within a specific preweaning interval for developing tolerance to the gut bacteria (Knoop et al., 2017). Likewise, toll-like receptor 5 (TLR5) mediated counter selection of colonizing flagellated bacteria is restricted to a critical time window in the neonatal life and has a significant influence in shaping the gut microbiome (Fulde et al., 2018). Thus it is evident that microbial colonization during early life is required to take place within a window of opportunity during which the gut microbiome establishes the host’s mucosal immune homeostasis in a way that cannot be accomplished in later life. The immune development was found to be substantially different in mice that were conventionalized in adulthood despite the achievement of immune homeostasis (Aidy et al., 2013).

Commensals have a dual role in both training the immune system and functional tuning, thus acting as adjuvants to the immune system. The studies on germ-free (GF) mice and colonized mice have revealed the significance of the microbiome in the development of the immune system. GF animals have no commensal microflora, and their immune responses have been found not to be affected by the molecules of pathogenic and beneficial microorganisms. Specific pathogen-free (SPF) animals of the same strain have simple flora. The mucosal immune system GF animals is underdeveloped (Macpherson and Harris, 2004). They have severe defects in the development of gut-associated lymphoid tissues (GALT) (Round and Mazmanian, 2009) and antibody production. They have hypoplastic Peyer’s patches (PP) with fewer germinal centers and reduced number of IgA-secreting plasma cells and CD4+ T cells. Apart from the underdeveloped mucosal immune system, their spleen and lymph nodes are structureless with poorly formed B and T cell zones (Fig. 2.1) (Macpherson and Harris, 2004). Likewise, in immunocompetent mouse pups that had not been exposed to milk IgA, the FDC network and primary follicles in the spleen were not properly developed (Rosado et al., 2018).

Figure 2.1 Characteristics of germ-free mice.

BM, bone marrow; CLP, common lymphoid progenitor; DC, dendritic cells; IEC, intestinal epithelial cells; IEL, intraepithelial lymphocytes; ILC, innate lymphoid cells; ILF, isolated lymphoid follicles; iNKT, invariant natural killer T cells; LP, lymphocytes; MAIT, mucosa-associated invariant T cells; MC, macrophages; MHC II, major histocompatibility complex class II; MLN, mesenteric lymph nodes; MN, monocytes; MZ, marginal zone; NK, natural killer cell; N-NP, neonatal neutrophils; NP, neutrophils; PP, Peyer’s patches; RELM-β, resistin-like molecule β; SCFAs, short-chain fatty acids; SI, small intestine; SP, spleen; Treg, T regulatory cells; TCR, T cell receptor; Tfh, T follicular cells.

These defects seemed to be rectified by colonizing GF animals with commensal bacteria (Round and Mazmanian, 2009). This can be done just by keeping an SPF mouse in a cage along with GF animals (Macpherson and Harris, 2004). The induction of commensal bacteria results in the development of organized GALT, an increase in CD4+ T cell numbers, and the generation of secretory IgA (Macpherson and Harris, 2004). Another study showed that spleens from GF mice that were colonized with Bacteroides fragilis appeared normal along with well-developed lymphocyte zones (Mazmanian et al., 2005). Although most abnormalities in GF animals could be corrected by induction of commensal microbes, certain cellular defects could be rectified only within a short time period in early life.

2.3 Influence of the microbiota on the development and function of specific immune cell subsets in early life

The microbiota plays a crucial role in the development and function of individual immune cell subsets in early life. Moreover, microbial exposure that occurs within a critical developmental time window in early life is crucial for the proper development of immune system and individual immune cell subsets.

2.3.1 Myeloid cells

Germ-free mice are deficient in resident myeloid cell populations (macrophages, monocytes, and neutrophils) in the spleen and bone marrow. Complex molecular signals from the microbiota enhance the hematopoietic differentiation of myeloid cells (Khosravi et al., 2014). Likewise, antibiotics have shown to suppress hematopoiesis by depleting the intestinal microbiota (Josefsdottir et al., 2017).

Newborn infants are highly susceptible to infection. This is mainly due to the immaturity and hyporesponsiveness of neonatal immune cells. This immaturity of innate immune cells in neonates is due to lack of microbial priming. The infant gut microbiome provides appropriate stimuli for priming of innate immune cells in early life (Yu et al., 2018). The gut bacteria direct the innate immune cell development by promoting hematopoiesis. Depletion of the microbiota in mice by using antibiotic treatment resulted in the reduction of granulocytes in bone marrow and decreased bone marrow cellularity (Josefsdottir et al., 2017).

The microbiota plays an important role in promoting neutrophil homeostasis in early life. The transfer of normal microbiota to neonatal mice born to antibiotic exposed mouse dams induced interleukin 17 (IL-17) production by intestinal innate lymphoid cells (ILCs) and increased plasma granulocyte colony stimulating factor (G-CSF) levels and neutrophil numbers in circulation and bone marrow (Deshmukh et al., 2014). Recent studies have shown that neutrophil ageing, which results in the production of a functionally overly active subset of neutrophils, is induced by the microbiome(Zhang et al., 2015). Furthermore, microbiota depletion inhibits neutrophil extracellular trap (NET) formation (Zhang et al., 2015).

The intestinal microbiota regulates the steady state cellular life span of neutrophils and inflammatory monocytes (Hergott et al., 2016). In microbiota-depleted mice there were significantly reduced numbers of F4/80+ monocytes and macrophages in the intestines (Ekmekciu et al., 2017). The bone marrow monocytes from gut flora–depleted mice using broad-spectrum antibiotic treatment displayed lower migratory capacity and maturation (Emal et al., 2017).

Gestational colonization increased the numbers of early postnatal intestinal F4/80+CD11c+ mononuclear cells in mice (de Agüero et al., 2016). The gut microbiome drives the influx of monocyte precursors that differentiate into mature macrophages at the time of weaning. This process needs to be continued throughout adult life to maintain a normal intestinal macrophage pool (Bain et al., 2014). Depletion of the microbiota causes defects in the bactericidal capacity of alveolar macrophages (Brown et al., 2017). Likewise, macrophages that were isolated from antibiotic-treated mice displayed reduced expression of genes related to antiviral immunity. They also exhibited a defective response to type I and II interferons (IFNs) and impaired ability to control viral replication (Abt et al., 2012).

Following antibiotic treatment, the various dendritic cell (DC) subsets have been found to be reduced in mucosal and systemic sites (Ekmekciu et al., 2017; Thackray et al., 2018). Some Lactobacillus species have been found to enhance the priming and maturation of DCs, and a decrease in this can result in delayed immune response in neonates (Sanidad and Zeng, 2020). Likewise, the first exposure to the microbiota after birth induces tumor necrosis factor (TNF) secretion by monocytes and macrophages. This initiates the differentiation and functional maturation of the neonatal splenic type I preconventional DC (pre-cDC1) compartment (Köhler et al., 2020).

2.3.1.1 Erythrocytes in the spleen

CD71+ erythroid cells are enriched in the spleen from birth to 2 weeks of age but are reduced in adult life. Their numbers in this neonatal stage are not influenced by the microbiota. Neonatal CD71+ cells exhibit immunosuppressive properties and suppress the aberrant inflammation that is associated with microbial colonization in early life, and their elimination alleviates these protective benefits. In particular, the intestinal CD11b+ and CD11c+ cells from CD71+ cell–depleted neonatal mice produced increased levels of TNF-α and costimulatory molecule expression. The removal of commensal bacteria, in turn, corrected the elevated TNF-α production and costimulatory molecule expression induced by CD71+ cell depletion (Elahi et al., 2013). This information shows the reciprocal interactions between the immune system and the microbiota in early life.

2.3.1.2 Myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSCs) are a cell population having T cell–suppressive properties. They are rarely found in healthy adults but are found to be elevated in neonates (Gervassi et al., 2014). The elevated levels of MDSCs in the first few weeks of life may be a mechanism to enhance tolerance during microbial colonization among neonates (Fike et al., 2019; He et al., 2018).

2.3.2 Lymphoid cells

The depletion of the microbiota using antibiotic treatment results in severe reduction in the common lymphoid progenitors (CLPs) in the bone marrow along with reduction in the peripheral lymphocytes (Josefsdottir et al., 2017; Thackray et al., 2018). GF mice also display reduced number of CLPs (Iwamura et al., 2017). This shows that the microbiome has an important role in the differentiation and function of lymphoid cells.

2.3.2.1 T cell repertoire

The intestinal T cell receptor (TCR) repertoire is shaped during infancy. In normal SPF mice at birth, the repertoire is polyclonal, which changes toward a more restrictive oligoclonal repertoire after weaning and into adulthood. This may be due to the selective clonal expansion that happens. The T cell clones that react to commensal and food antigens are deleted, and those that react to pathogenic antigens are selected and expanded. Thus aberrant responses to commensal flora and food antigens are avoided. Here, bacteria drive the clonal selection and expansions after weaning either independently or along with food antigens. Likewise, in GF mice the TCR repertoire was oligoclonal at birth and remains oligoclonal at and after weaning. Nevertheless, in GF mice, because microbial colonization is absent, there are far fewer intestinal epithelial lymphocytes, and it is unlikely that any specific clone would have undergone selection and expansion. However, in SPF mice the repertoire is oligoclonal, since there are numerous intraepithelial lymphocytes (IELs), specific clones would have undergone selection and expansion (Probert et al., 2007).

Furthermore, intestinal microbes have also been found to influence the thymic T cell repertoire. The impaired thymic development of the transcription factor PLZF expressing innate lymphocytes in GF neonatal mice is corrected by colonization with B. fragilis. The intestinal bacteria regulate the thymic distribution of PLZF+ cells through plasmacytoid dendritic cells, which migrate from the colon to the thymus (Ennamorati et al., 2020).

The gut immune maturation is based on colonization with host-specific bacterial species that induces the expansion of T cell numbers in intestinal secondary lymphoid organs such as PPs and mesenteric lymph nodes (MLNs) and epithelial compartment (IEL and lamina propria) in mice. Segmented filamentous bacteria (SFB) could partially restore intestinal T cell numbers and were not fully protective against Salmonella infection, suggesting that other microbes are needed for complete immune maturation and host defense (Chung et al., 2012).

2.3.2.2 CD4+ T cells

Recent studies have shown that neonatal gut microbiome dysbiosis enhances CD4+ T cell dysfunction (Fujimura et al., 2016). The depletion of the microbiota resulted in the reduction of CD4+ T cells in the peripheral blood, small intestinal, and colonic lamina propria of mice. Reconstitution with microbiota was able to reverse these effects. In contrast, bone marrow CD4+ T cells increased, and splenic CD4+ T cells were not affected by microbiota depletion (Ekmekciu et al., 2017; Josefsdottir et al., 2017).

Commensal gut flora has been shown to drive the expansion of CD4+ T cells in the intestines of adult mice (Niess et al., 2008). The expansion of CD4+ T cells was observed in the colonic lamina propria of conventionalized adult mice. Specifically, SFB was able to induce the differentiation of a wide range of proinflammatory and regulatory CD4+ T cells in the intestines (Gaboriau-Routhiau et al., 2009). However, the situation in neonatal intestines is quite different. The CD4+ T lymphocytes exhibited an immature phenotype throughout the postnatal period in spite of the rapid establishment of the neonatal gut microbiome. The maternal secretory IgA and neonatal T regulatory cells act together to impede the postnatal maturation of CD4+ T cells. This reduced maturity of CD4+ T cells in the neonatal intestines may protect from the microbiota-induced maturation of cross-reactive T lymphocytes and autoimmunity in the postnatal period. This might also provide a time window in early life to reinstate self-tolerance and sustain a broad