Microbial Symbioses with Humans — The Human Microbiome, Health, and Disease Explained | Chapter 24 from Brock Biology of Microorganisms

How do the trillions of microbes that live on and inside our bodies shape our health, disease risk, and development? Chapter 24 of Brock Biology of Microorganisms dives into the human microbiome—exploring the complex symbiotic relationships between humans and their resident microbes. This summary covers the composition and dynamics of human-associated microbial communities, the consequences of microbial imbalance, and promising interventions for promoting well-being.

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The Gastrointestinal and Oral Microbiomes

The gastrointestinal (GI) tract is the most densely populated microbial ecosystem in the human body. While the stomach contains relatively few microbes—mostly acid-tolerant species—bacterial density increases dramatically in the small and especially the large intestine. Strict anaerobes like Clostridium and Bacteroides predominate, supporting digestion, production of short-chain fatty acids (SCFAs), and synthesis of essential vitamins.

The oral cavity is home to biofilm-forming bacteria nourished by saliva. Key genera include Streptococcus, Veillonella, and Actinomyces. Dental plaque develops through a succession of microbial colonizers, with metabolic cooperation ensuring community stability. The upper respiratory tract is also colonized by diverse microbes, while the lower tract is typically sterile.

Urogenital Tract, Skin, and the Human Virome

The urogenital tract and skin harbor distinct microbial communities:

• Vaginal microbiota are dominated by Lactobacillus, which maintain a low pH and help prevent infections.
• Penile microbiota composition varies with circumcision status.
• Skin microbiota differ by body site and include Propionibacterium, Corynebacterium, and Staphylococcus species.

The human virome comprises DNA viruses, bacteriophages, and even dietary plant viruses. Bacteriophages are particularly abundant and may help protect against bacterial pathogens. The recently discovered crAssphage is a major player in the gut virome.

Development of the Human Microbiome

Microbial colonization begins at birth. Whether delivered vaginally or by C-section influences the initial composition of the infant microbiome. Breast milk further shapes the infant gut by promoting the growth of Bifidobacterium via human milk oligosaccharides. The infant gut transitions from facultative anaerobes to a complex community of strict anaerobes, stabilizing around age three and resembling those of family members. Elderly individuals often experience reduced microbiome diversity, which has been linked to frailty. Animal models, particularly mice, are useful for studying microbiome effects, but differences from human physiology must be considered.

Microbiome Dysbiosis and Associated Disorders

A stable and diverse microbiome supports health, while dysbiosis (disrupted microbial balance) can promote disease:

• Obesity: Associated with altered ratios of Firmicutes to Bacteroidetes and changes in SCFA production.
• Inflammatory Bowel Disease (IBD): Characterized by a weakened intestinal barrier and reduced microbial diversity.
• Dental caries: Linked to acid-producing bacteria and low microbial diversity in dental plaque.
• Acne: Associated with specific strains of Propionibacterium acnes.
• Vaginal dysbiosis: Disruptions can lead to vaginosis and are influenced by hormonal cycles.

Dysbiosis can cause inflammation, "leaky gut," and altered immune function.

Modulating the Human Microbiome: Probiotics, Prebiotics, and Fecal Transplants

Strategies to restore and enhance the microbiome are advancing rapidly:

• Fecal transplants effectively treat recurrent Clostridioides difficile infections and show promise for some metabolic disorders.
• Antibiotics can disrupt microbial communities, reduce diversity, and encourage growth of resistant pathogens.
• Probiotics: Live beneficial microbes (e.g., Bifidobacterium, Lactobacillus) that support gut health, interfere with pathogen signaling, and treat some infant conditions.
• Prebiotics: Nondigestible fibers that selectively promote the growth of beneficial microbes.
• Synbiotics: Combined pro- and prebiotic therapies; these can reduce infant sepsis and promote overall health.

Early-life interventions may have lasting health effects.

Glossary: Key Terms from Chapter 24

• Microbiome: The collective genetic material of all microbes in a host.
• Dysbiosis: A disruption of the normal microbial community.
• SCFAs: Short-chain fatty acids (acetate, butyrate, propionate) produced by fiber fermentation.
• Fecal transplant: The transfer of gut microbiota from a healthy donor to a patient.
• Virome: All the viruses associated with a host organism.
• Probiotic / Prebiotic / Synbiotic: Strategies to support and enhance healthy microbial communities.

Conclusion: Microbial Symbioses and Human Well-Being

Chapter 24 highlights the pivotal role of microbial communities in maintaining health, preventing disease, and shaping our biology from birth through old age. Advances in microbiome science are transforming medicine and opening new frontiers in nutrition, disease prevention, and personalized therapy.

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