Chapter 24: Microbial Symbioses with Humans

The human body hosts trillions of microorganisms collectively known as the microbiota, which inhabit multiple anatomical sites including the gastrointestinal tract, oral cavity, skin, and urogenital system. The gastrointestinal microbiome is the most densely populated microbial habitat and plays critical roles in digestion, nutrient metabolism, and the production of biologically important compounds such as short chain fatty acids, vitamins, and amino acids. Distinct microbial communities exist along different regions of the digestive tract, with the large intestine functioning as an anaerobic fermentation chamber dominated by bacteria such as Bacteroides and Firmicutes that specialize in polysaccharide degradation. The oral microbiome forms complex biofilm communities such as dental plaque, where early colonizing bacteria establish structured microbial consortia that support metabolic cooperation. Additional body sites support specialized microbial ecosystems, including Lactobacillus dominated communities in the vagina that maintain acidic conditions protective against infection, and diverse microbial populations on the skin shaped by microenvironments such as sebaceous, moist, or dry regions. The human virome, composed of bacteriophages and animal viruses, also contributes to microbial ecosystem dynamics and can influence bacterial physiology and host health. The chapter further explores how the human microbiome develops from birth, beginning with microbial colonization during delivery and early infancy, with factors such as delivery mode and breastfeeding shaping early microbial succession until a stable adult microbiome emerges. Disruptions to microbial community balance, known as dysbiosis, are associated with numerous diseases including obesity, inflammatory bowel disease, dental caries, and skin disorders. Finally, the chapter discusses strategies for modulating the microbiome to restore microbial homeostasis, including fecal microbiota transplantation, probiotics, prebiotics, and synbiotics, highlighting how understanding human microbial symbioses has become central to modern medicine, nutrition, and microbiome based therapeutics.