Introduction to the Human Microbiota

The human microbiota is a community of microbes, primarily bacteria, that inhabit multiple body regions including the skin, mouth, gut, and urogenital tract. Together, these microbial populations support key functions essential for health. The total number of microbial cells ranges from 10¹³ to 10¹⁴, comparable to or exceeding human somatic cells, making humans supraorganisms dependent on microbial communities.

Through microbial antagonism, the normal microbiota protects the host by physically occupying binding sites and consuming nutrients, outcompeting invading pathogens. Gut microorganisms synthesize vitamins and suppress pathogens through bacteriocin production. This protective balance can be disrupted by antibiotics, potentially allowing harmful species like Clostridioides difficile to overgrow.

Microbial colonization begins at birth and is shaped by maternal vaginal flora in vaginal births or skin and environmental microbes in cesarean deliveries. As a person grows, diet, age, environment, and antibiotic use influence microbial diversity and stability. Temperature, pH, oxygen levels, and body secretions also affect microbial growth, with the gut microbiota stabilizing after three years but remaining responsive to environmental influences.

Resident microbiota are long-term, stable populations that establish and persist in specific body sites, forming the core microbial community. Transient microbiota are present only for short durations and do not establish permanent colonization. Both populations contribute to the overall microbiota, but resident microbiota provide consistent protective and metabolic functions.

Symbiotic relationships within the microbiota are diverse: commensals benefit without affecting the host, mutualistic microbes offer reciprocal advantages such as E. coli synthesizing essential vitamins, and parasitic organisms harm the host. However, normally benign microbes can become opportunistic pathogens under certain conditions, demonstrating the dynamic nature of host-microbe interactions.

The host-microbiome relationship is fundamental to development, immunity, and disease susceptibility. The microbiota functions akin to a vital organ, essential for maintaining health and homeostasis. Understanding the microbiome holds promise for clinical advancements including disease risk biomarkers, tailored microbial therapies, personalized medicine, and custom-designed probiotics.

Culture-independent methods such as 16S rRNA gene sequencing and metagenomics have revealed extensive microbial diversity and facilitated comparisons across individuals and body sites. These technologies revealed that although species-level composition varies widely, similarities often emerge at higher taxonomic levels. The Human Microbiome Project and global metagenomic initiatives demonstrated that the microbiome's genetic capacity far exceeds the human genome.