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Microorganisms colonize various regions of the human body, including the mouth, nasal passages, throat, stomach, intestines, urogenital tract, and skin. The total number of microbial cells is estimated to range from 10¹³ to 10¹⁴—comparable to, or exceeding, the number of human somatic cells. This host–microbiome relationship has led to the conceptualization of humans as supraorganisms, wherein microbial communities perform vital roles in development, immunity, and disease susceptibility.
Understanding the human microbiome holds immense promise for clinical advancements. It could lead to biomarkers that forecast disease risk, tailored microbial therapies, personalized medicine, and custom-designed probiotics. Nonetheless, interpreting the complex host–microbe interactions remains a significant challenge. These interactions are not unidirectional; human lifestyle, diet, and health can influence the microbiota, obscuring causality.
The study of the microbiome has revealed extensive microbial diversity, primarily through culture-independent methods such as 16S rRNA gene sequencing and metagenomics. These technologies have facilitated comparisons of microbiomes across individuals and body sites, showing that although species-level composition varies widely, similarities often emerge at higher taxonomic levels.
Some studies have reported microbial DNA in the placenta, though whether a true in-utero microbiome exists remains debated. At birth, the newborn's microbiota is shaped by maternal vaginal flora, especially Lactobacillus species in vaginal births, or by skin and environmental microbes in cesarean deliveries. It continues evolving rapidly in the first three years. The gut microbiota stabilizes thereafter but remains responsive to environmental influences.
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, under certain conditions, normally benign microbes can become opportunistic pathogens.
The microbiota also confer protective benefits through microbial antagonism, preventing pathogen overgrowth by competing for resources and altering the local environment. Disruption of this balance, often by antibiotics, can lead to overgrowth of harmful species such as Clostridioides difficile.
Recent efforts like the Human Microbiome Project and global metagenomic initiatives have revealed a remarkable genetic capacity in the human microbiome, far exceeding the human genome. This supports the argument that the microbiome functions akin to a vital organ, essential for maintaining health and homeostasis.
