Human Microbiome: Trillions of Bacteria Live In and On Us

The human body hosts approximately 38 trillion bacterial cells—roughly equal to the number of human cells (previous estimates of 10:1 ratios are outdated). These microbes, collectively termed the microbiome, reside on skin, in the oral cavity, gastrointestinal tract, and urogenital mucosa.

Key Roles

• Digest fiber and produce short-chain fatty acids
• Synthesize vitamins (K, B12, folate)
• Train and modulate the immune system
• Protect against pathogens via colonization resistance
• Influence metabolism, mood, and brain function (gut-brain axis)

Diversity and balance are hallmarks of a healthy microbiome. Diet, antibiotics, stress, and lifestyle shape its composition.