Our bodies host vast numbers of microbes, especially in the gut, and microbial cells rival or exceed the number of human cells depending on how you count them, a picture refined by recent estimates and large‑scale microbiome studies.
What it means
The microbiome refers to the communities of bacteria, archaea, fungi and viruses living on and inside us, with the highest densities in the gastrointestinal tract; these organisms perform essential roles in digestion, immunity and metabolism and together form a substantial portion of the cellular biomass associated with a human body.
How many microbes
Older statements claimed microbial cells outnumber human cells by ten to one, but more careful reviews and measurements revised that ratio to something much closer to parity, with modern estimates often near one to one or about 1.3 microbial cells per human cell, and total microbial counts reaching into the tens to hundreds of trillions depending on the individual and measurement method.
Variation between people
Microbial load and composition vary widely between individuals and over time, shaped by diet, age, antibiotics, environment and genetics; large longitudinal studies show each person’s microbiome is both highly individual and partly stable across years, which influences health and disease risk in personalised ways.
Why the counts are tricky
Estimating cell numbers depends on definitions (which microbes to count, whether to include extracellular microbes), sampling methods and assumptions about cell size and density; methodological differences explain why published figures range widely and why simple soundbites (like ‘‘10 to 1’’) are misleading.
Importance
Whether microbes outnumber human cells by a small or large margin, the crucial point is that the microbiome profoundly affects physiology, immunity and metabolism, making it central to modern medicine and a major focus of research into personalised health and disease prevention.
Quick related facts
- Definition: the microbiome includes bacteria, archaea, fungi and viruses that live on and in humans.
- Counts: earlier 10:1 claims have been revised; current best estimates are near parity or about 1.3:1 microbes to human cells.
- Total microbes: totals can reach tens to hundreds of trillions depending on individual and method.
- Role: microbiome influences digestion, immunity and metabolic health.