What is the approximate number of microorganisms in the human body?

Microorganisms in the Human Body

The human body harbors trillions of microorganisms, with microbial cells outnumbering human cells by approximately 10 to 1, and the vast majority residing in the gastrointestinal tract. 1

Quantitative Overview

Total Microbial Load

• Approximately 39 trillion microbial cells inhabit the human body, representing a super-organism where microbial cells vastly outnumber our own somatic and germ cells 2
• The microorganism population is 10 times larger than the total number of human somatic and germ cells 1
• More than 70% of all microbes in the human body are located in the colon 3

Genetic Diversity

• The intestinal metagenome contains 150 times as many genes as the human genome 1
• The human gut microbiome harbors over 20 million genes 2
• Research has catalogued 3.3 million unique bacterial genes in the human gut, representing 150 times more genes than our own genome 4
• A core microbiome containing more than 6,000 functional gene groups is present in the majority of human guts surveyed 4

Anatomical Distribution

Gastrointestinal Tract Dominance

• The intestinal tract houses the majority of microorganisms in the body 1
• Approximately 100 trillion strains comprising more than 1,000 bacterial species maintain symbiotic relationships with the host in the gastrointestinal tract 5
• About 1,000 bacterial species are living in the human gut 4

Other Body Sites

• Significant microbial communities also inhabit the skin, vagina, and respiratory tract 1
• The airways, including the lungs, contain bacterial numbers similar to those found in the upper intestine, contrary to previous beliefs that the respiratory tract was sterile 1
• Each area of the body represents a discrete ecological niche with distinct microbial communities showing strong phylogenetic clustering 1

Taxonomic Composition

Dominant Phyla

• The gut microbiota consists mainly of the phyla Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria 5
• Firmicutes and Bacteroidetes constitute 70-90% of the total microbial abundance 5
• Two bacterial phyla, Firmicutes and Bacteroidetes, account for more than 90% of bacterial cells in the healthy adult intestine 3

Core Microbiota

• Most people share a core microbiota comprising 50-100 bacterial species at the phylotype level 4
• The degree of genetic variation among gut microbes is orders of magnitude greater than that of the human host—any two E. coli from the gut differ by approximately 40%, whereas human genomes differ by only 0.1% 1

Clinical Significance

Important caveat: The composition of the microbiome exhibits high variability between individuals at the species and strain level, making it comparable to a fingerprint 3. This individual variation, combined with differences based on anatomic location, delivery route at birth, diet, geography, and other environmental factors, means that while these quantitative estimates provide a general framework, actual microbial loads can vary substantially between individuals 1.