How many bacteria live inside the human body?

The answer

The human body harbors approximately 38 trillion bacterial cells compared to roughly 30 trillion human cells, making the ratio remarkably close to 1:1 rather than the previously cited 10:1 estimate. These bacteria collectively contain about 3.3 million unique genes, compared to only about 20,000 human genes. The gut microbiome alone weighs approximately 2 kilograms and produces about 90% of the body's serotonin, a neurotransmitter critical for mood regulation. Gut bacteria also synthesize essential vitamins including K and B12, train the immune system to distinguish friend from foe, and influence everything from weight gain to mental health through the gut-brain axis.