Your Body Has More Bacterial Cells Than Human Cells

Your body has roughly 30 trillion human cells and 38 trillion bacteria, so bacteria slightly outnumber human cells, with most concentrated in the gut.

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For years, people repeated the idea that the human body contained about ten bacterial cells for every human cell. A detailed 2016 analysis revised that picture. For a reference 70 kilogram adult male, the researchers estimated about 30 trillion human cells and about 38 trillion bacterial cells, putting the bacterial-to-human ratio near 1.3 to 1. That means human cells account for roughly 44 percent of the combined human and bacterial cell count, while bacterial cells make up roughly 56 percent. The exact numbers vary with body size, age and other factors, so the percentage is an estimate rather than a fixed rule for every person. The striking part is where the bacteria are found. Most of the bacterial cells are concentrated in the colon, while smaller populations live on places such as the skin and in the mouth and other parts of the body. The numbers also do not mean bacteria make up most of a persons body mass. Human fat and muscle cells are much larger, while bacteria are tiny, and the researchers estimated the total bacterial mass at only about 0.2 kilograms for the reference adult. The study therefore changed the popular picture from a supposed ten-to-one bacterial majority to a much closer balance in cell numbers.

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