Do Bacteria Cells Outnumber Human Cells by 10:1?

It has long been claimed that the number of bacteria cells in our bodies outnumbers the number of human cells by up to a 10 to 1 ratio. An article published in the January 13 issue of Cell, however, disputes this. According to Ron Sender, Shai Fuchs and Ron Milo’s Are We Really Vastly Outnumbered? Revisiting the Ratio of Bacterial to Host Cells in Humans, the ratio is closer to 1:1. The 10:1 estimate relied on an erroneous assumption about the the relative size of human and bacteria cells. According to a Science Daily summary of the research,

How many microbes inhabit our body on a regular basis? For the last few decades, the most commonly accepted estimate in the scientific world puts that number at around ten times as many bacterial as human cells. In research published in the journal Cell, a recalculation of that number by Weizmann Institute of Science researchers reveals that the average adult has just under 40 trillion bacterial cells and about 30 trillion human ones, making the ratio much closer to 1:1.

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The original estimate that bacterial cells outnumber human cells in the body by ten to one was based on, among other things, the assumption that the average bacterium is about 1,000 times smaller than the average human cell. The problem with this estimate is that human cells vary widely in size, as do bacteria. For example, red blood cells are at least 100 times smaller than fat or muscle cells, and the microbes in the large intestine are about four times the size of the often-used “standard” bacterial cell volume. The Weizmann Institute scientists weighted their computations by the numbers of the different-sized human cells, as well as those of the various microbiome cells. They also weighted their calculations for the quantities of “guest” bacteria in different organs in the body. For example, the bacteria in the large intestine dominate, in terms of overall numbers, all the other organs combined.

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