Israeli Study: Number of Bacteria in Body Far Fewer Than Thought

Researchers estimate there are some 40 trillion bacteria in the body – nearly a 1:1 ratio with human cells, not the 10:1 ratio scientists had presumed.

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A petri dish with bacteria culture (illustrative)
A petri dish with bacteria culture (illustrative)Credit: Dreamstime

The human body hosts many fewer bacteria than previously thought, a new study by Israeli researchers has concluded.

For decades, there has been a consensus in the scientific community – though it’s not clear where this consensus came from – that human bodies have about 10 times as many bacteria as they do human cells.

But a new calculation by researchers at the Weizmann Institute of Science concluded that the ratio is actually much closer to one to one – about 40 trillion bacteria and 30 trillion human cells.

Resident bacteria play important roles in both preserving health and causing sickness. The unique composition of each person’s resident bacteria – known as the microbiome – is one of the reasons for differences in how easily people gain weight, how they respond to medicines and many other differences, leading some to view it as a “second genome.” Indeed, one of medicine’s new frontiers is adapting treatments to each individual’s microbiome.

Accordingly, much research has been devoted to understanding the microbiome. And recently, mathematical and statistical models from the world of computer science have become a hot new tool for doing so.

The growing importance of understanding the body’s bacteria led Prof. Ron Milo, Dr. Shai Fuchs and student researcher Ron Sender of the Weizmann Institute to wonder whether the dominant assumption about the ratio between bacteria and human cells was actually correct. Their answer appears in a book titled “Cell Biology by the Numbers,” which Milo recently published, together with Prof. Rob Phillips of Caltech.

The researchers argue that the accepted estimate is seemingly derived from the assumption that a bacterium is 1,000 times smaller than a human cell. But not all cells are the same size, and not all bacteria are the same size. For instance, a human blood cell is only 100 times bigger than a “standard” laboratory bacterium. Moreover, bacteria that live in the large intestine are about four times bigger than the average bacterium.

Based on these and other similar facts, the researchers performed a computation in which the quantity of blood cells was weighted according to both their individual size and the ratio of their size to that of the body’s bacteria. They also took into account differences in the number of bacteria hosted by different organs of the body. The large intestine, for instance, contains a great many bacteria, but most organs contain far fewer.

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