(August 30th, 2016) How many bacteria inhabitate the human body? For decades, people believed that bacteria would outnumber our own cells by at least 10:1. A new study corrects the ratio to 1:1 for men and 2:1 for women.
It is nearly common knowledge that the large majority of cells in the human body are actually non-human. What might be frightening to hygiene fanatics and fascinating to microbiologists is simply biology: we live in symbiosis with the bacteria on our skin, mouth or gut. How many bacteria there are, however, is a long-standing miracle and important not only to number nerds but also to doctors and pharmacists.
Researchers from the Weizmann Institute of Science in Israel have now tackled the issue systematically and revisited the “textbook ratio” of ten bacterial cells for every human cell. The three biologists Ron Sender, Shai Fuchs and Ron Milo began historically, discovering that this long-believed 10:1 ratio stems from a back-of-the-envelope calculation from the 1970s. Milo explains that this calculation 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.”
Therefore, he and his team now made a new calculation. First, they took a close look at microbial colonies in certain places. Bacteria populate the whole human body but they crowd together especially well on surfaces like the skin, the genitals and the gastrointestinal tract, including mouth and colon.
For these bacteria hotspots, they dug out the sizes from the literature. For example, the average human gastrointestinal tract weighs 1kg, which equals to 1l volume. Having also read up the bacterial density on these organs (for example 1011 cells/ml in the colon), the scientists started calculating: take volume (ml) and density (cells/ml) and multiply it. This gives you the total number of cells in the entire volume, here in each organ. Only the skin is an exception: here, you use density (cells/cm2) and area (cm2). With these parameters, Milo and his colleagues calculated that the vast majority of bacteria live in the colon and that the other organs contain orders of magnitudes fewer bacterial cells. And because they came up with 3.8x1013 cells for the colon, they regard this as the number of total bacterial cells in the body of a 70kg “reference man”.
So what about the number of human cells? One can imagine two ways of calculating the total number of human cells in the body. Either, an order of magnitude rough calculation, assuming that each kg of the human body consists only of “representative” mammalian cells with a volume of 1,000–10,000 µm3; cells with a density similar to water would weigh 10-12–10-11 kg. Using these numbers, you'd arrive at 1013–1014 human cells in a human body of 100 kg. Or, doing a detailed inventory of all cell types in the human body.
The latter brings the astonishing fact home to us that some cell types are the absolute rulers in our body realm: with 84%, red blood cells make up the majority of our body's cells, followed with 5% by platelets and bone marrow cells (3%). Other cell types are negligibly small in number. Hence, by adding up the published numbers of these cells in the human body, you arrive at 3x1013 human cells in a 70 kg “reference man”.
Luckily, both approaches lead to a similar number, so Milo and his colleagues agreed that the “reference man” consists of 3x1013 body cells. Knowing both values for bacteria as well as human cells, one arrives at a magical ratio of close to 1:1 for the “reference man”. Importantly, the “reference woman” has twice as many bacteria than her own body cells. This can be explained by a lower blood volume and a slightly larger colon size - and of course, the more diverse genital microbiota. Interestingly, elderly or obese men do not differ much from the “reference man”.
Milo is enthusiastic about his findings. “It is truly important to understand our microbiome and research into this fascinating field is crucial for biomedical research. In the life sciences, which involve 'messy', highly dynamic and variable systems, researchers sometimes tend to rely on qualitative rather than quantitative statements.” He believes that “educated estimates can serve as an extremely powerful tool for understanding the lives of cells.”