When people ask me what the microbiome is, part of my answer usually includes the fact that there are 10 times as many bacteria in the body as human cells in the body. Unfortunately, I may no longer be able to use that statistic. A recent study out of the Weizmann Institute in Israel states that the number of bacteria may actually be very similar to the number of human cells in the body.
The authors of the study found that the 10:1 ratio of bacterial to human cells goes back to a 1977 study by Dwayne Savage and an earlier 1972 paper estimating the number of bacterial cells in the human body. The Weizmann scientists redid the estimate and found that there were about 39 trillion bacterial cells in the body. They also estimated the number of human cells in the body, about 84% of which are red blood cells, finding there to be about 30 trillion human cells in the body.
While this results in about 1.3 bacterial cell per human cell, the numbers may vary significantly from person to person and could change significantly with each defecation. They estimate that the range of bacterial cells goes from about 30 to 50 trillion in each individual. Women may also have a higher ratio of bacterial cells than human cells because they have fewer human cells, specifically red blood cells.
While this study does not take into account fungi, viruses, and archaea which all make up the human microbiome and would increase the ratio of microbes to human cells, the often stated ratio of 10:1 for bacterial cells to human cells is most likely not accurate. While I will no longer be able to use this fun fact in my description of the microbiome, it does not take away from the importance of bacterial cells in human health.