Ten Times More Microbial Cells than Body Cells in Humans?

Science rights itself by critical reanalysis of facts. Most scientists are hypercritical and very resistant to new claims in the absence of unscrutinized data. But once in a while some fake “fact” gets into the literature that is difficult to expunge. A recent case in point: the frequent assertion that the number of cells in the human microbiota is ten times as numerous as the number of cells in the human body (J. Fox, Microbe, December 2013, p. 494; M. Pollan, New York Times Magazine, May 15, 2013; Science 342:1440, 2013; Cell Metab. 12:111, 2010). Let's think about it. The number of cells in the human microbiota is likely to be highly variable depending on nutritional input, body size, age, ethnicity, culture and environment. There could also be significant variability in body cell number among diverse humans in diverse environments and at different stages of life. Furthermore, it is difficult to determine the number of cells in the human body accurately. Finally, there is no a priori reason for thinking that the ratio of microbial to human cells is constant among humans or even for one individual in the course of a lifetime. Thus, the ratio of these two variable numbers is likely to be highly variable.