Host: Jeff Fox with special guests, Ron Milo and Shai Fuchs.

Ron Milo of Weizmann Institute of Science in Rehovot, Israel, and Shai Fuchs at the Hospital for Sick Children in Toronto, Canada, talk with Jeff Fox about their efforts, with Ron Sender at Weizmann, to redetermine the ratio of microbial to human cells. This ratio, widely cited as being 10 to 1, is closer to even, they find, while arguing that it may prove helpful in the long run to have a better and more rigorous grasp of how many cells there are in both the host and the microbiome.

Milo, Fuchs, and Sender update the widely-cited 10:1 ratio, “showing that the number of bacteria in our bodies” is instead “of the same order as the number of human cells. Indeed, the numbers are similar enough that each defecation event may flip the ratio to favor human cells over bacteria.”

Thus, the total number of bacteria in the ″reference man″ is about 3.9 x 1013 with an uncertainty of 25%, and a variation over the population of 52%. For human cells, they find that the hematopoietic lineage of cells plays a “dominant role, accounting for about 90% of all body cells. They also revise estimates to the a new total of 3.0 x 1013 human cells in a 70-kg ″reference man″ with a 2% uncertainty.

This story was featured in the May 2016 issue of Microbe Magazine.

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Image: Colored transmission electron micrograph of methicillin-resistant Staphylococcus aureus (MRSA). MRSA are among the drugresistant pathogens that are drawing researchers to look at how such resistance moves through the environment (see p. 201). (Image © Credit: Biomedical Imaging Unit, Southhampton General Hospital/Science Source.)