Friday, January 22, 2016

Who wins by count: Man Vs Microbiome


So often I have heard of talks and references stating that the total number of microbial cells in our body exceeds our own cell count by a ratio of 1:10. The original estimate was published in 1972 by Thomas Luckey. I myself like other microbiologists have quoted this estimate in a variety of my communication and classes. But the recent buzz has had me thinking. The new study claims that this is a myth and the real numbers are not anywhere near the estimated figure.

Table 1: Bacterial count estimates. Source
Microbiome was a big deal a few years ago. However, with better Sequencing and bioinformatic capabilities microbiome studies are becoming more common. The human microbiome project (HMP) has currently achieved to catalogue a huge amount of data. The aim of HMP was to generate resources that would enable the comprehensive characterisation of the human microbiome and analysis of its role in human health and disease. The current literature is pretty much aware of the average set of organisms that constitutes the microbiome.

Fig 1: Counting human cells
The original study was based on the estimates from stool samples. The new study by Milo etal, suggests that this is probably an overestimate. The new estimate is approximately 1.3:1, with an error factor of about 2 degrees. Yet that is way beyond the original estimate. The paper mined earlier data from multiple studies and put up the probable bacterial count from different body areas. See Table 1. My first set of question are based on the method of estimate. When I talk about the microbiome, I always want to think about all the microbial forms. I want to include fungus and viruses as well. It appears to me that this is obviously missing in the estimates. But even if I consider that there are at least a 100 phage for every bacteria, the estimate still doesn't shoot up to the classic 1:10 counts.

The study has provided a better number for quoting. But what is still an amazement, even a conserved estimate would be somewhere in a 1:1 ratio. That means there is at least one microbe for every cell the humans have. Needless to say, with defecation, bathing and other routine activities the ratio will change for a temporary period of time. Simplified into one line, “The numbers are similar enough that each defecation event may flip the ratio to favour human cells over bacteria”. Peer Bork comments, "It is good that we all now have a better estimate to quote. But I don’t think it will actually have any biological significance".
Ron Sender, Shai Fuchs & Ron Milo (2016). Revised estimates for the number of human and bacteria cells in the body bioRxiv DOI: 10.1101/036103

Abbott, A. (2016). Scientists bust myth that our bodies have more bacteria than human cells Nature DOI: 10.1038/nature.2016.19136

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