If you’ve had the pleasure of attending a microbiology or human microbiome talk within the last year, you’ve likely heard the following statistic: there are ten times as many bacteria in your body then there are eukaryotic cells. It’s an astounding fact that we cling to and regurgitate to impress our friends and make a splash at parties. But is it true? If you want to double check the math, go to the start.
From one source to another, I followed the rabbit hole of reviews and papers to find at the end of this journey a pair of papers by Dobzhansky, T and Lukey, D. Two publications published in the early 1970s which made estimates of the number of bacterial and eukaryotic cells based on weight. The eukaryotic estimate of 1013 cells, I could not track down. The paper was not available in electronic format. Luckily the bacterial estimate made by Lukey, D. et al is easily assessable. The statement in question was made in the opening remarks of the second International Symposium on Intestinal Microecology:
“The composition of this system is surprising. Adult man carries 1012 microbes associated with his epidermis and 1014 microbes in his alimentary tract (Fig. 1). The latter number is based upon 1011 microbes/g contents of an alimentary tract with a capacity of approximately 1 liter. The 1013 cells (2) in his body are a distinct numerical minority of the total being that we call man.” – T. D. Luckey. Introduction to intestinal microecology. The American Journal of Clinical Nutrition 25: 1972, pp. 1292-1294.
Figure 1, if you’re curious, is a drawing of man with an arrow pointing to his gut aptly labelled “Alimentary Tract: 1014 Bacteria”.
I am not doubting the validity of the statement or the calculations of the original authors. Instead, I want to bring to light the inevitable problem that arises when you propagate citations from one paper to another. The original message runs the risk of being lost or misinterpreted. Sadly, this is not an isolated incident and I have been on this garden path before. I have found citation which are overly vague, unrelated to the direct conclusion of the original paper, or rely on an interpretation of the data that is either written in speculation or never made at all. Propagating citations in this manner is potentially destructive to the scientific process so the next time you cite a paper which cited another, make sure to double check their statements.
Yuriy Baglaenko
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