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Microorganisms constitute the bulk of all the biomass on Earth. I weighed myself yesterday, and wondered how much less I would weigh if I were completely free of bacteria and microbes, inside and out.

Approximately how much weight and volume do microbes occupy within the average human body? How were these values obtained?

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up vote 7 down vote accepted

@AlanBoyd's calculations are reasonable, I think the estimate is off though. The human microbome includes other bacteria which are not necessarily E.coli equivalent.

The human microbome projects give estimates that microbes are 1-3% total body mass. i.e. several pounds of bacteria.

The GI tract alone has most of the microbome mass - faeces is ~60% intestinal flora/fauna by dry weight, which for many adults alone must be hundreds of grams at any given moment.

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The bacterial flora weighs approximately 90 g and, assuming bacterial cell density is approximately 1 g ml-1, occupies a volume of 90 ml.

The human body has 1013 human cells and hosts 9x1013 bacterial cells.

One E. coli cell has a mass of 0.95×10−15 kg (wet weight).

Assuming that E. coli is typical with respect to mass of human microbial flora:

mass of bacterial cells in one human body = (0.95×10−15 * 9x1013) kg = 0.0855 kg = 86 g

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Just human intestines have over a kilogram of bacteria! A good enema will leave you 2kg lighter and that's mostly bacteria. – jkadlubowska Dec 3 '12 at 21:09
The number I read where more like 1-2 orders of magnitude more bacterial cells than cells in our body. – Chris Dec 20 '13 at 9:56

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