How much weight/volume do microbes occupy within the human body?

Question

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?

Answer 1

@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.

Answer 2

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