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

Researchers from the NIH working on the Human Microbiome Project Consortium found in 2012 that microorganisms make up only 1-3% of your body mass.

The researchers collected samples from about 15 sites from the bodies of 242 study subjects, and analyzed all genome sequences from specimens the collected (human and microbial). Using computers, the researchers were able to exclude human DNA and focus in on certain genomic signatures indicative of microbes, identifying more than 10,000 bacterial species that occupy the human ecosystem accounting for between 81% and 99% of all the normal flora on healthy adults.

However, there is not always consensus on these figures and new research is being done every year. A 2016 article in Cell found that the often cited figure that microorganisms outnumber human cells 10:1 is rather inaccurate, since "both the numerator (number of microbial cells) and the denominator (human cells) of this 10:1 ratio are based on crude assessments," often tracing back to a single "back-of the envelope" estimate made in a 1972 study. The true ratio, the authors argue, is closer to 1:1, though microorganisms do likely still outnumber human cells. However, if you compare the number of bacteria to the number of nucleated human cells (which excludes red blood cells, among other cell types) you do get a ratio close to 10:1. Using their revised data, these researchers estimated in a PLOS Biology article that the total mass of microorganisms in the average human adult is 0.2 kg, or about 0.5 lbs.