Question: What is the average amount of bacteria in your poo?

Ciarán O'Brien answered on 12 Nov 2014:

It’s hard to get an exact number, because there are still a lot of gut bacteria that we know very little about. We can take an educated guess by analysing the DNA of the bacteria found in poo, though.

Bacteria have a gene called the 16s gene while other living things don’t. Each species of bacteria has a slightly different 16s gene, and that’s what we use to tell species apart. We can also get a good idea of how many bacteria are present in something by how many copies of this 16s gene we find (although that’s not perfect, as some bacteria might have 2 or 3 copies of a 16s gene in them).

Using this, scientists have been able to count about 90-95% of the bacteria that live in poo. Most of the bacteria come from 2 different phyla (big groupings of species with some common traits), bacteroidetes and firmicutes. I read a paper from 1998 where scientists managed to count the bacteria from these phyla. they got about 54,000,000,000 bacteroidetes and 72,000,000,000 firmicutes, so that’s 126,000,000,000 bacteria and we haven’t counted them all yet, the real number is probably closer to 130,000,000,000, absolutely huge numbers.

The kicker? Those numbers were for 1 GRAM of poo. People tend to produce closer to 400 or 500 grams of poo in one, um, sitting, so the amount of bacteria in any given poo is probably very close to:

52,000,000,000,000

That can change a lot depending on what you eat, whether you’ve been sick or taken antibiotics, but a healthy person’s poo would have somewhere around that number of bacteria.