Question: Does the bacteria content of a person's poo change the smell?

Ciarán O'Brien answered on 17 Nov 2014:

It can, yes!

All the different bacteria in your gut have their own preference for the kind of food they like to eat. Some prefer sugary foods, others like fatty foods, and so on. Eating a varied diet will help keep all the numbers of the different bacteria in check, and everyone’s happy. So if you change what you eat, you change the bacterial content of your gut, and those changes can be seen in someone’s poo.

There’s a type of bacteria called sulphate reducing bacteria. They live in your gut in relatively small numbers most of the time. One of the chemicals they produce from growing is hydrogen sulphide, which is what gives rotten eggs their smell. If you have a lot of sulphate reducing bacteria in your gut, they’ll produce more hydrogen sulphide, and your poo will end up being a whole lot smellier as a result.

You could combine these bacteria with ones we call methanogens (they produce methane gas as a side effect of growing) to make things smellier. Methane doesn’t smell of much by itself, but it’s great for carrying smellier gases with it. To encourage those bacteria to grow you need to eat more cellulose, which is found in plants.

So, eat lots of egg yolks (containing sulphur) and lots of veggies, and you’ll change the numbers of certain bacteria in your poo, which will make it the smelliest poo in Christendom!