Hannah Critchlow put this question to microbiologist Professor Kevin Kerr, from Harrogate District Hospital...

Well, you could turn the question around the other way and ask why do cheeses smell like feet? And if you did, you'll come up with the same answer. And the answer is, bacteria - bacteria called Brevibacteria, which are used to ripen or mature certain types of cheeses can also be found growing harmlessly on the skin of humans especially areas where it's warm and moist such as sweaty feet. As the bacteria grow, they produce compounds which smell, well, cheesy and the chemicals responsible for this powerful pungent odour are sulphur containing compounds known as S-methyl thioesters.

If you've eaten asparagus, you may well find that your wee smells funny too and again, it's those S-methyl thioesters that you've got to thank, or blame, for your smelly wee. You may find something that actually looks like cheese underneath your toenails. So, does this mean that our bacterial friend, Brevibacterium is actually making cheese under your nails? Well, it may look like cheese, but it certainly wouldn't taste like cheese because what's going on here is that you've probably got a fungal infection in your nail, and the name for that is onychomycosis. Onychomycosis can be tricky to treat, so it's best to see your family doctor to discuss the available treatment options.

Hannah - And onychomycosis toenail infections are caused by fungi known as dermatophytes. These dastardly dermatophytes produce an enzyme which acts on the nail. Bad news for the nail, but good news for the fungus, because an enzyme called keratinase breaks down the keratin in the nail to provide tasty treats for the dermatophytes. So, the cheesy appearance, the yellow and thickening nail represents the damage done to the nail by the fungus feasting on it. But the cheesy aroma coming from fetid feet is from Brevibacterium festering between sweaty unwashed toes and producing stinky S-methyl thioesters. And you are likely to get both the Brevibacterium cheesy pong in combination with the fungi eating hardened toenail as both seem to like warm, moist environments.