Why do shoes smell? Shoes smell because bacteria break down the sweat and dead skin cells from our feet, producing waste products that smell bad.
Bacteria live all over our skin, not just on our feet. These bacteria are called the skin microbiota. You may have heard of the gut microbiota, which is the name for all of the 100 trillion bacteria that live in your gut. Just like those bacteria, the ones that live on our skin are harmless and are actually beneficial to us. The bacteria that live in our gut have a constant source of food. They help us to break down many of the things that we eat. We aren’t able to break down some complex carbohydrates, many vitamins, and a lot of fiber. Our bacteria do that for us and in return for the sugars they get, they give us short-chain fatty acids. The bacteria on the skin don’t have access to a constant source of food, and they have to find their energy in other ways. They do it by feeding on dead skin, to release the fats it contains. They breakdown some of the compounds that are in our sweat. They are halotolerant, which means they can survive the very salty environment of our sweat, which would kill many other bacteria. They can also breakdown the oil, called sebum, that our skin produces.
There are many different types of bacteria on our skin, and they live in certain areas because they specialize. Some fare better on dry parts of the skin, and some do better in moist areas. The bacteria on the skin help us by making the skin acidic, which kills viruses and other harmful bacteria. They can also trigger the immune system to react to invading microbes that are trying to get through the skin. In tests, mice without bacteria on their skin did not respond to invading microbes, but mice with skin bacteria did. However, the skin microbiota can sometimes go out of balance and cause problems. If you have ever suffered from acne, that is the bacteria called Propionibacterium acnes that feeds on the sebum you produce. It releases lipase to digest the sebum and that can cause an inflammatory reaction in the skin, which is a spot.
One of the biggest problems with these bacteria is the smell they produce. Most of them don’t, but some of them produce a smell when they break down compounds we produce. The bacteria that live near our sweat glands are an example. We produce two kinds of sweat. We have the general sweat that comes out of our sweat glands, and we have a milkier sweat that comes out of apocrine glands that are attached to hair follicles under our arms and in some other places. This sweat contains proteins, sugars, and fats. A bacterium called Staphylococcus aureus loves this sweat and it breaks it down to get at the energy in it. The product it breaks it down into has a very strong smell, and this is the bad smell of sweat. Our sweat on its own doesn’t smell of anything. A similar thing happens on our feet.
The main bacteria that lives on our feet is called Brevibacterium linens. This is another bacteria that lives on the proteins, fats, and sugars that our sweat glands produce. You may not think it, but there are more sweat glands on the foot per cm than any other place on the body and most of them are at the front of the foot. When Brevibacterium linens breaks down the sweat, they produce methanethiol, which has a distinctive, sweaty foot smell. It is also used in some cheeses, which is why some cheeses smell like feet, or some feet smell like cheese. It is easy to clean and air other parts of the body, but we tend to spend most of our day in socks and shoes, which is a nice warm environment for the bacteria. Unless you have shoes and socks that wick away the sweat, it is going to build up in the shoes. Most of us clean our feet and wash off most of the old sweat, but it is very hard to clean our shoes. The bacteria stay in there, happily eating the sweat that is still in the shoe. The only way to get rid of the smell is to remove the bacteria or to remove the sweat, both of which are tricky to do. And this is why shoes smell, and this is what I learned today.
Photo by Melisa Godfreyson: https://www.pexels.com/photo/photo-of-pair-of-blue-sneakers-1003685/
Sources
https://www.nih.gov/news-events/nih-research-matters/bacteria-skin-boost-immune-cell-function
https://www.fiosgenomics.com/microbiome-vs-microbiota
https://www.health.harvard.edu/staying-healthy/can-gut-bacteria-improve-your-health
https://www.nature.com/articles/nrmicro.2017.157
https://www.webmd.com/skin-problems-and-treatments/skin-microbiome
https://www.nike.com/jp/en/a/how-to-remove-shoe-smell
https://en.wikipedia.org/wiki/Brevibacterium_linens
https://www.nationalgeographic.com/lifestyle/article/how-to-get-rid-of-shoe-odor
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