Foot Odor Microbes
By Heath Carmichael
What microbes are associated with foot odor? What benefits, if any, do these microbes serve for humans? What metabolic processes do these microbes have that lead to the odors they produce? What diseases might be caused by these microbes?
Bromodosis is the medical term for having foot odor. Many people worldwide suffer from Bromodosis. Bromodosis is caused from microbes that metabolize dead skin cells from peoples feet to smelly molecules. This is an even bigger problem for people who run because sweat provides an ideal growth environment for foot microbes. Some think that the odor is caused by sweat. This is untrue because sweat is odorless, but the microbes that live on feet grow better because of the moisture provided by sweat. Another contributor to foot odor is the type of socks that people choose to wear. Socks constructed of synthetic material like nylon and polyester provide less ventilation for peoples feet and therefore increase the amount of sweat produced. Wool or cotton socks do a much better job at providing more ventilation for feet so less sweat is produced. A study was done to test the amount of microbial growth on cotton t-shirts versus synthetic t-shirts and the results were that Firmicutes, Actinobacteria and Proteobacteria were found in both types of shirts, but Micrococci were found in much more abundance on synthetic t-shirts (Callewaerta et al. 2014). Micrococci are gram positive actinobacteria and when under anaerobic conditions they can produce acid from glucose (Microbewiki, Micrococcus). The anaerobic conditions on synthetic shirts were most likely due to the poor ventilation of these shirts compared to the ventilation of cotton t-shirts. Wearing the same shoes consistently is also a cause of foot odor because it does not allow proper time for shoes to “air out” and reduce the amount of moisture inside the shoes.
The smell is caused by the breakdown of fatty acids and peptides into smaller acids, most commonly acetic acid, serine, alanine and glycine (Ara et al. 2005). This study also attempted to identify what microbes are to blame for the production of these smelly molecules. Propionibacteria and Staphylococcus were present in large amounts, but the amount did not change between smelly feet and feet that did not produce the same odors. Both these microbes are part of the normal human flora so their abundance is not unusual. Propionibacteria is a gram positive actinobacteria that lives primarily under anaerobic conditions, so its main metabolic processes are fermentation to produce lactic acid and propanoic acid. However, the amount of Bacilli increased on odorous feet which implies that an increase in Bacilli could lead to an increase in smell. Staphylococcus aureus is a gram positive firmicute that can take up a number of different sugars to metabolize to lactate using the Embden-Meyerhof-Parnas pathway and has been shown to produce acids that contribute to foot odor (Microbewiki, Staphylococcus aureus). Staphylococcus epidermis has also been shown to lactic acid that contribute to sweat odor (Sawano 2000). It has also been shown that Bacillus subtilis natto produces odors related to sweat by producing isobutyric and isovaleric acid form the amino acid leucine using a leucine dehydrogenase (Takemura et al. 2000). It was found that a similar leucine dehydrogenase is found in many microbes present in the human flora (Ada 2006). However, the leucine dehydrogenase enzyme has been found to have the highest activity in Bacillus sphaericus (Oshima et al. 1978). This enzyme is used to oxidize the L forms of leucine, valine, isoleucine, norvaline, norleucine, and aminobutyrate by removing the amine group from these amino acids and reducing NAD+ to NADH. The D enantiomers of these amino acids are capable of competitively inhibiting the leucine dehydrogenase enzyme. The removal of the amine group from these amino acids contributes to the odor produced by the bacteria that have the leucine dehydrogenase enzyme because NH3 and NH4+ have very distinct smells. While leucine dehydrogenase is most present in Bacillus sphaericus it is also found in many other types of Bacillus as well as in Corynbacterium. Corynbacterium is gram positive actinobacteria that is chemoorganotrophic, and is a fermenter under anaerobic conditions. This amine production as well as the mixed acid fermentation that comes from other microbes present on feet that break down proteins and carbohydrates in dead skin cells are the main contributing factors to foot odor.
Brevibacteria is another type of bacteria that is commonly associated with foot odors (Microbewiki, Brevibacterium linens). They ingest dead skin cells and break down the amino acid methionine to methanethiol which produces a funky odor. This type of bacteria also is used in the production of cheeses by metabolizing lactate in to lactic acid as well as metabolizing casein proteins to amine and sulfur compounds. What is interesting about this is that the odors associated with feet can also be associated with cheese. Common skin microbes also play an important part in the production of cheese. A famous example is Propionibacterium freundenreichii, which is used to produce Swiss cheese. These bacteria break down the lactic acid produced by other microbes using fermentation to produce propanoic acid and carbon dioxide. The propanoic acid helps to give Swiss cheese its smell and flavor. The carbon dioxide production helps to give the cheese its characteristic holes (Button, Dutton. 2012). Researchers have used the similarity of skin odor producing microbes and cheese producing microbes to test the effectiveness of using types of cheese as attractants for mosquitos. Mosquitos and other blood metabolizing insects are able to find their hosts by sensing chemical cues that come from their targets. Some attractants are Carbon Dioxide, from humans breath, volatile compounds from feces and urine such as octanol, as well as smelly acids like acetone and butanone (Knols, Meijerink. 1997). This research is of particular important for Africa, where malaria carrying mosquitos and tsetse flies are very common. Malaria is a blood born virus carried by mosquitoes. A person is infected when they are bit by a mosquito that carries the virus. The phage then replicates in the liver of human hosts and then attacks the host red blood cells. Some cases of malaria can be mild, with symptoms similar to a cold like a fever, sweats and an enlarged liver. However, cases of malaria can be very severe and life threatening such as cases of cerebral malaria, severe anemia and acute kidney failure (CDC, “Malaria Disease”). In fact, the washing of ones feet with antibacterial soap changes the biting pattern of Anopheles gambiae, a malaria carrying mosquito. Returning to the information that some of the bacteria on human feet are used to produce cheese, researchers have been testing the attraction of malaria mosquitos to cheese. Limburger cheese production most closely resembles the microbial community present on human feet and as a result malaria carrying mosquitos showed the highest affinity for this type of cheese. This research has implications for public health because malaria and other insect pathogens have plagued sub-saharan Africa for many years. If widespread malaria mosquito traps could be constructed across the continent, diseases that come from these pathogens could be exterminated. One study shows that the number of mosquitos caught in traps using Limburger cheese increased when compared to a control compound used in traps (Knols, Jong. 1996). In 2015 over 214 million cases of malaria occurred worldwide and 438,000 people died because of this disease, many in sub-saharan Africa (CDC, “Malaria Disease”).
Other Foot Microbe Diseases
The bacterial growth environment present on feet can also lead to a number of infections that range from annoying to dangerous. Cellulitis, or staph infection, is a common infection of the feet as a result of Staphylococcus aureus or Streptococci infecting layers of the skin (Mistry. 2013). These microbes are able to enter the skin via cuts or breaks in the feet. The bacteria is able to infect deeper layers of the skin such as subcutaneous fat and the dermis. The result of this infection is usually an irritating rash, but can have more serious symptoms in immunocompromised people. Diabetics are especially are at risk for more serious symptoms because they are at risk to develop a diabetic foot as a result of infection. Diabetic feet can develop as a result of bacterial infection and can cause diabetic ulcers. These ulcers present as a hole in a persons foot as a result of infection. The feet are a particularly risky part of the body for some diabetics because of the ideal bacterial growth environment that exists on feet. Choice of footwear is even more important for people with diabetes because a lack of ventilation can cost them more than just odorous feet. If a foot infection becomes especially nasty a diabetic person may have parts of their toes or feet amputated as a way to stop the infection from spreading to the blood stream (Paola et al. 2015). For most people cellulitis is easily treated with antibiotics, but it is important to consistently wash ones feet with antibacterial soap after a cut opens in order to avoid infection because the microbes that cause infection are also present largely in the normal human skin flora.
Another danger to human feet are fungal infections. Athletes foot is a famous fungal infection and has similar symptoms to Cellulitis. Redness and itching are the major symptoms and the most likely areas of infection are between the toes and underneath the feet. The fungi that are usually to blame for this disease are Epidermophyton, Trichophyton, and Microsporum (CDC, “Athletes Foot”). Athletes foot is referred to as such because it is usually spread in locker rooms or public showers. Both of these environments contain plenty of moisture which produce excellent growth environments for these fungi and can be easily spread to the feet of people that come in to contact with the fungi. Once a person has become infected with Athletes foot they may treat it with antifungal creams and by keeping the foot very dry as maintaining a moist environment will only contribute to the growth of the fungi. It is common that people will wear sandals when they are infected with Athletes foot in order to reduce the amount of moisture and make the living environment less beneficial for the fungi.
Toenails are excellent growth environments for microbes and fungi as well. Onychromycosis, or toenail fungus is a fungal infection located in the toenails. It is most commonly caused by dermatophyte fungi coming in to contact with a toenail. The symptoms are toenail discoloration, a brittle texture, inflammation around or underneath the toenail, and an odor. Having hang nails or breaks in the skin close to the toenail can increase the likelihood of developing onychromycosis due to the fact that it provides a place for the fungi to infect. It is also possible to have fungal infections in the fingernails, but it is more common to have nail fungus in the toes. This is because there is less blood circulation to the toenails than fingernails so the body is less capable of fighting off fungal infection in the toes than in the fingers. Another reason is the living environment for microbes and fungi is much better on the toenails than on the fingernails. The moisture and darkness produce ideal conditions for these fungi to grow. Treatments are usually either oral antifungal drugs or antifungal nail creams that can be applied directly to the nails. In serious cases it is even possible for the nail to be removed surgically in order to more directly treat the fungi, but it can take a long time for a new nail to grow in its place. The prevention for nail fungus is very similar to the way that other foot problems are treated, consistent cleaning as well as reducing the moisture on the feet and nails (“Nail Fungus”. Mayo Clinic).
 Jessica C Zweers, Imrich Barák, Dörte Becher, Arnold JM Driessen, Michael Hecker, Vesa P Kontinen, Manfred J Saller, L'udmila Vavrová and Jan Maarten van Dijl. "Towards the Development of Bacillus subtilis as a Cell Factory for Membrane Proteins and Protein Complexes". Microbial Cell Factories. 2008. 7:10
 Katsutoshi Ara, Masakatsu Hama, Syunichi Akiba, Kenzo Koike, Koichi Okisaka, Toyoki Hagura, Tetsuro Kamiya, and Fusao Tomita. “Foot odor due to microbial metabolism and its control”. NCR Research Press. 2005.
Luca Dalla Paola, Anna Carone, Lucian Vasilache, Marco Pattavina. “Overview on diabetic foot: a dangerous, but still orphan, disease”. European Heart Journal. 2015. A64-A68.