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”).
 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.