The top and sole of a foot may share a sock, yet their bacterial habitats and odor compounds appear to differ. A study of several foot locations found more bacteria on the sole, with Staphylococcus accounting for a large proportion. Isovaleric acid, a compound involved in foot odor, was detected on the soles of eight of the 10 people tested, but not on the tops of their feet.
The study combined counting bacteria by location, examining community composition, and measuring volatile compounds released from the skin. Rather than treating the foot as a single site, it explored the spatial distribution of odor production.
Measuring bacterial numbers, types, and volatiles separately
The first investigation involved 16 people, eight women and eight men. After a standardized wash, they did not wash their feet or use foot products for 24 hours and wore supplied socks. Researchers then sampled four sites on the top and four on the sole of each foot.
The sampling fluid was cultured to count staphylococci, aerobic corynebacteria, and Gram-positive anaerobic cocci. Culture captures organisms that grow under the conditions provided. Therefore, for 10 people, five women and five men, bacterial DNA was also collected from one top-of-foot and one sole location, and community composition was analyzed using 16S rRNA gene sequences.
Volatile compounds were measured in 10 people, six men and four women. For this part, participants did not wash their feet for 48 hours after washing and changed into a fresh pair of supplied socks partway through. Thin silicone-based patches that take up compounds were placed on the top and sole for 30 minutes, and the collected compounds were heated and analyzed by GC–MS. Sampling procedures and timing differed from the bacterial measurements, so the participant counts cannot be added together and treated as one study population.
More bacteria on the sole, greater diversity on top
All three cultured bacterial groups were more abundant on the sole than on the top of the foot. Within the sole, counts tended to be higher toward the front near the bases of the toes than near the heel, although some site comparisons for anaerobic cocci were not significant. Staphylococci were the most abundant, followed by corynebacteria and anaerobic cocci.
DNA analysis also showed clear differences. At the sampled sole site, Staphylococcus accounted for an average of 98.6%, compared with 26.9% on top. The top of the foot had a more diverse community, including 12.1% Corynebacterium. These are relative community proportions derived from the analyzed sequences, not a claim that 98.6% of odor was attributable to staphylococci.
Overall community composition differed between the top and sole, but the difference between women and men was not significant. Cultured counts also showed no sex difference at most sites, except that anaerobic cocci were more abundant in women at two forefoot sole sites. In this sample, the major differences concerned sampling location rather than sex.
Isovaleric acid was detected on soles, alongside higher levels of other acids
Among volatile compounds, acetic, butyric, and valeric acids were detected at higher levels on the sole than on top. Particularly notable was isovaleric acid, associated with the cheesy, acidic character of foot odor. It was detected on eight of the 10 participants’ soles and on none of their top-of-foot samples.
Not detected means that the compound did not reach an amount measurable by the sampling and analytical methods used. It does not demonstrate that the top of the foot is completely odorless. The study also measured compound amounts; it was not a test in which assessors smelled different foot locations and scored intensity or unpleasantness.
Earlier research has shown that staphylococci on feet can use the amino acid leucine to produce isovaleric acid. Because this compound was detected on the staphylococcus-rich sole, the authors interpreted bacterial distribution as contributing to the distribution of foot-odor compounds.
The sole’s environment and bacterial activity raise further questions
The sole and top differ in factors such as the thickness of the outer skin layer, skin lipids, and pressure. The authors propose that such environmental differences may affect where bacteria live. For example, lower lipid levels on the sole might matter for organisms requiring lipids. The present comparison alone cannot identify which factor is decisive.
Organisms such as Kytococcus, which have attracted attention in foot-odor research as degraders of the outer skin layer, were also uncommon on the soles in this study. The authors point out that odor-related acids can arise even without large numbers of particular keratin-degrading bacteria. Foot odor appears difficult to explain as a problem involving only one bacterial type.
On the other hand, the unusually high proportion of staphylococci on soles may partly reflect the strictly standardized washing and sock conditions. Bacteria and volatiles could not be collected simultaneously from the same skin location, and directly linking particular bacteria to the amounts of compounds produced in each person remains a topic for future investigation.
The spaces between the toes were not measured because there was no method for standardized sampling of bacterial counts per unit area there. Nor did the study compare deodorizing products, washing methods, or skin care, so its results cannot establish the effectiveness of particular measures.
Summary
The sole had more culturable bacteria than the top of the foot, and its community was dominated by Staphylococcus. The top had a more diverse bacterial community. Acetic, butyric, and valeric acids were also more abundant on the sole; isovaleric acid was detected on eight participants’ soles but not on the tops of their feet.
Understanding foot odor benefits from looking beyond total bacterial counts to consider which organisms live where and what compounds emerge. The study relates the distributions of bacteria and compounds and indicates a route toward testing that relationship more directly.
Research affiliations and conflicts of interest: At the time of the study, nine of the 12 authors were affiliated with Unilever. The paper declares no conflicts of interest. No separate research-funding disclosure section was found.
Reference
Stevens D, Cornmell R, Taylor D, et al. Spatial variations in the microbial community structure and diversity of the human foot is associated with the production of odorous volatiles. FEMS Microbiology Ecology. 2015;91(1):1–11. doi:10.1093/femsec/fiu018 (published online in 2014)
