Foot malodour has a distinct cheesy, acidic note that has been
attributed to volatile fatty acids (VFAs), and, in particular, to isovaleric
acid.[1,11,12] Staphylococcus species isolated from the human
axilla (under-arm) have been shown to convert branched aliphatic
amino acids, such as L-leucine, to short-chain methyl-branched
VFAs, including isovaleric acid,[13] and it has since been reported
that the same mechanism is responsible for generation of this
VFA on the foot.[14] The volatile sulfur compound (VSC) methanethiol,
presumably a product of L-methionine metabolism, has
also been implicated in foot odour, with Brevibacterium species
proposed as the causal organisms on the basis of their ability to
carry out this biotransformation.[6] However, foot isolates of
K. sedentarius can also generate methanethiol in vitro, and there
is in fact no direct evidence to link this VSC or either of these
organisms to malodour.[6] What has been established is that the
intensity of foot malodour is associated with an increase in total
microbial load, along with elevated pH and minor pitting of the
stratum corneum.[6] As expected, elevated hydration levels, as a
consequence of the use of occlusive footwear, lead to an increase
in microbial numbers. Under these circumstances, it seems likely
that elevated proteolytic and callus-degrading activity by the foot
microbiota leads to the liberation of high levels of peptides and
amino acids which, in turn, serve as substrates for the production
of malodour.