As with the search for the agent of influenza earlier this century, several studies have appeared to incriminate a particular microbe, only to be discredited, partially at least, by further work. There was high excitement a few years ago, when one research team discovered that so-called brevibacteria, commonly found nestling between our toes, produced methanethiol. This substance has a cheesy smell exactly like that which emanates every evening from sweaty socks on the train home. But the circumstantial evidence against brevibacteria has not been borne out by subsequent studies, which have shown no consistent correlation between their presence and the offending stink.
Dr Keith Holland and colleagues at the University of Leeds have been particularly interested in another bacterium, called Micrococcus sedentarius. This certainly seems to be to blame for the affliction pitted keratolysis, which often develops in people such as soldiers and miners who wear occluded footwear for long periods. The condition is characterised by pits in the stratum corneum (dead outer layer) of the toes and sole of the foot. Normally extremely resistant to attack, the stratum corneum can become eroded under dank, airless conditions, especially in those areas that bear the greatest weight.
The Leeds researchers have been pursuing a twin-track approach - investigating precisely how Mic. sedentarius causes pitted keratolysis, but also trying to discover whether it or some other microbe or mixture of microbes is responsible for the stench sometimes issued by 'normal feet'.
They began with the right feet of 19 male volunteers, all of whom worked in an office, laboratory or factory. All had good foot hygiene and none was using products likely to affect the bacterial population of the skin. Evaluated for foot odour by an 'experienced assessor', nine of the men had a consistently low-level smell and 10 a high-level smell. The investigators also took washings of the feet to isolate any resident bacteria, and used a pH meter to measure the acidity or alkalinity of the soles of the feet.
Surprisingly, perhaps, Dr Holland and his co-workers found Mic. sedentarius, which is strongly suspected of being the initiator of pitted keratolysis, on the feet of individuals who were free of this condition. Yet biochemical tests showed that the bacterium behaved exactly as one would expect if it were capable of eroding the tough stratum corneum. Added to protein (the main consituent of stratum corneum), it produced two different enzymes that attacked the protein. Similar proteases are used to tenderise steaks as well as to remove the hairs from hides.
Even more convincing was the discovery that Mic. sedentarius broke down fragments of tissue from calloused feet - kindly provided by chiropodists in and around Leeds.
So why does this same microbe make inroads into human feet only when they are encased for long periods in the same footwear? The most recent findings from Leeds indicate that the answer is in the environment of an occluded foot. Normally, Mic. sedentarius is sparse in numbers and produces little of the protein- digesting enzymes. Gradually, however, as the foot becomes damper, alkalinity increases and this triggers the bacterium to grow more quickly and to generate greater quantities of enzymes. As a result, pits begin to form in the soles and other parts of the feet.
It is possible Mic. sedentarius also contributes methanethiol to the pungent smell associated with severe cases of pitted keratolysis. In the 19 Leeds feet, there was a marked association between odour and the presence of pits. At the same time, there was no correlation between the degree of stink and the presence of either Mic. sedentarius or brevibacteria.
What Keith Holland and his collaborator did find was a clear relationship between smelliness and two other groups of bacteria - staphylococci and aerobic coryneform bacteria. High population densities of these organisms do, they believe, predispose an individual to foot odour. Again, it is probably the increasing alkalinity within unchanged shoes and socks that promotes the proliferation of these types of bacterium.
So the game is afoot, the quarry in sight. Backed by Scholl International Research and Development, Dr Holland and his colleagues are now investigating further the microbes found most commonly on their 10 most pungent feet. The aim is to define more exactly the environmental conditions under which they synthesise odoriferous substances and foot-rotting enzymes. The yomping soldiery should benefit in the form of preventive measures. But so, too, on a vastly greater scale, should the possessors of sweaty feet and their long-suffering companions.
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