A huge variety of fungi call your feet home
Human skin is a hardy, water-resistant covering that keeps important biological stuff from falling out of the body. It's also a camping ground for millions of bacteria (picked out in magenta, above), fungi (seen in blue-green, above) and yeast that mostly hang out minding their own business and getting on with their lives. Sometimes, though, in particularly warm and moist areas, they grow a bit overexuberantly and cause problems.
Athlete's foot and toenail infections are two common examples of fungi gone wild. One team of researchers from the National Institutes of Health in Bethesda, USA, were interested to find out if the levels and types of fungi found on the foot predisposed it to fungal disease. So, they recruited 10 healthy adults, swabbed them in 14 different anatomical areas, including the feet, and ran genetic analyses on the sampled fungi.
8 of those 14 swabs were taken from core body parts - the inner ear, behind the ear, chest, back, nostril, scalp, the crease between the thigh and torso and the space between the eyebrows. These areas were totally dominated by fungi from the Malassezia genus, with an average of 6 different types observed.
3 of those 14 swabs were taken from the arm - at the elbow crease, palm and forearm - and these showed a richer level of fungal variety, with an average of 25 types identified.
The final 3 swabs were taken from the feet - the heel, toenail and between the toes - and returned a huge spectrum of fungal types. On average, 50 different varieties were found.
When some of the volunteers returned a few months later to be freshly swabbed, the core body and arm sites still housed the same types of fungi. On the feet, though, while there was still a huge assortment of fungi present, they were not the same communities that had previously been sampled.
This suggests that the normal exposure of the foot to the outside world causes constant shifts in microbial ecosystems that present opportunities for pathogenic fungi to become established. So, the relatively high propensity for fungal outbreaks on the foot - where up to 60% of "healthy" people have underlying infections - could definitely be explained by the highly changeable nature of the fungi that live there.