The correct answer is gram-negative bacteria (choice “d”); Pseudomonas is the most likely culprit. Candida albicans (choice “a”), a yeast, is an unlikely cause of this problem and even more unlikely to show up on a bacterial culture. Coagulase-positive staph aureus (choice “b”) is typically associated with infections involving the acute onset of redness, pain, swelling, and pus formation, not the indolent, chronic, low-grade process seen in this case. Trichophyton rubrum (choice “c”) is a dermatophyte, the most common fungal cause of athlete’s feet. The bacterial culture could not have grown a dermatophyte, which needs special media and conditions to grow.
Gram-negative interweb impetigo is a relatively common dermatologic entity, which can be caused by any number of organisms found in fecal material. Pseudomonas, Klebsiella, Proteus, and Acinetobacter are among the more common culprits. These types of infections tend to be much more indolent than the more common staph- and strep-caused cellulitis, which are more likely to create acute redness, swelling, pain, and pus.
Both types of bacterial infections need certain conditions in order to develop. These include excessive heat, sweat, and perhaps most significantly, a break in the skin barrier. Ironically, these fissures are often caused by dermatophytes, in the form of tinea pedis, which is, of course, far better known for causing rashes of the foot.
But tinea pedis is more likely to be found between the third and fourth or the fourth and fifth toes. It creates itching and maceration but rarely causes diffuse redness or edema, and even more rarely leads to pain (unless there is a secondary bacterial infection). As mentioned, given the indolence of this infective process, a culture result showing staph or strep was unlikely.
The culture in this case showed Proteus, for which the minocycline was predictably effective. The rationale for obtaining the acid-fast culture was the possibility of finding Mycobacteria species such as M fortuitum, which is known to cause chronic indolent infections in feet and legs. These, however, more typically manifest with solitary eroded or ulcerated lesions. (Minocycline would have been effective against this organism.)
The use of the topical econazole served two purposes: While this was clearly not classic tinea pedis, it was still possible a dermatophyte or a yeast could have played a role in the creation of the initial fissuring; econazole will help control this, long term. Econazole also has significant antibacterial action and is particularly useful to help prevent future flares.