In vitro susceptibility testing of Malassezia pachydermatis: technical issues and interpretive controversies

Malassezia pachydermatis is a normal inhabitant yeast of canine skin and ear canals and occasionally acts as an opportunistic secondary pathogen in dermatitis and otitis externa in dogs. The CLSI developed standard methods for susceptibility testing of yeasts both by broth dilution and disk diffusion methods. However, these methods are applicable to Candida spp. and Cryptococcus spp., while adjustments are needed for Malassezia species, including M. pachydermatis. These adjustments are necessary in order to meet some special requirements of M. pachydermatis: growth medium supplemented with lipid sources and dispersing agents to overcome the problem of cellular clumping which occurs due to the butyrous nature of this yeast; increased inoculum size and time of incubation to counteract the slow growth rate of Malassezia compared to that of Candida; altering the definition of MIC endpoint. Unfortunately, previous authors appear to have addressed these issues with a variety of approaches, which has resulted in a great variability of results, both in terms of absolute results and of interpretive considerations. The aim of this study was to evaluate the susceptibility of some clinical M. pachydermatis strains to miconazole and itraconazole by a broth microdilution method with RPMI 1640+MOPS and Christensen’s urea broth and to compare the results with available literature data, to assess the current status of antifungal susceptibility testing as far as M. pachydermatis is concerned.