Evaluation of the antifungal susceptibility of Malassezia pachydermatis to clotrimazole, miconazole and thiabendazole using a modified CLSI M27-A3 microdilution method.
BACKGROUND
In this study, we evaluated the antifungal susceptibility of Malassezia pachydermatis to clotrimazole (CTZ), miconazole (MCZ), and thiabendazole (TBD), azole derivatives employed in aural formulations labeled for treatment of canine otitis.


METHODS
The procedure for in vitro testing was based on the indications of the Clinical and Laboratory Standards Institute (CLSI) M27-A3 microdilution method. A lipid-enriched medium was employed to enhance the yeast growth (Christensen's urea broth, with 0.1% Tween 80 and 0.5% Tween 40 as the lipid sources), while the inoculums size corresponded to approximately 1-5 × 10(5) yeast cells/mL. Microplates were incubated at 37°C and read 48 h after inoculation. Azole MICs inhibiting fungal growth were the lowest drug concentrations that showed an optical density of ≤ 50% of the (drug-free) growth control, as assessed by spectrophotometer (630 nm filter).


RESULTS
All isolates were inhibited by the three azoles, with different minimum inhibitory concentration (MIC) values. Most isolates were inhibited by drug concentrations of 2-8 (CTZ), 1-4 (MCZ), or 16-32 (TBD) μg/mL. These results are partially in agreement with the findings of previous studies, in which substantially higher/lower MICs were occasionally reported. This is likely because of the different methodologies employed. Such discrepancies may not apply to clinical situations, where the compounds are applied topically.


CONCLUSIONS
The concept that clinical failure is linked to increased MICs is debatable, because significantly higher concentrations (in most cases at least 1,000 × the MIC) of the antifungals that were included in our study are routinely used in formulated products.
Department of Animal Productions, Epidemiology and Ecology, School of Veterinary Medicine, University of Turin, Italy. andrea.peano@unito.it
Andrea
A
Peano
Massimo
M
Beccati
Elisa
E
Chiavassa
Mario
M
Pasquetti
D000935:Antifungal Agents / Q000494:pharmacology*; D003022:Clotrimazole / Q000494:pharmacology*; D025141:Drug Resistance, Fungal; D008290:Malassezia / Q000187:drug effects*; D008825:Miconazole / Q000494:pharmacology*; D008826:Microbial Sensitivity Tests; D013827:Thiabendazole / Q000494:pharmacology*
D000935:Antifungal Agents; D008825:Miconazole; D003022:Clotrimazole; D013827:Thiabendazole