Comparative evaluation of antifungal susceptibility testing methods for Rhizopus species isolates.

Background and purpose: The mainstay of treatment for COVID-19-associated mucormycosis was liposomal Amphotericin B. Other antifungal agents, such as posaconazole and isavuconazole, were used as well. The Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing recommend broth microdilution methods for antifungal susceptibility testing. In this regard, the present study aimed to see what potency and zone diameters correlate with the gold standard broth microdilution method.

Materials and methods: All the isolates were identified by matrix-assisted laser desorption ionization-time-of-flight. In total, 127 isolates of 83 Rhizopus oryzae complex and 44 isolates of Rhizopus microsporus complex were selected. Anti-fungal susceptibility testing by disc diffusion and E-test was performed on Mueller Hinton Agar and compared with the CLSI broth microdilution method of Anti-fungal susceptibility testing.

Results: Percentage agreement was found to be more in the case of the E test than the disc diffusion method. In the case of R. oryzae, posaconazole had 98.79% agreement with broth microdilution followed by Isavuconazole (97.59%), Itraconazole (96.38%), and Amphotericin B (91.56%).

Conclusion: Disc diffusion correlates well with broth microdilution, although its correlation is weaker when compared to the E test. Effective concentration of Amphotericin B discs for antifungal susceptibility testing depends on the specific Rhizopus species.