In vitro susceptibility of Pythium insidiosum and a Lagenidium sp to itraconazole, posaconazole, voriconazole, terbinafine, caspofungin, and mefenoxam.

OBJECTIVE

To evaluate the radial growth assay for use in in vitro susceptibility testing of Pythium insidiosum and a Lagenidium sp and to assess susceptibility of representative isolates to itraconazole, posaconazole, voriconazole, terbinafine, caspofungin, and mefenoxam.

SAMPLE POPULATION

6 isolates each of P insidiosum and Lagenidium sp.

PROCEDURES

Isolates were plated in triplicate onto agar supplemented with antifungal compounds at concentrations of 0.025 to 8 microg/mL. Isolates on dimethyl sulfoxide- and water-supplemented agar served as control samples. Effect of antifungal concentration on colony diameter was assessed with a mixed linear model. Assay variability was assessed with the coefficient of variation.

RESULTS

Colony growth was uniform (mean intra-assay and interassay coefficients of variation were < 5%). Minimal inhibition was evident with voriconazole and posaconazole at 8 microg/mL. Terbinafine at 8 microg/mL significantly reduced growth of P insidiosum and at > or = 1 microg/mL significantly reduced growth of the Lagenidium sp. Caspofungin and mefenoxam (concentrations > or = 1 microg/mL and > or = 0.025 microg/mL, respectively) significantly reduced growth of both pathogens. Mefenoxam at 0.1 microg/mL caused > 50% growth inhibition in 11 of 12 isolates and at 1 microg/mL caused > 90% inhibition in all isolates.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that the radial growth assay was a simple, reproducible technique for susceptibility testing of P insidiosum and a Lagenidium sp. Azoles had limited activity, whereas terbinafine and caspofungin caused significant but minimal to moderate inhibition. Only mefenoxam had a profound effect on both pathogens at concentrations likely to be achievable in tissues.