Evaluating the resistance to posaconazole by E-test and CLSI broth microdilution methodologies of Candida spp. and pathogenic moulds

Abstract

E-test methodology was compared with Clinical and Laboratory Standards Institute (CLSI) broth microdilution, particularly concerning the detection of resistance to posaconazole among clinical fungal isolates. The susceptibility of a large set of fungal strains (n = 300) was evaluated following 24 and 48 h in two different culture media (RPMI 1640 and Sabouraud agar). Fungal strains were highly susceptible to posaconazole; however, few less susceptible strains were found, mostly regarding Candida albicans, Candida glabrata, Acremonium sp., Cladosporium sp. and Scedosporium apiospermum. Broth microdilution and E-test methods provided similar results for posaconazole-susceptible strains, while the less susceptible fungal strains (10.3% of the strains showed MIC > or =2 microg/mL) resulted in higher discrepancies between the two methodologies, particularly concerning Candida spp. E-test susceptibility values were critically affected by the pH of the culture media. Sabouraud medium provided similar susceptibility results for moulds to those for RPMI, soon after 24 h. Posaconazole resistance was rare in this study, but routine susceptibility methods, such as the E-test, should be able to detect fungal strains with reduced susceptibility. E-test methodology still needs improvements to recognise accurately strains less susceptible to posaconazole.