New Insights into the Cyp51 Contribution to Azole Resistance in Aspergillus Section Nigri.

Invasive aspergillosis (IA) is a severe condition mainly caused by Aspergillus fumigatus, although other species of the genus, such as section Nigri members, can also be involved. Voriconazole (VRC) is the recommended treatment for IA; however, the prevalence of azole-resistant Aspergillus isolates has alarmingly increased in recent years, and the underlying resistance mechanisms in non-fumigatus species remain unclear. We have determined the in vitro susceptibility of 36 strains from section Nigri to VRC, posaconazole (POS), and itraconazole (ITC), and we have explored the role of Cyp51A and Cyp51B, both targets of azoles, in azole resistance. The three drugs were highly active; POS displayed the best in vitro activity, while ITC and VRC showed MICs above the established epidemiological cutoff values in 9 and 16% of the strains, respectively. Furthermore, expression studies of cyp51A and cyp51B in control condition and after VRC exposure were performed in 14 strains with different VRC susceptibility. We found higher transcription of cyp51A, which was upregulated upon VRC exposure, but no correlation between MICs and cyp51 transcription levels was observed. In addition, cyp51A sequence analyses revealed nonsynonymous mutations present in both, wild-type and non-wild-type strains of A. niger and A. tubingensis Nevertheless, a few mutations were exclusively present in non-wild-type A. tubingensis strains. Altogether, our results suggest that azole resistance in section Nigri is not clearly explained by Cyp51A protein alteration or by cyp51 gene upregulation, which indicates that other mechanisms might be involved.