Colistin interacts synergistically with echinocandins against Candida auris

Antifungal combination is an interesting approach for the treatment of several fungal infections but there is currently little evidence to support combined therapy in Candida auris infections. The antibacterial colistin has recently been shown to interact synergistically with antifungals against Candida spp., including azole-resistant isolates. The current study evaluated the in vitro interaction between colistin and either caspofungin or micafungin against 15 C. auris isolates by a checkerboard methodology based on the European Committee on Antimicrobial Susceptibility Testing (EUCAST) reference method. Results were analysed by two approaches: calculation of the fractional inhibitory concentration index (FICI) and response surface analysis based on the Bliss model. The minimum inhibitory concentration (MIC) range (geometric mean [Gmean]) of caspofungin and micafungin was 0.25 to 1 µg/mL (0.691 µg/mL) and 0.03 to 0.125 µg/mL (0.114 µg/mL), respectively. No activity was observed for colistin alone with MIC of >64 µg/mL for all the isolates. When colistin was combined with caspofungin, synergistic interactions were observed for all strains with FICI values of 0.08 to 0.14. In contrast, indifferent interactions were observed for the combination of colistin with micafungin with FICI values of 0.51 to 1.01. Synergy was also demonstrated using the Bliss model against all isolates for the colistin-caspofungin combination and in 60% of isolates for the colistin-micafungin combination. Antagonism was not observed for any combination.