In vitro polymyxin activity against clinical multidrug-resistant fungi

Abstract

Background: Although antifungals are available and usually used against fungal infections, multidrug-resistant (MDR) fungal pathogens are a growing problem for public health. Moreover, fungal infections have become more prevalent nowadays due to the increasing number of people living with immunodeficiency. Thus, previously rarely-isolated and/or unidentified fungal species including MDR yeast and moulds have emerged around the world. Recent works indicate that polymyxin antibiotics (polymyxin B and colistin) have potential antifungal proprieties. Therefore, investigating the in vitro activity of these molecules against clinical multidrug-resistant yeast and moulds could be very useful.

Methods: In this study, a total of 11 MDR yeast and filamentous fungal strains commonly reported in clinical settings were tested against polymyxin antibiotics. These include strains belonging to the Candida, Cryptococcus and Rhodotorula yeast genera, along with others belonging to the Aspergillus, Fusarium, Scedosporium, Lichtheimia and Rhizopus mould genera. The fungicidal or fungistatic action of colistin against clinical yeast strains was determined by the time-kill study. Further, a checkerboard assay for its combination with antifungal agents, usually used in clinical practices (amphotericin B, itraconazole, voriconazole), was carried out against multi-drug resistant fungal strains.

Results: Polymyxin B and colistin exhibited an antifungal activity against all MDR fungal strains tested with MICs ranging from 16 to 128 μg/ml, except for the Aspergillus species. In addition, colistin has a fungicidal action against yeast species, with minimum fungicidal concentrations ranging from 2 to 4 times MICs. It induces damage to the MDR Candida albicans membrane. A synergistic activity of colistin-amphotericin B and colistin-itraconazole associations against Candida albicans and Lichtheimia corymbifera strains, respectively, and colistin-fluconazole association against Rhodotorula mucilaginosa, was demonstrated using a checkerboard microdilution assay.

Conclusion: colistin could be proposed, in clinical practice, in association with other antifungals, to treat life-threatening fungal infections caused by MDR yeasts or moulds.

Keywords: Candida albicans; MDR-fungi; Molds; Polymyxin antibiotics; Repurposing-drug.

Fig. 1

Time-kill kinetics of colistin against four fungal strains (C. albicans, C. krusei, C. neoformans and R. mucilaginosa). The colistin concentrations used are as following: (▲) control (no colistin added), (♦) 0.5X MIC, (■) 1X MIC,(●) 2X MIC, (■) 4X MIC

Fig. 2

Fluorescence microscopy of Candida albicans H6 after treatment with 5 μg/ml of propidium iodide. a: fluorescence image of cells treated with 256 μg/ml (2X MIC) of colistin for 24 h. b: Brightfield image of cells treated with colistin for 24 h. Scale bar: 2 μm

Fig. 3

Plots of the checkerboard assays for the combinations of colistin with 3 antifungals (fluconazole, itraconazole and amphotericin b). Each dot presents the MICs of colistin (x-axis) and the antifungal agent (y-axis) used in the combination against R. mucilaginosa (a), C. albicans H5 (b), L. corymbifera (c and e) and C. albicans H6 (d and f)