Genotypic and phenotypic characterization of Candida albicans Lebanese hospital isolates resistant and sensitive to caspofungin

Abstract

The fungus Candida albicans is both a commensal and an opportunistic human pathogen, present as part of the normal human microflora causing serious mucosal, and systemic life threatening infections. The antifungal drug caspofungin of the echinocandin family is the latest generation of antifungal drugs to be developed. It functions by inhibiting glucan synthase thus weakening the fungal cell wall leading to death. Recently reports of resistance to caspofungin have been reported mainly through mutations in the FKS encoded subunits of glucan synthase at hot spot 1 (amino acids 641 to 649, FSTLSLRDP) and hot spot 2 (amino acids 1357 to 1364, DWIRRYTL). Our study aimed at sequencing both hot spots from 16 C. albicans Lebanese hospital isolates resistant and sensitive to caspofungin to determine whether mutations in these hot spots are present, and whether such mutations also impart resistance to our isolates. In addition, we wanted to determine any relationship between resistance and pathogenicity related attributes such as virulence, resistance to cell wall disrupting agents, biofilm formation, and cell wall chitin deposition. Five isolates were found to contain mutations with the mutations restricted to resistant strains. Within hot spot 1 substitution at positions S642, T643, L644, R647, and D648 were found, while within hot spot 2 substitutions at positions L1364, T1363, and R1360, W1358 and R1361 were identified with some of the mutations not previously documented. Strains that were resistant to caspofungin also showed increased resistance to Congo red but decreased biofilm formation and attenuated virulence in a mouse model of infection. Caspofungin sensitive strains showed decreased resistance to Congo red yet increased virulence and biofilm formation. Chitin content analysis showed that caspofungin resistant strains had elevated levels of chitin resulting in cell wall thickening that counters the effect of caspofungin, while sensitive strains showed decreased chitin content. Our results demonstrate an inverse correlation between resistance and virulence whereby resistance is due to thickening of the cell wall preventing the cell from gaining virulence attributes, while a more susceptible cell wall increases susceptibility to drugs but allows increased virulence.