The alternative oxidase of Candida albicans causes reduced fluconazole susceptibility.

Objectives: To evaluate the effect of Candida albicans mitochondrial respiratory status on antifungal azole susceptibility.

Methods: The inhibitors cyanide and salicylhydroxamic acid (SHAM) were each combined with azoles to examine the effect of the combinations on C. albicans. C. albicans strains deleted for the alternative oxidase (Aox) were also examined for susceptibility to azoles and for the generation of intracellular reactive oxygen species (ROS). A chequerboard microdilution assay was performed on several C. albicans clinical strains including azole-resistant isolates to explore the combined effect of fluconazole and inhibitors of Aox.

Results: The induction of the alternative respiratory pathway by cyanide decreased susceptibility to azoles, while the inhibition of alternative respiration by SHAM increased azole susceptibility. It was found that ROS production was increased in the absence of Aox in C. albicans upon treatment by antifungals such as miconazole and benomyl. The combination of fluconazole with SHAM resulted in a synergistic effect on the killing of C. albicans clinical isolates.

Conclusion: These results demonstrate that the induction of the alternative respiratory pathway confers reduced susceptibility to antifungal azoles, potentially through a mechanism that involves decreased intracellular ROS production during exposure to antifungal agents.