Candida albicans uses multiple mechanisms to acquire the essential metabolite inositol during infection

Candida albicans is an important cause of life-threatening systemic bloodstream infections in immunocompromised patients. In order to cause infections, C. albicans must be able to synthesize the essential metabolite inositol or acquire it from the host. Based on the similarity of C. albicans to Saccharomyces cerevisiae, it was predicted that C. albicans may generate inositol de novo, import it from the environment, or both. The C. albicans inositol synthesis gene INO1 (orf19.7585) and inositol transporter gene ITR1 (orf19.3526) were each disrupted. The ino1 Delta/ino1 Delta mutant was an inositol auxotroph, and the itr1 Delta/itr1 Delta mutant was unable to import inositol from the medium. Each of these mutants was fully virulent in a mouse model of systemic infection. It was not possible to generate an ino1 Delta/ino1 Delta itr1 Delta/itr1 Delta double mutant, suggesting that in the absence of these two genes, C. albicans could not acquire inositol and was nonviable. A conditional double mutant was created by replacing the remaining wild-type allele of ITR1 in an ino1 Delta/ino1 Delta itr1 Delta/ITR1 strain with a conditionally expressed allele of ITRI driven by the repressible MET3 promoter. The resulting ino1 Delta/ino1 Delta itr1 Delta/P-MET3::ITR1 strain was found to be nonviable in medium containing methionine and cysteine (which represses the promoter), and it was avirulent in the mouse model of systemic candidiasis. These results suggest a model in which C. albicans has two equally effective mechanisms for obtaining inositol while in the host. It can either generate inositol de novo through Ino1p, or it can import it from the host through Itr1p.