Candida glabrata is unusual with respect to its resistance to cationic antifungal proteins

Abstract

Naturally occurring salivary antifungal proteins have been of major interest, due to their potential to provide the basis for peptide antimycotics effective in combating fungal infections in the oral cavity or elsewhere. We tested the fungistatic activity of a number of cationic antifungal proteins, with major emphasis on histatin 5, a basic protein secreted by the human parotid and submandibular glands. Histatin 5 inhibited the growth of Candida albicans and that of other medically important Candida species, such as C. kefyr, C. krusei, and C. parapsilosis, with IC 50 values in the range of 10-20 µg/ml. Two Cryptococcus neoformans strains were also sensitive (IC 50 5.2 and 5.6 µg/ml). On the other hand, three C. glabrata strains (ATCC 90030, 2001 and 64677) were entirely insensitive to histatin 5 (IC 50 >225 µg/ml). Four genetically very similar species to C. glabrata, Candida castelli (CBS 4332), Saccharomyces cerevisiae (S288C, BY4741 and CBS 1171), Kluyveromyces delphensis (CBS 2170) and Kluyveromyces bacillisporus (CBS 7720) were all sensitive to histatin 5 (IC 50 2.6-64.6 µg/ml). C. glabrata was also insensitive to other members of the histatin family; histatin 1, 3 and P-113 (IC 50 values in all cases >225 µg/ml). In addition, two entirely different cationic antifungal proteins originating from frog skin, PGLa and magainin 2, also showed a strong reduced activity toward this fungus. Besides the well-described inherent resistance of C. glabrata to azole-derived antifungal agents, our studies indicate that this species is also able to withstand the otherwise detrimental activities of cationic antifungal proteins.

Antifungal Activity of the Noncytotoxic Human Peptide Hepcidin 20 against Fluconazole-Resistant Candida glabrata in Human Vaginal Fluid

“…However, C. glabrata has been shown to be scarcely susceptible to a wide spectrum of AMPs of different derivations; among these are histatin, cathelicidins, defensins, and magainin (7)(8)(9)(10).…”

Roles of Cellular Respiration, Cg CDR1 , and Cg CDR2 in Candida glabrata Resistance to Histatin 5

“…While histatins are highly effective against diverse classes of fungi, they are significantly less effective against C. glabrata (9, 24). Furthermore, other, structurally unrelated, cationic antifungal peptides, belonging to the magainin and defensin family, also show a selectively reduced activity against this fungus (6,9,11). The cellular basis for the apparent insensitivity of C. glabrata to various antifungal agents as different as azoles and cationic antifungal proteins is as yet unknown.…”

“…While it is unknown whether histatin 5 could be transported by the CgCDR1 and CgCDR2 pumps that are so effective in exporting fluconazole, it was assessed whether the mutants lacking one or both of these genes would show an increased sensitivity to histatin 5. Growth inhibition assays with histatin 5 were conducted in diluted SDB (9), since salts in RPMI medium interfere with histatin activity (9). As indicated in Table 1, the efficacy of fluconazole in RPMI medium and in diluted SDB was in most cases comparable, allowing the comparison of C. glabrata sensitivities to fluconazole and histatin 5 in diluted SDB.…”

“…(13). c The growth inhibition assay was conducted as described by Helmerhorst et al (9). d PPB, potassium phosphate buffer, pH 7.0.…”

“…d PPB, potassium phosphate buffer, pH 7.0. The killing assay was conducted as described by Helmerhorst et al (9). e ND, not determined.…”