Pathogenic mechanisms of Candida glabrata in oral candidiasis, especially because of its inability to form hyphae, are understudied. Since both Candida albicans and C. glabrata are frequently co-isolated in oropharyngeal candidiasis (OPC), we examined their co-adhesion in vitro and observed adhesion of C. glabrata only to C. albicans hyphae microscopically. Mice were infected sublingually with C. albicans or C. glabrata individually, or with both species concurrently, to study their ability to cause OPC. Infection with C. glabrata alone resulted in negligible infection of tongues; however, colonization by C. glabrata was increased by co-infection or a pre-established infection with C. albicans. Furthermore, C. glabrata required C. albicans for colonization of tongues, since decreasing C. albicans burden with fluconazole also reduced C. glabrata. C. albicans hyphal wall adhesins Als1 and Als3 were important for in vitro adhesion of C. glabrata and to establish OPC. C. glabrata cell wall protein coding genes EPA8, EPA19, AWP2, AWP7, and CAGL0F00181 were implicated in mediating adhesion to C. albicans hyphae and remarkably, their expression was induced by incubation with germinated C. albicans. Thus, we found a near essential requirement for the presence of C. albicans for both initial colonization and establishment of OPC infection by C. glabrata.

Oropharyngeal candidiasis (OPC) is caused by the opportunistic fungi Candida albicans and is prevalent in immunocompromised patients, individuals with dry mouth, or patients with prolonged antibiotic therapies that reduce oral commensal bacteria. Human salivary histatins, including histatin 5 (Hst 5), are small cationic proteins that are the major source of fungicidal activity of saliva. However, Hsts are rapidly degraded in vivo, limiting their usefulness as therapeutic agents despite their lack of toxicity. We constructed a conjugate peptide using spermidine (Spd) linked to the active fragment of Hst 5 (Hst 5 4 -15 ), based upon our findings that C. albicans spermidine transporters are required for Hst 5 uptake and fungicidal activity. We found that Hst 5 4 -15 -Spd was significantly more effective in killing C. albicans and Candida glabrata than Hst 5 alone in both planktonic and biofilm growth and that Hst 5 4 -15 -Spd retained high activity in both serum and saliva. Hst 5 4 -15 -Spd was not bactericidal against streptococcal oral commensal bacteria and had no hemolytic activity. We tested the effectiveness of Hst 5 4 -15 -Spd in vivo by topical application to tongue surfaces of immunocompromised mice with OPC. Mice treated with Hst 5 4 -15 -Spd had significant clearance of candidal tongue lesions macroscopically, which was confirmed by a 3-to 5-log fold reduction of C. albicans colonies recovered from tongue tissues. Hst 5 4 -15 -Spd conjugates are a new class of peptide-based drugs with high selectivity for fungi and potential as topical therapeutic agents for oral candidiasis.