Oral candidiasis, commonly referred to as “thrush,” is an opportunistic fungal infection that commonly affects the oral mucosa. The main causative agent, Candida albicans, is a highly versatile commensal organism that is well adapted to its human host; however, changes in the host microenvironment can promote the transition from one of commensalism to pathogen. This transition is heavily reliant on an impressive repertoire of virulence factors, most notably cell surface adhesins, proteolytic enzymes, morphologic switching, and the development of drug resistance. In the oral cavity, the co-adhesion of C. albicans with bacteria is crucial for its persistence, and a wide range of synergistic interactions with various oral species were described to enhance colonization in the host. As a frequent colonizer of the oral mucosa, the host immune response in the oral cavity is oriented toward a more tolerogenic state and, therefore, local innate immune defenses play a central role in maintaining Candida in its commensal state. Specifically, in addition to preventing Candida adherence to epithelial cells, saliva is enriched with anti-candidal peptides, considered to be part of the host innate immunity. The T helper 17 (Th17)-type adaptive immune response is mainly involved in mucosal host defenses, controlling initial growth of Candida and inhibiting subsequent tissue invasion. Animal models, most notably the mouse model of oropharyngeal candidiasis and the rat model of denture stomatitis, are instrumental in our understanding of Candida virulence factors and the factors leading to host susceptibility to infections. Given the continuing rise in development of resistance to the limited number of traditional antifungal agents, novel therapeutic strategies are directed toward identifying bioactive compounds that target pathogenic mechanisms to prevent C. albicans transition from harmless commensal to pathogen.

A majority of infections caused by Candida albicans—the most frequent fungal pathogen—are associated with biofilm formation. A salient feature of C. albicans biofilms is the presence of the biofilm matrix. This matrix is composed of exopolymeric materials secreted by sessile cells within the biofilm, in which all classes of macromolecules are represented, and provides protection against environmental challenges. In this review, we summarize the knowledge accumulated during the last two decades on the composition, structure, and function of the C. albicans biofilm matrix. Knowledge of the matrix components, its structure, and function will help pave the way to novel strategies to combat C. albicans biofilm infections.

The aim of this study was to evaluate miltefosine and four synthetic compounds (TCAN26, TC19, TC106 and TC117) for their in vitro inhibitory activity against Candida albicans planktonic and biofilm cells and investigate whether these compounds are able to inhibit the biofilm formation and to reduce the viability of mature C. albicans biofilm cells. The XTT reduction assay and transmission and scanning electron microscopy were employed to determine the inhibitory effects of the test compounds in comparison with amphotericin B and fluconazole against both planktonic cells and sessile cells in biofilms. C. albicans planktonic cells were susceptible to miltefosine, TCAN26 and TC19, all alkylphospholipid compounds. Miltefosine and TCAN26 present a fungicidal activity with similar values of MIC and minimum fungicidal concentration (MFC), ranging from 2 to 8 mg/L. Cell treatment with sub-inhibitory concentrations of alkylphospholipids induced several ultrastructural alterations. In relation to biofilms, miltefosine reduced formation (38%–71%) and mature biofilms viability (32%–44%), at concentrations of 64 mg/L. TCAN26 also reduced biofilm formation (24%–30%) and mature biofilm viability (15%–20%), at concentrations of 64 mg/L. Although amphotericin B reduced biofilm formation similarly to miltefosine (51%–74%), its activity was lower on mature biofilms (24%–30%). Miltefosine antibiofilm activity was significantly higher than amphotericin B, on both formation and mature biofilms (P<0.05 and P<0.0001, respectively). Fluconazole was the least effective compound tested. Promising antibiofilm activity was displayed by miltefosine and other alkylphosphocholine compounds, which could be considered a putative option for future treatment of candidaemia associated with biofilm formation, although further evaluation in in vivo systems is required.

Cryptococcus spp. are common opportunistic fungal pathogens, particularly in HIV patients. The approved drug miltefosine (MFS) has potential as an alternative antifungal against cryptococcosis; however, the mechanism of action of MFS in Cryptococcus is poorly understood. Here, we examined the effects of MFS on C. neoformans and C. gattii yeasts (planktonic and biofilm lifestyles), to clarify its mechanism of action. MFS presented inhibitory and fungicidal effects against planktonic Cryptococcus cells, with similar activity against dispersion biofilm cells, while sessile biofilm cells were less sensitive to MFS. Interestingly, MFS had post-antifungal effect on Cryptococcus, with a proliferation delay of up to 8.15 h after short exposure to fungicidal doses. MFS at fungicidal concentrations increased plasma membrane permeability, likely due to direct interaction with ergosterol, as suggested by competition assays with exogenous ergosterol. Moreover, MFS reduced the mitochondrial membrane potential, increased ROS production, and induced DNA fragmentation and condensation, all of which are hallmarks of apoptosis. Transmission electron microscopy analysis showed that MFS-treated yeasts had a reduced mucopolysaccharide capsule (confirmed by morphometry in light microscopy), plasma membrane irregularities, mitochondrial swelling and a less conspicuous cell wall. Our results suggest that MFS increases plasma membrane permeability in Cryptococcus via interaction with ergosterol, and also affects the mitochondrial membrane, eventually leading to apoptosis, in line with its fungicidal activity. These findings confirm the potential of MFS as an antifungal against C. neoformans and C. gattii, and warrants further studies to establish clinical protocols for MFS use against cryptococcosis.

The newly emerged Candida species C. auris has been associated with an exponential rise in invasive disease in health care facilities worldwide with a mortality rate approaching 60%. C. auris exhibits a high level of transmissibility, multidrug resistance, and persistence in hospital environments, yet little is known about its pathogenesis largely due to limited data from animal studies. We used clinically relevant murine models of infection to comparatively evaluate the host niche-specific pathogenic potential of C. auris and C. albicans . Findings demonstrated that C. auris adheres more avidly, forming robust biofilms on catheters implanted in mice. However, although C. auris adhered to oral tissue ex vivo , it failed to colonize the oral cavity in vivo . In contrast, in the intraperitoneal infection model, C. auris persisted longer in the peritoneal cavity and kidneys. Understanding the host-pathogen factors contributing to the rise of C. auris as a nosocomial pathogen is critical for controlling the spread of this species.