Although Candida glabrata is an important human pathogenic yeast, its pathogenicity mechanisms are largely unknown. Immune evasion strategies seem to play key roles during infection, since very little inflammation is observed in mouse models. Furthermore, C. glabrata multiplies intracellularly after engulfment by macrophages. In this study, we sought to identify the strategies that enable C. glabrata to survive phagosome biogenesis and antimicrobial activities within human monocyte-derived macrophages. We show that, despite significant intracellular proliferation, macrophage damage or apoptosis was not apparent, and production of reactive oxygen species was inhibited. Additionally, with the exception of GM-CSF, levels of pro- and anti-inflammatory cytokines were only marginally increased. We demonstrate that adhesion to and internalization by macrophages occur within minutes, and recruitment of endosomal early endosomal Ag 1 and lysosomal-associated membrane protein 1 indicates phagosome maturation. However, phagosomes containing viable C. glabrata, but not heat-killed yeasts, failed to recruit cathepsin D and were only weakly acidified. This inhibition of acidification did not require fungal viability, but it had a heat-sensitive surface attribute. Therefore, C. glabrata modifies the phagosome into a nonacidified environment and multiplies until the host cells finally lyse and release the fungi. Our results suggest persistence of C. glabrata within macrophages as a possible immune evasion strategy.

Candida albicans frequently causes superficial infections by invading and damaging epithelial cells, but may also cause systemic infections by penetrating through epithelial barriers. C. albicans is a remarkable pathogen because it can invade epithelial cells via two distinct mechanisms: induced endocytosis, analogous to facultative intracellular enteropathogenic bacteria, and active penetration, similar to plant pathogenic fungi. Here we investigated the contributions of the two invasion routes of C. albicans to epithelial invasion. Using selective cellular inhibition approaches and differential fluorescence microscopy, we demonstrate that induced endocytosis contributes considerably to the early time points of invasion, while active penetration represents the dominant epithelial invasion route. Although induced endocytosis depends mainly on Als3-E–cadherin interactions, we observed E–cadherin independent induced endocytosis. Finally, we provide evidence of a protective role for serum factors in oral infection: human serum strongly inhibited C. albicans adhesion to, invasion and damage of oral epithelial cells.

f Candida glabrata is both a human fungal commensal and an opportunistic pathogen which can withstand activities of the immune system. For example, C. glabrata can survive phagocytosis and replicates within macrophages. However, the mechanisms underlying intracellular survival remain unclear. In this work, we used a functional genomic approach to identify C. glabrata determinants necessary for survival within human monocyte-derived macrophages by screening a set of 433 deletion mutants. We identified 23 genes which are required to resist killing by macrophages. Based on homologies to Saccharomyces cerevisiae orthologs, these genes are putatively involved in cell wall biosynthesis, calcium homeostasis, nutritional and stress response, protein glycosylation, or iron homeostasis. Mutants were further characterized using a series of in vitro assays to elucidate the genes' functions in survival. We investigated different parameters of C. glabrata-phagocyte interactions: uptake by macrophages, replication within macrophages, phagosomal pH, and recognition of mutant cells by macrophages as indicated by production of reactive oxygen species and tumor necrosis factor alpha (TNF-␣). We further studied the cell surface integrity of mutant cells, their ability to grow under nutrient-limited conditions, and their susceptibility to stress conditions mirroring the harsh environment inside a phagosome. Additionally, resistance to killing by neutrophils was analyzed. Our data support the view that immune evasion is a key aspect of C. glabrata virulence and that increased immune recognition causes increased antifungal activities by macrophages. Furthermore, stress resistance and efficient nutrient acquisition, in particular, iron uptake, are crucial for intraphagosomal survival of C. glabrata.

Candida glabrata is one of the most common causes of candidemia, a life-threatening, systemic fungal infection, and is surpassed in frequency only by Candida albicans. Major factors contributing to the success of this opportunistic pathogen include its ability to readily acquire resistance to antifungals and to colonize and adapt to many different niches in the human body. Here we addressed the flexibility and adaptability of C. glabrata during interaction with macrophages with a serial passage approach. Continuous co-incubation of C. glabrata with a murine macrophage cell line for over six months resulted in a striking alteration in fungal morphology: The growth form changed from typical spherical yeasts to pseudohyphae-like structures – a phenotype which was stable over several generations without any selective pressure. Transmission electron microscopy and FACS analyses showed that the filamentous-like morphology was accompanied by changes in cell wall architecture. This altered growth form permitted faster escape from macrophages and increased damage of macrophages. In addition, the evolved strain (Evo) showed transiently increased virulence in a systemic mouse infection model, which correlated with increased organ-specific fungal burden and inflammatory response (TNFα and IL-6) in the brain. Similarly, the Evo mutant significantly increased TNFα production in the brain on day 2, which is mirrored in macrophages confronted with the Evo mutant, but not with the parental wild type. Whole genome sequencing of the Evo strain, genetic analyses, targeted gene disruption and a reverse microevolution experiment revealed a single nucleotide exchange in the chitin synthase-encoding CHS2 gene as the sole basis for this phenotypic alteration. A targeted CHS2 mutant with the same SNP showed similar phenotypes as the Evo strain under all experimental conditions tested. These results indicate that microevolutionary processes in host-simulative conditions can elicit adaptations of C. glabrata to distinct host niches and even lead to hypervirulent strains.

The human pathogenic fungus Candida albicans is a frequent cause of mucosal infections. Although the ability to transition from the yeast to the hypha morphology is essential for virulence, hypha formation and host cell invasion per se are not sufficient for the induction of epithelial damage. Rather, the hypha‐associated peptide toxin, candidalysin, a product of the Ece1 polyprotein, is the critical damaging factor. While synthetic, exogenously added candidalysin is sufficient to damage epithelial cells, the level of damage does not reach the same level as invading C. albicans hyphae. Therefore, we hypothesized that a combination of fungal attributes is required to deliver candidalysin to the invasion pocket to enable the full damaging potential of C. albicans during infection. Utilising a panel of C. albicans mutants with known virulence defects, we demonstrate that the full damage potential of C. albicans requires the coordinated delivery of candidalysin to the invasion pocket. This process requires appropriate epithelial adhesion, hyphal extension and invasion, high levels of ECE1 transcription, proper Ece1 processing and secretion of candidalysin. To confirm candidalysin delivery, we generated camelid VHHs (nanobodies) specific for candidalysin and demonstrate localization and accumulation of the toxin only in C. albicans‐induced invasion pockets. In summary, a defined combination of virulence attributes and cellular processes is critical for delivering candidalysin to the invasion pocket to enable the full damage potential of C. albicans during mucosal infection. Take Aways Candidalysin is a peptide toxin secreted by C. albicans causing epithelial damage. Candidalysin delivery to host cell membranes requires specific fungal attributes. Candidalysin accumulates in invasion pockets created by invasive hyphae. Camelid nanobodies enabled visualisation of candidalysin in the invasion pocket.