Candida albicans Hyphal Formation and Virulence Assessed Using a Caenorhabditis elegans Infection Model

Abstract

Candida albicans colonizes the human gastrointestinal tract and can cause life-threatening systemic infection in susceptible hosts. We study here C. albicans virulence determinants using the nematode Caenorhabditis elegans in a pathogenesis system that models candidiasis. The yeast form of C. albicans is ingested into the C. elegans digestive tract. In liquid media, the yeast cells then undergo morphological change to form hyphae, which results in aggressive tissue destruction and death of the nematode. Several lines of evidence demonstrate that hyphal formation is critical for C. albicans pathogenesis in C. elegans. First, two yeast species unable to form hyphae (Debaryomyces hansenii and Candida lusitaniae) were less virulent than C. albicans in the C. elegans assay. Second, three C. albicans mutant strains compromised in their ability to form hyphae (efg1⌬/efg1⌬, flo8⌬/flo8⌬, and cph1⌬/cph1⌬ efg1⌬/efg1⌬) were dramatically attenuated for virulence. Third, the conditional tet-NRG1 strain, which enables the external manipulation of morphogenesis in vivo, was more virulent toward C. elegans when the assay was conducted under conditions that permit hyphal growth. Finally, we demonstrate the utility of the C. elegans assay in a screen for C. albicans virulence determinants, which identified several genes important for both hyphal formation in vivo and the killing of C. elegans, including the recently described CAS5 and ADA2 genes. These studies in a C. elegans-C. albicans infection model provide insights into the virulence mechanisms of an important human pathogen.Candida albicans is the most common human fungal pathogen; however, our knowledge of its virulence mechanisms is incomplete, and our best antifungal agents are often ineffective in treating severe candidiasis (3). Infections with Candida species account for 70 to 90% of all invasive mycoses (32) and can be associated with devastating consequences, particularly in intensive care units where mortality rates reach 40% (24, 34). The drug resistance of pathogenic fungi exacerbates this problem and often limits therapeutic options (35). The identification of virulence pathways that can be targeted with novel antifungal therapies is urgently needed (31,38,46).One approach to understand the genetic mechanisms of virulence is to use invertebrates, such as the nematode Caenorhabditis elegans, as model hosts (43). Studies of C. elegans infection with Pseudomonas aeruginosa and Cryptococcus neoformans, for example, have led to the identification of evolutionarily conserved mechanisms of host immunity and microbial virulence (1, 21, 50). However, efforts to design an accurate nonmammalian model of C. albicans pathogenesis have been stymied, in part because it has been difficult to capture the role of Candida dimorphism in these systems.Morphogenesis in C. albicans is intricately related to pathogenesis and thus has been intensively studied. C. albicans hyphae are important for tissue destruction and host invasion (3). As such, C. albicans mutants and non-albicans Cand...

Efficacy of 7‐benzyloxyindole and other halogenated indoles to inhibit Candida albicans biofilm and hyphal formation

“…Previously, it was shown that hyphal form of C. albicans kills by piercing nematode cuticles, and that the yeast form is non‐lethal (Pukkila‐Worley et al ., 2009). Similarly, it was reported that the survival rates of C. albicans ‐infected nematodes were increased by treatment with gymnemic acid (Vediyappan et al ., 2013), retigeric acid (Chang et al ., 2012) or polyphenolic compounds such as magnolol and honokiol (Sun et al ., 2015).…”

Use of Caenorhabditis elegans as a non-mammalian model system to study Candida virulence

“…In some ways, this has similarity to its proliferation in the human gastrointestinal tract and gut translocation to cause invasive disease 15 . Pukkila-Worley, Peleg and colleagues have also shown the successful use of C. elegans in elucidating C. albicans dimorphism 7 . By employing hyphal defective mutant strains (EFG1 and FLO8) we showed that hyphae formation is necessary for the pathogenesis of Candida as confirmed by attenuated virulence toward C. elegans.…”

“…One of the most important virulence factors that contribute to C. albicans pathogenesis is its ability to switch, reversibly, between yeast to filamentous (hyphal) forms 3 . The hyphal form of C. albicans is thought to be intricately related to its pathogenesis, with some studies showing that it is necessary for tissue destruction and invasion 3,[5][6][7] .…”

“…The study further demonstrated the convenience of using the C. elegans-C. albicans infection model to screen C. albicans mutants for genes implicated in virulence. The screening identified the role of two novel genes; ADA2 and CAS5 in Candida virulence 7 . Interestingly, an ADA2 deletion mutant showed normal filamentation in vitro but showed attenuated filamentation inside C. elegans, highlighting the utility of C. elegans as a substitute in vivo host for characterisation of C. albicans virulence 7 .…”

“…The screening identified the role of two novel genes; ADA2 and CAS5 in Candida virulence 7 . Interestingly, an ADA2 deletion mutant showed normal filamentation in vitro but showed attenuated filamentation inside C. elegans, highlighting the utility of C. elegans as a substitute in vivo host for characterisation of C. albicans virulence 7 . Other Candida virulence factors have also been identified using C. elegans, including the co-transcription factor known as Mediator Med31 and the mitochondrial outer membrane SAM (Sorting and Assembly Machinery) complex subunit (Sam37), which were both shown to have an impact on C. albicans virulence 16,17 .…”