The Adhesin Scf1 Mediates Candida auris Colonization

Abstract

Candida auris is an emerging human fungal pathogen that can rapidly spread and cause outbreaks of invasive infections. Santana et. al. discovered that a novel Surface Colonization Factor (Scf1), and a conserved adhesin, Iff4109, mediates C. auris colonization on abiotic surfaces, skin and virulence in vivo.

Candida auris, an emerging multi-drug resistant fungal pathogen, has been recently classified as an urgent threat by the US Centers for Disease Control and the World Health Organization [1]. Unlike many other Candida species, C. auris efficiently colonizes human and mouse skin, inert surfaces including floors, bed rails, mobile phones, chairs and medical equipment such as glucometers, temperature probes, intravenous poles, blood pressure cuffs and oxygen masks [2–4]. The ability of C. auris to colonize skin and abiotic surfaces results in contamination of a patient’s environment leading to infection [5]. However, the factor(s) that mediate C. auris’s distinct colonization ability remains unknown.

In this issue of Science, Santana et. al. characterized a unique and conserved C. auris adhesion factor, Surface Colonization Factor (SCF1) that mediates colonization on abiotic surfaces and skin [6] (Figure 1). In Candida species, adhesin proteins such as Agglutinin-Like Sequence (ALS) and IFF/HYR (IPF Family F/Hyphally Regulated) play critical roles in attachment, colonization, and virulence. C. auris encodes 12 adhesin genes homologous to the ALS and IFF/HYR family proteins found in other Candida species [7]. In this study, the authors determined that of the 12 adhesin proteins, only the deletion of IFF4109 decreased C. auris adhesion, but the attachment was not abolished. To investigate if other adhesive factors regulate C. auris colonization, the authors generated 2560 insertional mutants and screened for their ability to attach to polystyrene microspheres. The authors found deletion of tnSWI1 or tnBCY1 resulted in a loss of adhesion. These proteins are predicted to be involved in basic metabolic processes like kinase activity, DNA binding and chromatin remodeling and thus it was posited that these genes played a role in the expression of the specific adhesion factor, responsible for C. auris surface adhesion. Upon comparing the transcriptome of tnSWI1 and tnBCY1 mutants with C. auris parent strain AR03082, it was found that open reading frame (ORF) B9J08_001458 was significantly downregulated. This ORF had similarities with canonical glycosylphosphatidylinositol (GPI) superfamily of fungal adhesins [8]. Deletion of B9J08_001458 ORF lead to adhesive defects in C. auris AR0382 and the authors named this ORF Surface Colonization Factor 1 (SCF1). The authors further confirmed that complementation with epitope-tagged SCF1 rescued adhesion to a polymer surface and epitope-tagged SCF1 protein localized to the C. auris cell surface consistent with its proposed role in adhesion. Deletion of IFF4109 in the Δscf1 mutant did not further reduce C. auris attachment to polymer surface, suggesting nonadditive roles.

Figure 1.

Adhesin proteins mediate C. auris colonization on abiotic polymer surface, biofilm formation, skin colonization, and virulence in vivo. Surface Colonization Factor (SCF1) is a glycosylphosphatidylinositol (GPI) – anchored fungal adhesin specific for C. auris colonization. IFF409 is a member of the conserved IFF/HYR family of adhesin present in C. auris. IFF4109 promotes adhesion by hydrophobic interaction, whereas SCF1 mediates adhesion by promoting cationic protein interactions with hydrated surfaces.

The authors determined SCF1 is conserved in C. auris and C. haemulonii, but not found in other Candida and Saccharomyces species. However, SCF1 is not essential for adhesin in C. haemulonii. Substantial variation was observed on SCF1 reliance for adhesion among various clinical isolates of C. auris belonging to five different clades. SCF1 transcript was less abundant in poorly adhesive C. auris AR0387 relative to highly adherent AR0382 and the authors found overexpression of SCF1 in C. auris AR0387 increased adhesion. Next, the authors examined the mechanisms of SCF1 mediated adhesion. While IFF4109 promotes adhesion to inanimate surfaces by hydrophobic interaction, SCF1 mediates adhesion by promoting cationic protein interactions with hydrated surfaces. SCF1 is rich in cationic and aromatic residues, and has functional homology with the bivalve adhesion system, and Vibrio adhesion BAP1 in Vibrio cholerae for nonspecific binding to abiotic substrates in ion rich aqueous environment. The cationic-aromatic residues in SCF1 are similar those in lipid binding proteins, and they have the ability to adhere to phosphatidylcholine, a negatively charged phospholipid found in phospholipid bilayers of mammalian cell membranes. Furthermore, purified SCF1 cationic peptides bound to polystyrene microspheres and phosphatidylcholine microparticles. Hence SCF1 promotes binding to lipid rich high salinity biotic niches. These findings correlate with previous observation that C. auris forms a high biofilm burden in high salinity artificial sweat media [9].

Although SCF1 and IFF4109 operate autonomously in polymer surface adhesion, C. auris requires both adhesins for biofilm formation in vitro and in vivo. Deletion of both SCF1 and IFF4109 were required to prevent biofilm formation, but expression of SCF1 alone in biofilm-incompetent C. auris isolates was sufficient to establish biofilm formation. In addition, the authors found deletion of SCF1 and IFF4109 reduced C. auris to colonize human skin explants ex vivo and mouse skin in vivo, whereas overexpression of SCF1 enhanced skin colonization. Mice infected with SCF1 and IFF4109 mutants showed reduced fungal dissemination and mortality, while mice infected with C. auris AR0387 overexpressing SCF1 had increased fungal dissemination and mortality.

Taken together, this elegant study by Santana et. al. forms a foundation for future studies to understand how C. auris efficiently colonizes abiotic surfaces and skin (Figure 1). This study brings to light several important questions regarding C. auris colonization. What are the factors in skin (sweat, salts, and ions) that regulate SCF1 expression? The difference in skin microbiome observed in C. auris positive and C. auris negative individuals suggest that skin commensals may regulate C. auris colonization on the skin [4]. Understanding if skin microbiota members and their metabolites regulate C. auris adhesin proteins is an important future area of investigation. Unlike the skin-tropic fungal Malassezia species, C. auris not only colonizes the skin surface, but can enter the dermis and persist in skin tissue for several months [2, 10]. This raises the possibility of SCF1 playing a role not only in the colonization but invasion of the dermis and persistence in skin tissue. C. auris rapidly spreads in healthcare settings because it colonizes and persist on abiotic surfaces and is resistant to common disinfectants like quaternary ammonium compounds. The findings from this study will facilitate studies on environmental factors that regulate C. auris colonization, and has the potential to prompt the development of novel control strategies to prevent C. auris colonization in health care settings.