P. BERRA¹, S. AMORIM-VAZ¹, H. BESANÇON¹, F. ISCHER¹, D. SANGLARD¹, A.T. COSTE<sup>1

1</sup>CHUV-Institut de Microbiologie

A large proportion of Candida albicans open reading frames(ORFs) have still unknown functions. One useful approach to unveil their function is to obtain transcriptional data of these ORFs in specific conditions. We have used an RNA enrichment approach to identify genes that are expressed in vivo in two different animal models (mice, Galleria mellonella larvae) (1). Among the most upregulated genes in vivo, 2 ORF with unknown functions were present (orf19.1363 and orf19.7455) and are the subject of the present study.
In this work, these 2 ORFs were characterized by reverse genetics. We investigated the phenotypes of the respective null mutants in vivo, but also their phenotypes on several in vitro conditions mimicking the conditions of the host environment such as oxidative-, pH-, CO2-, temperature-stresses, use of alternative carbon sources, iron and zinc deprivation and filamentation conditions. In addition, we used several drug stresses, biofilm and adhesion on plastic surface assays and co-cultures with immune cells.
This large in vivo/in vitro screen allowed us to determine that orf19.1363 was required to survive and invade the host. Indeed, the fungal burden following infection in G. mellonella was significantly lower than for the wild type strain. Plus, the mutant was less efficiently uptaken by murine macrophages than the wild type. No significant in vitro phenotype which could help to determine its exact role was identified, however the experiments highlighted a more specific role in vivo.
The study of two independent orf19.7455 deletion mutants revealed a role of this gene in virulence in G. mellonella, since the larvae survive slightly longer when infected by the mutants than by the wild type or revertant strains. These mutants also showed decreased biofilm formation efficiency in vitro and a specific rate of filamentation, with a faster initiation of filamentation decreasing with time.
Although the characterization of both mutants is not complete yet, this study highlights a framework to assess the function of the numerous remaining uncharacterized C. albicans genes specifically expressed in vivo.
1 – Amorim-Vaz S et al., 2015. MBio 6:e00942–e00915