Host-induced, stage-specific virulence gene activation in Candida albicansduring infection

An understanding of the complex interactions between pathogenic microbes an d their host must include the identification of gene expression patterns du ring infection. To detect the activation of virulence genes in the opportun istic fungal pathogen Candida albicans in vivo by host signals, we devised a reporter system that is based on FLP-mediated genetic recombination. The FLP gene, encoding the site-specific recombinase FLP, was genetically modif ied for expression in C. albicans and fused to the promoter of the SAP2 gen e that codes for one of the secreted aspartic proteinases, which are putati ve virulence factors of C. albicans. The SAP2P-FLP fusion was integrated in to one of the SAP2 alleles in a strain that contained a deletable marker th at conferred resistance to mycophenolic acid and was flanked by direct repe ats of the FLP recognition target (FRT). Using this reporter system, a tran sient gene induction could be monitored at the level of single cells by the mycophenolic acid-sensitive phenotype of the colonies generated from such cells after FLP-mediated marker excision. In two mouse models of disseminat ed candidiasis, SAP2 expression was not observed in the initial phase of in fection, but the SAP2 gene was strongly induced after dissemination into de ep organs. In contrast, in a mouse model of oesophageal candidiasis in whic h dissemination into internal organs did not occur, no SAP2 expression was detected at any time. Our results support a role of the SAP2 gene in the la te stages of an infection, after fungal spread into deep tissue. This new i n vivo expression technology (IVET) for a human fungal pathogen allows the detection of virulence gene induction at different stages of an infection, and therefore provides clues about the role of these genes in the disease p rocess.