Prevalence of biofilm formation in clinical isolates of Candida species causing bloodstream infection

Candida species are the fourth most common cause of nosocomial invasive infections. Biofilm formation is recognised as one virulence factor of Candida species. A total of 243 Candida albicans, 81 C. glabrata, 33 C. parapsilosis, 14 C. dubliniensis, 8 C. tropicalis, 8 C. lusitaniae, 5 C. krusei and 1 C. pelliculosa isolates causing bloodstream infections were evaluated for biofilm formation. The biofilm formed on silicone elastomer preincubated with human serum was quantified by estimation of the metabolic activity through XTT assay and visualised by light and scanning electron microscopy. Forty per cent of the C. albicans isolates formed biofilm compared to 88.7% of the non-albicans Candida isolates (P<0.0001). Among non-albicans Candida spp., biofilm formation was most commonly observed in C. tropicalis and C. lusitaniae (100%), followed by C. glabrata (95%), C. dubliniensis (85.7%) and C. parapsilosis (66.7%). A quantitative correlation was observed between the amount of biofilm observed microscopically, and that determined by metabolic activity measurements. The biofilms of all Candida species were composed of basal yeast cells with the exception of C. parapsilosis which produced biofilms consisting of pseudohyphae and aggregated yeast cells. These results suggest that biofilm formation as a virulence factor might have a higher significance for non-albicans Candida species than for C. albicans.