Secretion of serine proteases by planktonic- and biofilm-growing cells of Candida parapsilosis.

Candida parapsilosis is a common cause of candidiasis worldwide, with biofilm formation and secretion of aspartic proteases (Saps) as key virulence factors. Conversely, serine protease secretion by this fungus is poorly understood. In this study, we investigated the secretion of serine-type proteases by planktonic- and biofilm-forming cells of C. parapsilosis cultured in brain heart infusion (BHI) medium. Cell-free supernatant from the reference strain (ATCC 22019) was screened against various serine protease substrates, revealing pronounced activity toward N-benzoyl-Phe-Val-Arg-pNa (0.74 nmol pNA.mg^- 1.min^- 1), with optimal activity at pH 9.0 and temperatures between 32 °C and 40 °C. Proteolytic activity was significantly reduced by serine protease inhibitors PMSF (32.8%), TLCK (40.2%) and benzamidine (50.7%), while inhibitors of other protease classes had no effect, confirming its serine-type specificity. Notably, serine protease activity was detected in supernatants from cells grown in BHI but absent in those cultured in albumin-supplemented yeast carbon base medium, a known inducer of Saps, suggesting culture-dependent regulation of protease expression. Serine protease activity also increased over time, rising from 0.36 pNA.mg^- 1.min^- 1 at 24-hour to 1.14 pNA.mg^- 1.min^- 1 at 72-hour. Clinical isolates of C. parapsilosis exhibited significantly higher serine protease activity than the reference strain under optimal conditions. Serine-type protease activity was also detected in the supernatant of mature biofilms, showing a correlation with metabolic activity and biomass. Infection of Galleria mellonella larvae with C. parapsilosis isolates revealed no correlation between larval mortality and serine protease production. These findings suggest that C. parapsilosis serine proteases contribute to fungal growth and biofilm development, representing potential targets for antifungal intervention.