Production of biofilm by Candida and non-Candida spp. isolates causing fungemia: Comparison of biomass production and metabolic activity and development of cut-off points

Biofilm production in Candida spp. can be studied by measuring the biomass produced after application of crystal violet stain or by measuring metabolic activity with XTT. Our study is the first in which crystal violet and XTT are compared to analyze the ability of clinically relevant Candida and non-Candida species to produce biofilm. We studied 577 isolates causing fungemia in 512 patients admitted from January 2007 to July 2013. Based on the biomass production measured by crystal violet and the metabolic activity measured by XTT, strains were divided into terciles to establish tentative cut-offs to classify isolates as being low, moderate, or high biofilm-forming and as having low, moderate, or high metabolic activity. Considerable variability in biofilm production and metabolic activity was found both between species and within species. C. tropicalis showed the highest biomass production, whereas C. glabrata showed the highest metabolic activity, and non-Candida species isolates showed the lowest metabolic activity (P < 0.0023). The isolates were classified as low metabolic activity, moderate metabolic activity, and high metabolic activity according to their cut-offs by XTT (<0.097, 0.097-0.2, and >0.2) and as low biofilm-forming, moderate biofilm-forming, and high biofilm-forming according to their cut-offs by crystal violet (<0.44, 0.44-1.17, and >1.17). The overall categorical agreement between the procedures was 43.7%, which increased to >50% for C. albicans and C. parapsilosis. XTT and crystal violet are complementary procedures for the study of biofilm production.