Role of the biofilm matrix in resistance of Candida biofilms to antifungal agents

Abstract

The aim of this project was to investigate the possible role of the biofilm matrix as a barrier to drug diffusion in Candida biofilms and in mixed species fungal-bacterial biofilms. The penetration of antifungal agents through single- and mixed-species biofilms containing Candida was investigated using a novel filter disk bioassay. Fluconazole permeated all single-species Candida biofilms more rapidly than flucytosine. Drug penetration was more extensive with C. albicans than with the other species and the rates of diffusion of either drug through biofilms of three strains of C. albicans were similar. In all cases, after 3 to 6h the drug concentration at the distal edge of the biofilm was very high (many times the MIC). Nevertheless, drug penetration failed to produce complete killing of biofilm cells. These results indicate that poor antifungal penetration is not a major drug resistance mechanism for Candida biofilms under these conditions. It has been reported that the production of extracellular matrix by Candida biofilms growing under static incubation conditions is relatively minimal, but increases dramatically when developing biofilms are subjected to a liquid flow. In this study, Candida biofilms were grown under flow conditions in a modified Robbins device (MRD). Biofilms of C. albicans grown in the MRD produced more matrix material than those grown statically, and were significantly more resistant (P<0.001) to amphotericin B. Biofilms of C. tropicalis synthesized large amounts of matrix material even when grown statically, and such biofilms were completely resistant to both amphotericin B and fluconazole. Mixed-species biofilms of C. albicans and S. epidermidis RP62A, when grown statically or in the MRD, were also completely resistant to amphotericin B and fluconazole. Mixed-species biofilms of C. albicans and S. epidermidis M7, on the other hand, were completely drug resistant only when grown under flow conditions. Overall, these findings demonstrate that the matrix can make a significant contribution to drug resistance in Candida biofilms, especially under conditions similar to those found in catheter infections in vivo, and that the composition of the matrix material is an important determinant in resistance