Synergistic activity of antifungal drugs and lipopeptide AC7 against <em>Candida albicans</em> biofilm on silicone

The occurrence of Candida albicans device-associated infections is tightly correlated to the ability of this fungus to form biofilms. The presence of this three-dimensional structure protects cells from host defenses, and significantly increases their resistance to antifungal agents. Lipopeptide biosurfactants are microbial products with interesting antibacterial, antifungal and anti-adhesive properties. Aim of the present study was to investigate a possible synergistic effect of lipopeptide AC7BS in combination with amphotericin B or fluconazole against C. albicans planktonic cells, biofilm formation and 24 h-old biofilms on medical-grade silicone elastomer disks, in simulated physiological conditions. In co-incubation experiments, AC7BS alone was not effective. However, the combination of AC7BS with the antifungal compounds resulted in a synergistic increase in the efficacy of the drugs against planktonic cells and biofilm, leading to a reduction of MICs and SMICs₅₀. In pre-coating conditions, amphotericin B alone and AC7BS alone significantly inhibited C. albicans biofilms. When the two molecules were tested in association, a synergistic effect was observed on different phases of biofilm formation and a lower SMIC₅₀ was detected. The observed synergism could be related to the combination of the AC7BS anti-adhesive activity and the AMB antifungal effect, but also to the ability of the biosurfactant to affect membranes, thus facilitating AMB entry in the cells. These results suggest that AC7BS can be considered a potential inhibitor of C. albicans biofilm on medical insertional materials and its use as coating agent may potentiate the effect of antifungal compounds such as AMB, when applied in combination