BACKGROUND: Two of our long term efforts are to discover compounds with synergistic antifungal activity from metabolites of marine derived microbes and to optimize the production of the interesting compounds produced by microorganisms. In this respect, new applications or mechanisms of already known compounds with a high production yield could be continually identified. Surfactin is a well-known lipopeptide biosurfactant with a broad spectrum of antimicrobial and antiviral activity; however, there is less knowledge on surfactin's antifungal activity. In this study, we investigated the synergistic antifungal activity of C(15)-surfactin and the optimization of its production by the response surface method.
METHODOLOGY/PRINCIPAL FINDINGS: Using a synergistic antifungal screening model, we found that the combination of C(15)-surfactin and ketoconazole (KTC) showed synergistic antifungal effect on Candida albicans SC5314 when the concentrations of C(15)-surfactin and KTC were 6.25 µg/mL and 0.004 µg/mL, respectively. These concentrations were lower than their own efficient antifungal concentrations, which are >100 µg/mL and 0.016 µg/mL, respectively. The production of C(15)-surfactin from Bacillus amyloliquefaciens was optimized by the response surface methodology in shaker flask cultivation. The Plackett-Burman design found sucrose, ammonium nitrate and NaH(2)PO(4) x 2H(2)O to have significant effects on C(15)-surfactin production. The optimum values of the tested variables were 21.17 g/L sucrose, 2.50 g/L ammonium nitrate and 11.56 g/L NaH(2)PO(4)·2H(2)O. A production of 134.2 mg/L, which were in agreement with the prediction, was observed in a verification experiment. In comparison to the production of original level (88.6 mg/L), a 1.52-fold increase had been obtained.
CONCLUSION/SIGNIFICANCE: This work first found that C(15)-surfactin was an efficient synergistic antifungal agent, and demonstrated that response surface methodology was an effective method to improve the production of C(15)-surfactin.