Killing rate curve and combination effects of surfactin C produced from Bacillus subtilis complex BC1212 against pathogenic Mycoplasma hyopneumoniae

This study evaluated the killing rate as well as the antimycoplasmal effect of surfactins isolated from Bacillus subtilis complex BC1212, either alone or in combination with various antibacterials against Mycoplasma hyopneumoniae. Prior to the killing rate and the combination effect studies of surfactins, the minimal inhibitory concentrations (MICs) of various antibacterials and surfactins (consisting of surfactins A, B, C, and D) against M. hyopneumoniae were investigated. The MIC of all surfactins was found to be 64 mu g/ml. The MICs of colistin (COL), norfloxacin (NFX), oxytetracycline (OTC), streptomycin (SM) and tiamulin (TIA) were > 256, 0.063, 0.025, 4, and 0.015 mu g/ml, respectively. In the killing rate curve studies, surfactin C at 2x MIC and 4x MIC concentrations was found to reduce viability by > 3 log(10) c.c.u./ml within 2-4 h of incubation. Combination of surfactin C with other antibacterials showed additive interaction from the viewpoint of fractional inhibitory concentration (FIC) index of > 0.5 but <= 2 as a borderline. Taking all results into consideration, surfactin C may be useful as a preventive or therapeutic adjuvant in the pathogenesis of mycoplasmal infection.