Bacteriocin-like substances produced by Lactococcus lactis subsp. lactis CF4MRS isolated from fish intestine: Antimicrobial activities and inhibitory properties

In the present study, evaluation of antimicrobial activities of Lactococcus lactis subsp. lactis CF4MRS bacteriocin-like substances (BLIS) against various fish pathogens was performed using an agar well diffusion assay. The cell-free supernatant (CFS) was first pre-treated using four different bioassays. In the first treatment T1, CFS was treated with catalase, and the pH was adjusted to 6.5 with NaOH to eliminate the inhibitory effect of H2O2 and/or lactic acid. In T2, CFS was treated with only 1 mg/mL catalase. In T3, only the pH was modified and adjusted (6.5). For T4, no pretreatment was done on the CFS. Our results showed all tested pathogens: Pseudomonas fluorescens ATCC 13525, P. aeruginosa ATCC 10145, Klebsiella pneumonia ATCC 10031, Escherichia coli ATCC 25922, Aeromonas hydrophila ATCC 49140, Edwardsiella tarda BCRC 16703 and Serratia marcescens (Monash culture collection), were susceptible to L. lactis CFS (T4). This bacterial inhibition activity was presumably due to BLIS present in CFS. However, the CFS lost its antimicrobial activity when pH was adjusted and treated with enzyme catalase (T1 and T3). This inhibitory effect would be attributed to either organic acid or H2O2 produced by the bacterium. On the other hand, CFS treated with only catalase (T2) exerted similar inhibitory effect against the pathogens as showed by the untreated CFS (T4). BLIS in CFS were subsequently determined using HPLC method. Our results revealed that lactic acid in BLIS indeed plays the important role in bacterial inhibition, suggesting the bacteria could be potentially used in managing and controlling fish diseases.