Inhibition on certain food-borne bacteria in yoghurt by nisin-producing Lactococcus lactis WNC20

Abstract

The inhibitory effect of Lactococcus lactis WNC20 (Ll. 20), a nisin producing lactic acid bacteria, on food-borne bacteria was investigated by testing in yoghurt as a food model, as compared to commercial starter, LC. Experimental groups comprised of control A (Ll. 20 as starter culture), control B (commercial starter culture, LC), control C (food-borne pathogen), treatment A (Ll. 20 and food-borne pathogen) and treatment B (LC and food-borne pathogen). Starter culture inoculum of ~6-7- log cycle was used for all experiments. After 48 hours, Staphylococcus aureus ATCC25923 was not detected in treatment A after addition either at 2- or 3- log cycle. While, addition of S. aureus at 4- log cycle, S. aureus decreased 2- log cycle after 24 hours of incubation. However, no detection of S. aureus was observed in treatment B after 48 hours. With regard to, Bacillus cereus ATCC1729, Listeria monocytogenes DMST17303, Escherichia coli ATCC25922 and Salmonella Typhimurium ATCC13311, at 2-, 3- or 4- log cycles, were added into milk at the same time as the starter culture. Lis. monocytogenes was not detected in both treatment A and B after 48 hours incubation. B. cereus was inhibited at 16 hours incubation and increased to 5- log cycle at 48 hours in treatment A when added B. cereus at 2- and 3- log cycles. While, addition B. cereus 4- log cycle, it was inhibited after 16 hours and increased to 5- log cycle after 24 hours in treatment A. B. cereus in treatment B was not detected after 8 hours of incubation, added B. cereus starting at 2-, 3- and 4- log cycles. No viable cell of E. coli (<10 CFU/mL) in treatment A was detected after 72 hours, but in treatment B E. coli was inhibited after 48 hours. With regard to, Sal. Typhimurium starting at 2- and 3- log cycles was not detected after 24 hours of incubation yoghurt, and after 72 hours for adding 4- log cycle in treatment A. Whereas, in treatment B Sal. Typhimurium was not detected after 16 and 24 hours for adding 2-, 3- and 4- log cycles, respectively. All experiments found that pH always decreased whereas percentage of total acid always increased throughout the experiment. Nisin Z was also found from yoghurt Ll. 20, which showed inhibition on, Lb. plantarum TISTR850 growth by agar diffusion technique. Therefore, Ll. 20 could be used as starter in Nham preparatAAion in order to reduce food-borne bacteria contamination.