Antagonistic action of Lactobacillus lactis toward Salmonella spp. and Escherichia coli O157:H7 during growth and refrigerated storage

Cells of Lactobacillus lactis were added to trypticase soy broth that contained cells of Escherichia coli O157:H7 or cells of Salmonella spp. in order to determine if L lactis inhibited the pathogens. The inhibition of all pathogens was examined during growth at 37°C for 24 h. Inhibition of Salmonella spp. was also examined at refrigeration temperatures (6°C) for 5 days. One strain each of E. coli O157:H7, Salmonella Typhimurium, and Salmonella Enteritidis was examined. E. coli was enumerated on violet red bile agar, and Salmonella spp. were enumerated on brilliant green agar. In all experiments at 37°C, the L. lactis completely inhibited all pathogens, producing numbers that were not detectable after 24 h of incubation. There were significant (P > 0.05) increases in numbers of the pathogens in the control samples containing no L. lactis. There were significant (P < 0.05) declines in the pH of both control and L. lactis inoculated samples. There was a significantly (P < 0.05) larger decline in the pH of samples inoculated with L. lactis. Interaction studies with pH-neutralized broth indicated that acid production by L. lactis was primarily responsible for the inhibition. Numbers of Salmonella spp. incubated at 6°C did not decline significantly (P > 0.05) for control or inoculated samples, which suggests that this strain of L. lactis does not inhibit Salmonella spp. at refrigeration temperatures. Additionally, there were no significant (P > 0.05) changes in pH or in numbers of L. lactis during refrigerated storage.