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 degrees C for 24 h. Inhibition of Salmonella spp. was also examined at refrigeration temperatures (6 degrees 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 degrees 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 pi-I-neutralized broth indicated that acid production by L lactis was primarily responsible for the inhibition. Numbers of Salmonella spp. incubated at 6 degrees 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.