Purpose: This study was undertaken to determine the fate of S. enterica and L. monocytogenes as influenced by the presence of probiotics during a simulated gastrointestinal passage.
Methods: A 3-strain mixture of S. enterica or L. monocytogenes was co-inoculated with a commercial probiotic product into homogenized peanut butter at ca. 5 log CFU/ml. Homogenates inoculated with only the pathogens served as controls. To mimic gastric digestion, the pH of the samples was reduced to 1.4-1.9 and pepsin and lipase were added. Samples were incubated for 2h at 37°C. Afterwards, the pH of the samples was increased to 4.3-5.2 and pancreatin and bile were added. Samples were incubated as described above to simulate digestion in the upper small intestine. Finally, the pH of the samples was adjusted to 6.7-7.5 to simulate conditions in the lower small intestines and incubation was done at 37°C for 20h. Aliquots were collected periodically for bacterial enumeration.
Results: Average Listeria and Salmonella populations in samples without probiotics were higher than the populations co-inoculated with probiotics. After the 24h incubation period Listeria and Salmonella populations in the controls increased by 5.10 and 5.12 log CFU, respectively, while the two populations in samples with probiotics decreased by 2.30 and 1.34 log CFU, respectively. However, pathogen populations in samples with or without probiotics at sampling points prior to 24h were not significantly different (P > 0.05). Additionally, the average populations for the two pathogens in samples with probiotics were not significantly different (P > 0.05).
Significance: The survival and growth of Salmonella and Listeria could be controlled by the presence of probiotics in gastrointestinal system.