Survival of a Salmonella typhimurium poultry isolate in the presence of propionic acid under aerobic and anaerobic conditions

Propionic acid is commonly found as a fermentation product in the gastrointestinal tracts of food animals and has also been used to limit the microbial contaminants in animal feeds. Because propionic acid is known to have antibacterial activity, the propionic acid encountered by foodborne pathogens during their life cycles may play an important role in inhibiting the survival of the pathogens. The survival patterns of Salmonella typhimurium poultry isolate were determined both in aerobic and anaerobic tryptic soy broth (TSB; pH 5.0 or 7.0) containing various concentrations of propionic acid (0-200 mM). The levels of recovered cells were consistently greater at PH 7.0 compared to those at pH 5.0. For the first 4 days, the levels were significantly decreased by incubation under anaerobic conditions as compared to aerobic condition at PH 7.0 (P<0.05). However, there were fluctuations of cell populations with different patterns depending on both concentrations and growth conditions. To characterize the nature of the capability which allowed the cell multiplication following decreases in cell population during incubation at PH 7.0, the cells isolated from the outgrowth cultures were tested for survival in aerobic or anaerobic TSB (pH 5.0 or PH 7.0) containing propionic acid (50 mM). The outgrowth isolates did not show significant differences in the level of recovered cells in the presence of propionic acid when compared to the wild type strain (P>0.05), suggesting that the cells in the outgrowth cultures did not harbour mutation(s) conferring increased resistance to propionic acid. In addition, the level of recovered cells of isogenic rpoS mutant strain of S. typhimurium was not significantly different from that of the wild type strain in the same assay conditions (P<0.05). The results of this study show that the bactericidal activity of propionic acid on S. typhimurium can be affected by environmental conditions such as acidic pH levels and anaerobiosis in food materials and gastrointestinal tracts. However, S, typhimurium is also able to multiply in the presence of sub- lethal concentrations of propionic acid at neutral pH during prolonged incubation under both aerobic and anaerobic conditions. (C) 1998 Academic Press.