Reduction of Salmonella Typhimurium following exposure to multiple hurdle treatments of heated, acidified organic acid salt solutions occurs via membrane destabilization

The antimicrobial activity of organic acids in combination with non-chemical treatments was evaluated for inactivation of Salmonella Typhimurium. It was observed that the effectiveness of the multiple hurdle treatments were temperature and pH dependent and corresponded to the degree of organic acid lipophilicity. This led to the hypothesis that the loss in viability was due to cell membrane disruption. Evaluation of osmotic response, potassium ion leakage, and transmission electron micrographs confirmed effects on the cell membrane following treatment. Interestingly, all treatments, even those with no affect on viability, such as with sodium acetate, resulted in measurable cellular stress. Microarray experiments explored the specific response of S. Typhimurium to sodium acetate and sodium propionate, the most similar treatments in terms of pKa and ionic strength, and found little difference in the changes in gene expression following either treatment, despite their very different effects on viability. Taken together, the results reported support our hypothesis that following treatment with heated, acidified, organic acid salts, the loss of S. Typhimurium viability is at least in part due to membrane damage and the more lipophilic the organic acid the more effective the treatment. Overall, the data presented here indicate that a combined thermal, acidified sodium propionate treatment can provide a simple, yet effective antimicrobial treatment to combat Salmonella. We have strategically designed a multiple hurdle intervention employing various organic acid salts [sodium acetate (SA), sodium lactate, and sodium propionate (SP)], pH, and temperature hurdles into one treatment against S. Typhimurium. Previous research in our lab has shown similar treatments to be highly effective against S. Typhimurium. Here, we further investigate the range of effectiveness as well as the mechanism of action behind this multiple hurdle treatment using viability, osmotic response, potassium leakage, and transmission electron microscopy studies. Total bacterial RNA was isolated as previously described and the RNA samples were then converted to fluorescently-labeled cDNA and hybridized to S. Typhimurium microarrays version 8 (NIAID's Pathogen Functional Genomics Resource Center).