Influence of lactic acid and acetic acid on Salmonella spp. growth and expression of acid tolerance-related genes

Salmonella spp. is an important foodborne pathogen, often associated with meat products. This pathogen presents a complex tolerance mechanism in the presence of organic acids, which is regulated by a diversity of genes, including rpoS, nlpD and clpP. The present study aimed to measure the expression of such genes by Salmonella strains subjected to acid stress conditions, and associate these data with microbial growth. A culture collection composed of 79 strains of Salmonella spp. obtained from bovine and swine production chains was subjected to PFGE using XbaI, and 3 strains (serovars Derby, Typhimurium and Meleagridis) were selected for acid tolerance trials. The selected strains were inoculated in meat extract broth (MEB) added to lactic or acetic acids at a final pH of 4.0, 5.0 or 6.0, and incubated at 37°C for 6, 12, 24 and 48h. As controls, Salmonella strains were inoculated in MEB at pH7.0, and incubated in the same conditions. Bacterial populations were monitored by direct plating and gene expression using qPCR. Salmonella presented similar populations to controls and evident expression of rpoS at pH5.0 and 6.0. However, Salmonella populations were not detectable after 6h at pH4.0. The adaptability of Salmonella to pH5.0 and 6.0 emphasizes the importance of adequate monitoring of pH reduction during cleaning procedures in food industries, such as organic acid spraying in bovine carcasses. The data obtained demonstrated the relevance of rpoS in the acid tolerance mechanism of Salmonella strains, prompting further studies to investigate its expression in meat systems.