Selected genomic and phenotypic responses of Salmonella serovars to chlorine, chlorine dioxide, and cetylpyridinium chloride
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Non-typhoidal Salmonella enterica serovars continue to be the leading cause of foodborne illnesses in United States. Chlorine, chlorine related, and quaternary compounds are generally used for disinfecting carcasses and equipment in processing industries. The current study was aimed at understanding the inactivation kinetics of four Salmonella serovars to chlorine, chlorine dioxide and cetylpyridinium chloride (CPC). The transcriptomic responses to oxidative stress was investigated in stationary and log phase cells of S. Typhimurium. The study was also aimed at understanding the effect of the chemicals on the expression of virulence genes associated with the Salmonella Pathogenecity Island 1 (SPI1). The possible induction of the viable but nonculturable (VBNC) state in Salmonella due to CPC was also investigated. The inactivation parameters for each serovar and the chemical were estimated based on the Hom's model, ln (N/N0) = -k C^n T^m and it appeared that while disinfectant contact time was significant, biocide concentration in the overall disinfection was insignificant. This was true especially for chlorine and CPC with subtle differences observed between the serovars. The inactivation efficacy was, however, dependent on both concentration and the exposure time for chlorine dioxide. The highest degree of inactivation was obtained with chlorine followed by chlorine dioxide and CPC. Transcriptomic responses of S. Typhimurium revealed significant downregulation of several metabolic processes such as tricarboxylic acid cycle, oxidative phosphorylation, and amino acid biosynthesis in both log and stationary phase cells. Several stress related genes such as usp, rpoS and ompR were upregulated in the stationary phase cells. Majority of the virulence genes associated with the SPI1 were found to be downregulated for all the treatments. While treatment with chlorine and CPC caused downregulation of all the virulence genes, treatment with chlorine dioxide caused significant upregulation of few (hilC, invC, sipA and sipB) genes associated with the SPI1. Finally, the induction of VBNC state was not concluded as a result of treatment with CPC. However, significant percentage of cells (45 percent) with intact membrane was established based on the BacLight assayTM.