Differences in the Expression of Cold Stress–Related Genes and in the Swarming Motility Among Persistent and Sporadic Strains of Listeria monocytogenes

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Abstract
The persistence of certain Listeria monocytogenes strains in food-related environments suggests niche adaptation of these strains and therefore constitutes a major risk to consumer health and results in economic losses for the food producer. In this study, a set of 23 L. monocytogenes isolates, including a group of persistent and a group of sporadic strains, was evaluated regarding their swarming motility at 11°C. In each group, significant (p<0.05) differences in motility were observed. The transcript levels of nine cold stress-related genes were analyzed by real-time quantitative PCR in two representatives of persistent (CBISA3077) and sporadic (CBISA3049) strains isolated from the dairy environment, and significant (p<0.05) differences between the two strains were observed. The persistent strain showed significantly higher transcript levels of dtpT and sigB genes, and significantly lower levels of flaA, oppA, lmo1722, and lmo0866 genes. In the persistent strain, the upregulation of sigB, involved in the tolerance to low temperature and to osmotic stress, could account for the persistence of this strain in its original dairy environment. In a similar way, the downregulation of two helicase-encoding genes lmo1722 and lmo0866, in this strain, may be an evolutionary trait that could facilitate cold stress adaptation. Even though this analysis should be extended to more sporadic and more persistent strains, the results presented here strongly suggest gene expression networks differently adjusted, in the two strains, to the low-temperature environment from where they were collected. Moreover, our findings suggest that bacterial motility per se should not be considered a key feature for the persistence of L. monocytogenes in the food environment.

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