Stability of the Salmonella Typhimurium rcsC11 mutant under different stress conditions.

The virulence genes of Salmonella are modulated during infection by several regulatory systems, and the RcsCDB system is one of the most important of these. The S. Typhimurium EG14873 (rcsC11) strain harbours the rcsC11 point mutation, displaying a constitutive activation of this system, which is characterized by mucoid colonies and attenuated virulence phenotypes. In this work, the stability of the rcsC11 mutation was analysed under stress conditions. Under acid and anaerobic stresses, we observed the appearance of small and non-mucoid colonies of the rcsC11 strain. The sequencing of the rcsC gene from these colonies showed that the mutation is conserved. Moreover, we found that small colonies were also generated when the wild-type strain grew in acid and anaerobic conditions. It is worth noting that the transition from normal to atypical colonies of both strains only took place after several days of incubation and was not observed during eukaryotic cell infection. Therefore, the appearance of these atypical colonies is a characteristic feature of S. Typhimurium strains under stressful situations and does not involve a reversion of the rcsC11 allele and nor does it imply any risk to mammalian cells. Therefore, we propose that the S. Typhimurium rcsC11 strain is a good candidate for the development of attenuated vaccines.

The RcsCDB regulatory system plays a crucial role in the protection of Salmonella enterica serovar Typhimurium against oxidative stress.

Dps, the most abundant protein during the stationary growth phase, in Salmonella enterica is required for resistance to reactive oxygen species produced by the host during infection. It has been reported that in Salmonella dps expression is controlled by RpoS and Fur proteins. However, the regulation and function of Dps remain to be resolved. In the present work we demonstrate that activation of the complex RcsCDB regulatory system increases dps expression during exponential growth of Salmonella. In addition, we show that such dps upregulation produces high levels of H2O2 resistance. This phenotype allows the bacteria to avoid reactive oxygen species killing at early stages of growth, thus protecting its genetic material.