No abstract

Many Gram-negative pathogens utilize the type III secretion system (T3SS) to translocate virulence-promoting effector proteins into eukaryotic host cells. The activity of this system results in a severe reduction of bacterial growth and division, summarized as secretion-associated growth inhibition (SAGI). In Yersinia enterocolitica, the T3SS and related proteins are encoded on a virulence plasmid. We identified a ParDE-like toxin–antitoxin system on this virulence plasmid in genetic proximity to yopE, encoding a T3SS effector. Effectors are strongly upregulated upon activation of the T3SS, indicating a potential role of the ParDE system in the SAGI or maintenance of the virulence plasmid. Expression of the toxin ParE in trans resulted in reduced growth and elongated bacteria, highly reminiscent of the SAGI. Nevertheless, the activity of ParDE is not causal for the SAGI. T3SS activation did not influence ParDE activity; conversely, ParDE had no impact on T3SS assembly or activity itself. However, we found that ParDE ensures the presence of the T3SS across bacterial populations by reducing the loss of the virulence plasmid, especially under conditions relevant to infection. Despite this effect, a subset of bacteria lost the virulence plasmid and regained the ability to divide under secreting conditions, facilitating the possible emergence of T3SS-negative bacteria in late acute and persistent infections.

The two-component system DegS/U of Bacillus subtilis controls more than one hundred genes involved in several different cellular behaviours. Since the consensus sequence recognized by the response regulator DegU has not been clearly defined yet, mutations in either component have been crucial in the identification of the cellular targets of this regulatory system. Over the years, the degU32Hy mutant allele, that was supposed to mimic the activated regulator, has been commonly used to define the impact of this TCS on its regulated genes in domestic strains. SwrA encodes a small protein essential for swarming motility and for poly-γ-glutamate biosynthesis and is only present in wild strains. Previous work indicated that SwrA is partnering with DegU~P in exerting its role on both phenotypes. In this work, inserting a degS200Hy mutation in swrA+ and swrA- isogenic strains we demonstrate that SwrA modulates the action of DegU~P on two new phenotypes, subtilisin expression and competence for DNA uptake, with a remarkable effect on transformation. These effects cannot not be appreciated with the DegU32Hy mutant as it does not mirror the wild-type DegU protein in its ability to interact with SwrA.