The Vibrio vulnificus stressosome is dispensable in nutrient-replete conditions.

The stressosome is a protein complex that has been demonstrated to sense environmental stresses and mediate the stress response in several Gram-positive bacteria, through the activation of the alternative sigma factor SigB. The in vivo characterisation of this complex has never been performed in Vibrio vulnificus or any other bacteria that do not possess SigB. The elucidation of the role of the stressosome in V. vulnificus would provide elements to elaborate a functional model of the complex in a Gram-negative bacterium and identify the regulatory output in the absence of SigB. The stressosome locus is only found in 44% of Vibrio vulnificus isolates raising the question as to whether the role of stressosome is essential or modulatory in this bacterial species. In this work, the expression of the stressosome genes was proven in nutrient-replete conditions and the co-transcription as one operonic unit of the stressosome locus and its putative downstream regulatory locus was demonstrated. Moreover, the construction of a stressosome mutant lacking the four genes constituting the stressosome complex allowed us to examine the role of this complex in vivo. The initial established mutagenesis strategy relied on rifampicin-resistant V. vulnificus to select recombinant bacteria. Our data clearly showed that the influence of the RifR allele on stress and virulence characteristics overshadowed any effects of the stressosome. Therefore, we established an alternative mutagenesis strategy with a non-modified V. vulnificus parental strain and a DAP auxotrophic E. coli donor strain. Extensive phenotypic characterisation of the successfully-generated mutant in nutrient-replete conditions showed that the stressosome does not significantly contribute to the growth, of V. vulnificus. The stressosome did not modulate the response of V. vulnificus to the range of stresses tested: Ethanol, osmolarity, temperature, and salinity. Furthermore, the stressosome is dispensable for motility and exoenzyme production of V. vulnificus. ### Competing Interest Statement The authors have declared no competing interest.