Colicin Receptor CirA Enhances Salmonella Typhimuriums Resistance to Colicin Ib in the Absence of the Cognate Immunity Protein

Intestinal microbiota play a central role in colonisation resistance providing a fundamental barrier to infection to enteric pathogens. An important mechanism of colonisation resistance involves the production of antimicrobial peptides, such as colicins. Pore-forming colicins, synthesised by Escherichia coli (E. coli) strains, target competing bacteria in their environmental niche, whilst the producing cells are safeguarded by specific immunity proteins. Notably, non-typhoidal Salmonella Typhimurium strains can produce a narrow-spectrum protein toxin colicin IB (ColIb) providing a competitive edge against susceptible Enterobacteriaceae strains. However, the multi-drug resistant and systemically invasive iNTS (invasive non-Typhoidal Salmonella) S. Typhimurium D23580 strain poses an interesting case. The strain lacks colicin Ib production and the corresponding immunity protein, but its potential vulnerability in a colicin-rich gastrointestinal milieu remains uninvestigated. In this study, S. Typhimurium D23580 exhibited resistance to colicin Ib under tested conditions, despite the absence of the immunity gene imm. Intriguingly, in colicin Ib-producing S. Typhimurium strains, the imm gene appeared functionally redundant in contrast to our current understanding. ColIb binds to the outer membrane protein CirA and is translocated to the inner membrane where it forms a pore in sensitive bacteria dissipating the electrochemical potential. Through a series of experimental approaches, including the use of Escherichia coli and S. Typhimurium cirA deletion mutants, promoter-swap techniques, and gene complementation, we identified that the colicin resistance phenotype in S. Typhimurium was partly attributable to the CirA receptor. This finding suggests a complex interplay in the microbial resistance to colicins, highlighting the intricacies of microbial interactions within the gastrointestinal environment.