Colicin Receptor CirA EnhancesSalmonellaTyphimurium’s Resistance to Colicin Ib in the Absence of the Cognate Immunity Protein

Abstract

AbstractIntestinal 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 byEscherichia coli(E. coli) strains, target competing bacteria in their environmental niche, whilst the producing cells are safeguarded by specific immunity proteins. Notably, non-typhoidalSalmonellaTyphimurium strains can produce a narrow-spectrum protein toxin colicin IB (ColIb) providing a competitive edge against susceptibleEnterobacteriaceaestrains. However, the multi-drug resistant and systemically invasive iNTS (invasive non-TyphoidalSalmonella)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 geneimm. Intriguingly, in colicin Ib-producingS. Typhimurium strains, theimmgene 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 ofEscherichia coliandS. TyphimuriumcirAdeletion mutants, promoter-swap techniques, and gene complementation, we identified that the colicin resistance phenotype inS. 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.