The multi-drug efflux system AcrABZ-TolC is essential for infection of Salmonella Typhimurium by the flagellum-dependent bacteriophage Chi.

Bacteriophages are the most abundant biological entities in the biosphere. Due to their host specificity and ability to kill bacteria rapidly, bacteriophages have many potential healthcare applications, including therapy against antibiotic-resistant bacteria. Infection by flagellotropic bacteriophages requires a properly rotating bacterial flagellar filament. The flagella-dependent phage χ (Chi) infects serovars of the pathogenic enterobacterium Salmonella enterica However, cell surface receptors and proteins involved in other stages of χ infection have not been discovered to date. We screened a multi-gene deletion library of S. enterica serovar Typhimurium by spotting mutants on soft agar plates seeded with bacteriophage χ and monitoring their ability to grow and form a swim ring, a characteristic of bacteriophage-resistant motile mutants. Those multi-gene deletion regions identified to be important for χ infectivity were further investigated by characterizing the phenotypes of corresponding single-gene deletion mutants. This way, we identified motile mutants with varying degrees of resistance to χ. Deletions in individual genes encoding the AcrABZ-TolC multi-drug efflux system drastically reduced infection by bacteriophage χ. Furthermore, an acrABtolC triple deletion strain was fully resistant to χ. Infection was severely reduced but not entirely blocked by the deletion of the gene tig encoding the molecular chaperone trigger factor. Finally, deletion in genes encoding enzymes involved in the synthesis of the antioxidants glutathione (GSH) and uric acid resulted in reduced infectivity. Our findings begin to elucidate poorly understood processes involved in later stages of flagellotropic bacteriophage infection and informs research aimed at the use of bacteriophages to combat antibiotic-resistant bacterial infections.IMPORTANCEAntimicrobial resistance is a large concern in the healthcare field. With more multi-drug resistant bacterial pathogens emerging, other techniques for eliminating bacterial infections are being explored. Among these is phage therapy, where combinations of specific phages are used to treat infections. Generally, phages utilize cell appendages and surface receptors for the initial attachment to their host. Phages that are flagellotropic are of particular interest because flagella are often important in bacterial virulence, making resistance to attachment of these phages harder to achieve without reducing virulence. This study discovered the importance of a multi-drug efflux pump for the infection of Salmonella enterica by a flagellotropic phage. In theory, if a bacterial pathogen develops phage resistance by altering expression of the efflux pump then the pathogen would simultaneously become more susceptible to the antibiotic substrates of the pump. Thus, co-administering antibiotics and flagellotropic phage may be a particularly potent antibacterial therapy.