Genome-wide assay for S. Typhi genes involved in susceptibility to ciprofloxacin. Typhi_cipro

Antibiotic resistant Salmonella Typhi is a life-threatening problem in typhoid endemic regions. Resistance to ciprofloxacin, used to treat multidrug resistant (MDR) typhoid, is conferred by mutations in the topoisomerase genes. However, these mutations alone do not explain the different levels of resistance observed between strains. We have used transposon directed insertion-site sequencing (TraDIS) to perform a whole genome screen to search for genes involved in ciprofloxacin resistance in S. Typhi to understand resistance mechanisms and identify targets that may be used to counter these. We estimate that we have effectively assayed 88% of all the 4896 annotated genes in the S. Typhi strain, which also harbours the pHCM1 resistance plasmid, and identified some genes that were not known previously to be involved in resistance, and some of which had previously no known phenotype. TraDIS also identified several known resistance mechanisms, confirming the accuracy of the data. Sensitivity to ciprofloxacin was enhanced by inactivation of some efflux systems and other surface-associated functions, as well as some genes involved in DNA repair and expression regulation. Mutations that enhanced resistance were in genes predominantly coding for surface polysaccharide biosynthesis and other regulators. This demonstrates that the TraDIS technology is a sensitive tool to identify novel contributors to antibiotic resistance and sensitivity in bacteria. In addition, these data expand our understanding of how fluoroquinolone antibiotics, such as ciprofloxacin, impact on S. Typhi biology and provide insights on how bacteria may evolve to evade the action of antibiotics in general.