Characterisation of the role of the foxA gene in Salmonella iron acquisition and virulence

FoxA is an iron-regulated outer membrane receptor protein of Salmonella enterica serovar Typhimurium that is required for utilisation of the trihydroxamate siderophore ferrioxamines, which acts as source of iron. In this study various approaches were attempted in order to clone the entire foxA gene. This was successfully achieved by use of a homologous recombination using a suicide vector strategy. Three different S. Typhimurium foxA mutant strains, SL2MK (this study), RK102 and TML/nr, were tested for growth responsiveness to ferrioxamine B using a plate bioassay. No response was observed for any of the mutant strains while of growth was clearly observed for both of the wild type strains. Mutant strains carrying the complementing foxA+ plasmid showed restored growth stimulation by ferrioxamine, confirming that the observed phenotypes were dependent on the presence or absence of the functional foxA gene. An analysis of the role of FoxA in Salmonella virulence was undertaken. An in vitro assay involving bacterial interaction with human dendritic cells demonstrated that, while mutation of the foxA gene had little or no effect on the ability of S. Typhimurium to invade and survive within dendritic cells, the foxA mutant was significant attenuated in its ability to damage these cells. However, investigations of the involvement of the foxA gene in Salmonella virulence using a murine model of infection yielded consistent evidence which suggested that the foxA gene is not critical for the pathogenesis of Salmonella . The foxA mutant strain SL2MK was able to infect and reside in the liver and spleen of mice infected intravenously to an extent similar to the wild type parent strain SL1344/nr and to the complemented mutant strain CSL2MK. In addition the foxA mutant and its wild type parent strain were recovered in similar numbers from the Peyer's patches, mesenteric lymph nodes, spleens and livers of mice infected by the oral route with these strains.