Investigation of the role of porin proteins in Salmonella acquisition of host iron

Iron is essential for the growth and long-term persistence of most human pathogens, and growth within host tissues is often dependent on the bacteria being able to steal iron from the normally secure lung-iron sources, such as the iron-binding proteins transferrin (Tf) and lactoferrin (Lf). Proteomic analysis showed that Tf and Lf are bound by Salmonella and provisionally identified the porins OmpC and OmpD as responsible for their binding. Use of a Lambda red mutagenesis methodology created a single mutant of ompC and ompD genes as well as a double ompC and ompD mutant, which confirmed the porins’s role as Tf and Lf receptors. The porins mutants grew less well in Fe-limited media, while use of 55Fe-Tf showed that uptake of host-bound Fe was reduced, indicating a direct role for the Salmonella OmpC and OmpD porins in acquisition of Tf iron. Interestingly, although impaired in uptake of iron, siderophore production was not increased in the porin mutants.

The ompC and ompD porin mutants were not impaired in growth responsiveness to noradrenaline and dopamine, but there was no response in strains lacking OmpC to epinephrine. Analysis of the internalization of 3H-epinephrine showed that its uptake was reduced in the strains with a mutation in ompC, while 3H-noradrenaline uptake was unchanged. This indicates that OmpC porin is an entry point for the catecholamine epinephrine. Other investigations showed that deleting the gene for OmpC affected cell shape and size and so might have a role in Salmonella cell structure. The ability of the Salmonella porin mutants to infect macrophages cell was undertaken and was found to be reduced in the ompD single and particularly the double ompC and ompD mutant suggesting that the Salmonella OmpC and OmpD porins also contribute to the virulence of Salmonella in terms of invasion of human macrophages.