Salmochelins, siderophores of Salmonella enterica and uropathogenic Escherichia coli strains, are recognized by the outer membrane receptor IroN

Members of a family of catecholate siderophores, called salmochelins, were isolated by reversed-phase HPLC from Salmonella enterica serotype Typhimurium and structurally characterized by Fourier transform ion cyclotron resonance–MS/MS and GC–MS. The tentative structure of salmochelin 1 contained two 2,3- dihydroxybenzoylserine moieties bridged by a glucose residue, bound to the serine hydroxyl group of one moiety and the carboxylate of the second moiety. Salmochelin 2 contained in addition a second glucose residue linked to a third 2,3-dihydroxybenzoylserine moiety. Salmochelins were not produced by an iroBC mutant, which indicated that the IroB protein might be responsible for the glucosyl transfer predicted by sequence similarities to known glycosyltransferases. Uptake experiments with radiolabeled (55)Fe-salmochelin and growth promotion tests with salmochelins showed that the IroN outer membrane receptor, encoded in the iroA locus of S. enterica and uropathogenic Escherichia coli strains, was the main receptor for ferric salmochelin transport

Limited boundaries for extensive horizontal gene transfer among Salmonella pathogens

Recombination is thought to be rare within Salmonella, as evidenced by absence of gene transfer among SARC strains that represent the broad genetic diversity of the eight primary subspecies of this common facultative intracellular pathogen. We adopted a phylogenetic approach to assess recombination within the mutS gene of 70 SARB strains, a genetically homogeneous population of Salmonella enterica subspecies I strains, which have in common the ability to infect warm-blooded animals. We report here that SARB strains show evidence for widespread recombinational exchange in contrast to results obtained with strains exhibiting species-level genetic variation. Besides extensive allele shuffling, SARB strains showed notably larger recombinagenic patch sizes for mutS (at least ≈1.1 kb) than previously reported for S. enterica SARC strains. Explaining these experimental dichotomies provides important insight for understanding microbial evolution, because they suggest likely ecologic and genetic barriers that limit extensive gene transfer in the feral setting