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-MS͞MS 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.I n iron-poor environments, many bacteria secrete ironcomplexing agents called siderophores to satisfy their iron needs. For some pathogenic bacteria, siderophores are important virulence factors because iron is bound to transferrin and lactoferrin in body fluids. These proteins reduce the free Fe 3ϩ concentration to about one molecule per liter. Enterobacteria, including Escherichia coli and Salmonella enterica, often produce the catecholate siderophore enterochelin (also called enterobactin) (1, 2). It has been postulated that enterochelin is an inferior siderophore in serum because it adsorbs to hydrophobic sites in serum proteins, such as albumin (3). This fact has always been puzzling because iron supply for many pathogens plays a decisive role in the infection process, and enterochelin, a major siderophore synthesized by S. enterica, does not seem to enhance pathogenicity.Recently, the iroA locus, consisting of the two convergent operons iroN and iroBCDE, has been defined in S. enterica serotype Typhi and also in most other S. enterica serotypes (4, 5). The outer membrane siderophore receptor, IroN, is involved in the transport of several catecholate siderophores in S. enterica (5, 6). In the present investigation, we show that the presence of the iro gene cluster in S. enterica leads to glycosylation of the enterochelin building block 2,3-dihydroxybenzoylserine (DHBS), which makes the hydrophobic enterochelin molecules more hydrophilic, thereby possibly contributing to the observed pathogenicity of Salmonella strains. This siderophore was named salmochelin because it appears to be a characteristic siderophore of Salmonella strains. Interestingly, certain E. coli strains, e.g., the uropathogenic E. coli 563, also possess a very similar iro gene cluster on pathogenicity island III (7). The production and the tentative structural elucidation of salmochelins and their specific uptake via the outer membrane receptor IroN are described. In addition, it is shown that salmo...

Short-read, high-throughput sequencing technology cannot identify the chromosomal position of repetitive insertion sequences that typically flank horizontally acquired genes such as bacterial virulence genes and antibiotic resistance genes. The MinION nanopore sequencer can produce long sequencing reads on a device similar in size to a USB memory stick. Here we apply a MinION sequencer to resolve the structure and chromosomal insertion site of a composite antibiotic resistance island in Salmonella Typhi Haplotype 58. Nanopore sequencing data from a single 18-h run was used to create a scaffold for an assembly generated from short-read Illumina data. Our results demonstrate the potential of the MinION device in clinical laboratories to fully characterize the epidemic spread of bacterial pathogens

A genomic island encoding the biosynthesis and secretion pathway of putative hybrid nonribosomal peptidepolyketide colibactin has been recently described in Escherichia coli. Colibactin acts as a cyclomodulin and blocks the eukaryotic cell cycle. The origin and prevalence of the colibactin island among enterobacteria are unknown. We therefore screened 1,565 isolates of different genera and species related to the Enterobacteriaceae by PCR for the presence of this DNA element. The island was detected not only in E. coli but also in Klebsiella pneumoniae, Enterobacter aerogenes, and Citrobacter koseri isolates. It was highly conserved among these species and was always associated with the yersiniabactin determinant. Structural variations between individual strains were only observed in an intergenic region containing variable numbers of tandem repeats. In E. coli, the colibactin island was usually restricted to isolates of phylogenetic group B2 and inserted at the asnW tRNA locus. Interestingly, in K. pneumoniae, E. aerogenes, C. koseri, and three E. coli strains of phylogenetic group B1, the functional colibactin determinant was associated with a genetic element similar to the integrative and conjugative elements ICEEc1 and ICEKp1 and to several enterobacterial plasmids. Different asn tRNA genes served as chromosomal insertion sites of the ICE-associated colibactin determinant: asnU in the three E. coli strains of ECOR group B1, and different asn tRNA loci in K. pneumoniae. The detection of the colibactin genes associated with an ICE-like element in several enterobacteria provides new insights into the spread of this gene cluster and its putative mode of transfer. Our results shed light on the mechanisms of genetic exchange between members of the family Enterobacteriaceae.

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Salmonella typhimurium employs the specialized type III secretion system encoded in pathogenicity island 1 (SPI1) to translocate effector proteins into host cells and to modulate host cell signal transduction. The SPI1 type III system and the effector proteins are conserved among all salmonellae and are thought to be acquired by horizontal gene transfer. The genetic mechanisms mediating this horizontal transfer are unknown. Here, we describe that SopE, a SPI1-dependent translocated effector protein, is present in relatively few S. typhimurium isolates. We have isolated a temperate phage that encodes SopE. Phage morphology and DNA hybridization, as well as partial sequence information, suggest that this phage (SopE⌽) is a new member of the P2 family of bacteriophages. By lysogenic conversion this phage can horizontally transfer genes between different S. typhimurium strains. Strikingly, most of the isolates harboring SopE⌽ belong to the small group of epidemic strains of S. typhimurium that have been responsible for a large percentage of human and animal salmonellosis and have persisted for a long period of time. Our data suggest that horizontal transfer of type III dependent effector proteins by lysogenic infection with bacteriophages (lysogenic conversion) may provide an efficient mechanism for fine-tuning the interaction of Salmonella spp. with their hosts.