Aerobactin iron transport genes commonly encoded by certain ColV plasmids occur in the chromosome of a human invasive strain of Escherichia coli K1

Iron Acquisition Systems of Gram-negative Bacterial Pathogens Define TonB-Dependent Pathways to Novel Antibiotics

Iron is an indispensable metabolic cofactor in both pro- and eukaryotes, which engenders a natural competition for the metal between bacterial pathogens and their human or animal hosts. Bacteria secrete siderophores that extract Fe3+ from tissues, fluids, cells, and proteins; the ligand gated porins of the Gram-negative bacterial outer membrane actively acquire the resulting ferric siderophores, as well as other iron-containing molecules like heme. Conversely, eukaryotic hosts combat bacterial iron scavenging by sequestering Fe3+ in binding proteins and ferritin. The variety of iron uptake systems in Gram-negative bacterial pathogens illustrates a range of chemical and biochemical mechanisms that facilitate microbial pathogenesis. This document attempts to summarize and understand these processes, to guide discovery of immunological or chemical interventions that may thwart infectious disease.

Identification of the Wzx flippase, Wzy polymerase and sugar-modifying enzymes for spore coat polysaccharide biosynthesis in Myxococcus xanthus

The rod-shaped cells of Myxococcus xanthus, a Gram-negative deltaproteobacterium, differentiate to environmentally resistant spores upon starvation or chemical stress. The environmental resistance depends on a spore coat polysaccharide that is synthesised by the ExoA-I proteins, some of which are part of a Wzx/Wzy-dependent pathway for polysaccharide synthesis and export; however, key components of this pathway have remained unidentified. Here, we identify and characterise two additional loci encoding proteins with homology to enzymes involved in polysaccharide synthesis and export, as well as sugar modification and show that six of the proteins encoded by these loci are essential for the formation of environmentally resistant spores. Our data support that MXAN_3260, renamed ExoM and MXAN_3026, renamed ExoJ, are the Wzx flippase and Wzy polymerase, respectively, responsible for translocation and polymerisation of the repeat unit of the spore coat polysaccharide. Moreover, we provide evidence that three glycosyltransferases (MXAN_3027/ExoK, MXAN_3262/ExoO and MXAN_3263/ExoP) and a polysaccharide deacetylase (MXAN_3259/ExoL) are important for formation of the intact spore coat, while ExoE is the polyisoprenyl-phosphate hexose-1-phosphate transferase responsible for initiating repeat unit synthesis, likely by transferring N-acetylgalactosamine-1-P to undecaprenyl-phosphate. Together, our data generate a more complete model of the Exo pathway for spore coat polysaccharide biosynthesis and export.

Comparative accessory gene fingerprinting of surface water Escherichia coli reveals genetically diverse naturalized population

AIMS

To utilize comparative accessory gene fingerprinting to discriminate between naturalized and faecal Escherichia coli, with particular emphasis on strains from phylogroup B1.

METHODS AND RESULTS

Fourteen accessory genes that were potentially ecotype-specific were selected on the basis of comparative genomic DNA sequence analysis between faecal and environmental strains and also using a literature-based strategy. PCR assays were designed for each gene, and used to screen 107 faecal strains from various hosts and 106 environmental strains from surface water and sediment. While none of the 14 accessory genes were ecotype-specific, six of the genes were ecotype-enriched. Specifically, toxin-antitoxin system genes were more abundant among faecal strains, whereas genes involved in iron acquisition, complement resistance/surface exclusion, and biofilm formation were more abundant among environmental strains. These six genes were used to form composite fingerprints which revealed the presence of several ecotype-specific and -enriched fingerprints. Notably, some of the environmental strain-specific or -enriched fingerprints consisted of strains putatively belonging to clade ET-1, which has been previously recognized as a naturalized subpopulation.

CONCLUSIONS

Unlike single genes which did not reliably distinguish between faecal and naturalized phylogroup B1 E. coli strains, composite fingerprints of ecotype-enriched accessory genes may offer a novel method for distinguishing between these two populations.

SIGNIFICANCE AND IMPACT OF THE STUDY

Accessory gene fingerprinting may have important practical implications for improving the specificity of methods that are widely used for quantifying and identifying the sources of faecal contamination in surface water.

Plasmid-Encoded Iron Uptake Systems

Plasmids confer genetic information that benefits the bacterial cells containing them. In pathogenic bacteria, plasmids often harbor virulence determinants that enhance the pathogenicity of the bacterium. The ability to acquire iron in environments where it is limited, for instance the eukaryotic host, is a critical factor for bacterial growth. To acquire iron, bacteria have evolved specific iron uptake mechanisms. These systems are often chromosomally encoded, while those that are plasmid-encoded are rare. Two main plasmid types, ColV and pJM1, have been shown to harbor determinants that increase virulence by providing the cell with essential iron for growth. It is clear that these two plasmid groups evolved independently from each other since they do not share similarities either in the plasmid backbones or in the iron uptake systems they harbor. The siderophores aerobactin and salmochelin that are found on ColV plasmids fall in the hydroxamate and catechol group, respectively, whereas both functional groups are present in the anguibactin siderophore, the only iron uptake system found on pJM1-type plasmids. Besides siderophore-mediated iron uptake, ColV plasmids carry additional genes involved in iron metabolism. These systems include ABC transporters, hemolysins, and a hemoglobin protease. ColV- and pJM1-like plasmids have been shown to confer virulence to their bacterial host, and this trait can be completely ascribed to their encoded iron uptake systems.

Plasmids as scribbling pads for operon formation and propagation

Many bacterial genes are in operons and the process whereby operons are formed is therefore fundamental. To help elucidate this process, we propose in the Scribbling Pad hypothesis that bacteria have been constantly using plasmids for genetic experimentation and, in particular, for the construction of operons. This hypothesis simultaneously solves the problems of the creation of operons and the way operons are propagated. We cite results in the literature to support the hypothesis and make experimental predictions to test it.

Aerobactin synthesis genes iucA and iucC contribute to the pathogenicity of avian pathogenic Escherichia coli O2 strain E058

Aerobactin genes are known to be present in virulent strains and absent from avirulent strains, but contributions of iucC and iucA, which are involved in aerobactin synthesis, to the pathogenicity of avian pathogenic Escherichia coli (APEC) have not been clarified. In this study, effects of double mutants (iucA/iutA or iucC/iutA) compared to those of single mutants (iucA, iucC or iutA) of aerobactin genes on the virulence of APEC strain E058 were examined both in vitro (aerobactin production, ingestion into HD-11 cells, survival in chicken serum) and in vivo (competitive growth against parental strain, colonization and persistence). In competitive co-infection assays, compared to the E058 parental strain, the E058ΔiucA mutant was significantly reduced in the liver, kidney, spleen (all P<0.01), heart and lung (both P<0.001). The E058ΔiutA mutant also was significantly reduced in the liver, lung, kidney (all P<0.01), heart and spleen (both P<0.001). The E058ΔiucC mutant was significantly attenuated in the heart and kidney (both P<0.05) and showed a remarkable reduction in the liver, spleen and lung (P<0.01); meanwhile, both E058ΔiucAΔiutA and E058ΔiucCΔiutA double mutants were sharply reduced as well (P<0.001). In colonization and persistence assays, compared with E058, recovered colonies of E058ΔiucA were significantly reduced from the lung, liver, spleen and kidney (P<0.01) and significantly reduced in the heart (P<0.001). E058ΔiutA was significantly reduced from the heart, lung, liver, spleen and kidney (P<0.01). E058ΔiucC, E058ΔiucAΔiutA and E058ΔiucCΔiutA were significantly decreased in all organs tested (P<0.001). These results suggest that iutA, iucA and iucC play important roles in the pathogenicity of APEC E058.

(Patho-)Genomics of Escherichia coli

Escherichia coli represents a versatile and diverse enterobacterial species which can be subdivided into (i) nonpathogenic, commensal, (ii) intestinal pathogenic and (iii) extraintestinal pathogenic strains. This classification is mainly based on the presence or absence of DNA regions which are frequently associated with certain pathotypes. In most cases, this genetic information has been horizontally acquired and belongs to the flexible E. coli genome, such as plasmids, bacteriophages and genomic islands. These genomic regions contribute to the rapid evolution of E. coli variants as they are frequently subject to rearrangements, excision and transfer as well as further acquisition of additional DNA thus contributing to the creation of new (pathogenic) variants. Genetic diversity and genome plasticity of E. coli has been underestimated. The accumulating amount of sequence information generated in the era of "genomics" helps to increase our understanding of factors and mechanisms that are involved in diversification of this bacterial species as well as in those that may direct host specificity.

Virulence factors of septicemic Escherichia coli strains

Extraintestinal pathogenic Escherichia coli strains (ExPEC) are the cause of a diverse spectrum of invasive human and animal infections, often leading to septicemia. This review deals with the virulence genes of septicemic ExPEC strains. We discuss the meaning of a virulence gene and survey the genomic, genetic and physiological studies on these strains. Apparently, there are a few virulence factors, which are conserved in the septicemic strains, implying that they are essential for the infection. For the other virulence-related genes a high level of diversity is observed, demonstrating that all stages of the infection can be mediated by a number of alternative virulence factors. The variable profile of virulence genes in septicemic E. coli strains, as well as a prevalence of mobility-related sequences point out the existence of a "mix and match" combinatorial system.

The Yersinia high-pathogenicity island and iron-uptake systems in clinical isolates of Escherichia coli

The ability to acquire iron is crucial for bacteria during an infection. The capacity of 35 strains of Escherichia coli, isolated from clinical specimens, to use various strategies to obtain iron was analysed. The isolates employed several iron-uptake mechanisms, including production of enterobactin (86 %) and aerobactin (71 %). The majority of the isolates also excreted yersiniabactin, which is encoded by the Yersinia high-pathogenicity island (HPI). However, PCR analysis of the Yersinia HPI revealed diversity in its genetic organization. Use of human transferrin (91 %), lactoferrin (94 %), haemoglobin (80 %) and haemoglobin-haptoglobin complex (63 %) as the sole source of iron was common among E. coli isolates. Multiple iron-uptake systems may be of benefit to bacteria during an infection.

The Escherichia fergusonii iucABCD iutA genes are located within a larger chromosomal region similar to pathogenicity Islands

Three strains of Escherichia fergusonii (EF873, EF1496, EF939) of 50 strains tested produced the hydroxamate siderophore aerobactin. Screening of a cosmid library of the strain EF873 chromosomal DNA (in aerobactin nonproducing Escherichia coli VCS257) for aerobactin production identified iucABCD and iutA gene orthologues. The predicted IucABCD and IutA proteins showed 59-65% identity to the corresponding proteins of Shigella flexneri and E. coli. Aerobactin molecules synthesized by E. fergusonii and E. coli strains stimulated growth of aerobactin indicator strains harboring either E. coli or E. fergusonii iutA genes. In the 12 kb upstream and 17 kb downstream regions of the iuc and iut genes, 20 additional ORFs were identified. Their gene products showed homology to proteins from E. coli, S. flexneri, Klebsiella aerogenes, Pseudomonas aeruginosa and Vibrio cholerae. Probes recognizing DNA sequences from a region of more than 25 kb, which included the iucABCD and iutA genes, hybridized with chromosomal DNA of two aerobactin-producing strains (EF873 and EF939), but not with other nonproducing E. fergusonii strains tested. These data, together with the genetic organization of this region, suggest that E. fergusonii iucABCD iutA genes are a portion of a larger segment of DNA similar to pathogenicity islands of other bacteria.

Occurrence and characteristics of CS31A antigen-producing Escherichia coli in calves with diarrhoea and septicaemia in Argentina

CS31A is a K88-related non-fimbrial adhesin first described on Escherichia coli strains isolated from diarrhoeic and septicaemic calves. In this report, CS31A antigen was screened by immunological methods and confirmed by PCR among bovine E. coli isolates. In addition, CS31A-producing strains were characterized with respect to different fimbrial antigens, O-serogroup and other properties related to virulence. Faecal or tissue specimens of 100 diarrhoeic or septicaemic calves and 27 older cattle with different pathologies from 71 outbreaks or individual cases that occurred in Buenos Aires province, Argentina, were examined. CS31A + E. coli strains were isolated from 21 (21.0%) calves from 16 outbreaks or individual cases. No CS31A + E. coli was detected in samples from cattle more than 1 year old. Fimbriae F5, F41, F17a and F17b were not detected among the CS31A-producing strains. Three (14.3%) of the CS31A+ E. coli strains expressed the F17c fimbria. All of the 21 isolates exhibited at least one property of septicaemic strains (resistance to serum, production of aerobactin or colicins) but none of them demonstrated heat-stable enterotoxigenic activity. CS31A + E. coli isolates belonged to 10 serogroups, more commonly O8, O7, O17 and O21. The results obtained here confirm the worldwide distribution of CS31A antigen in bovine E. coli strains. However, CS31A + or CS31A + /F17c + E. coli were less frequently isolated than they were in North hemisphere countries.

Surface expression of O-specific lipopolysaccharide in Escherichia coli requires the function of the TolA protein

We investigated the involvement of Tol proteins in the surface expression of lipopolysaccharide (LPS). tolQ, -R, -A and -B mutants of Escherichia coli K-12, which do not form a complete LPS-containing O antigen, were transformed with the O7+ cosmid pJHCV32. The tolA and tolQ mutants showed reduced O7 LPS expression compared with the respective isogenic parent strains. No changes in O7 LPS expression were found in the other tol mutants. The O7-deficient phenotype in the tolQ and tolA mutants was complemented with a plasmid encoding the tolQRA operon, but not with a similar plasmid containing a frameshift mutation inactivating tolA. Therefore, the reduction in O7 LPS was attributed to the lack of a functional tolA gene, caused either by a direct mutation of this gene or by a polar effect on tolA gene expression exerted by the tolQ mutation. Reduced surface expression of O7 LPS was not caused by changes in lipid A-core structure or downregulation of the O7 LPS promoter. However, an abnormal accumulation of radiolabelled mannose was detected in the plasma membrane. As mannose is a sugar unique to the O7 subunit, this result suggested the presence of accumulated O7 LPS biosynthesis intermediates. Attempts to construct a tolA mutant in the E. coli O7 wild-type strain VW187 were unsuccessful, suggesting that this mutation is lethal. In contrast, a polar tolQ mutation affecting tolA expression in VW187 caused slow growth rate and serum sensitivity in addition to reduced O7 LPS production. VW187 tolQ cells showed an elongated morphology and became permeable to the membrane-impermeable dye propidium iodide. All these phenotypes were corrected upon complementation with cloned tol genes but were not restored by complementation with the tolQRA operon containing the frameshift mutation in tolA. Our results demonstrate that the TolA protein plays a critical role in the surface expression of O antigen subunits by an as yet uncharacterized involvement in the processing of O antigen.

Identification and characterization of a membrane permease involved in iron-hydroxamate transport in Staphylococcus aureus

Staphylococcus aureus was shown to transport iron complexed to a variety of hydroxamate type siderophores, including ferrichrome, aerobactin, and desferrioxamine. An S. aureus mutant defective in the ability to transport ferric hydroxamate complexes was isolated from a Tn917-LTV1 transposon insertion library after selection on iron-limited media containing aerobactin and streptonigrin. Chromosomal DNA flanking the Tn917-LTV1 insertion was identified by sequencing of chromosomal DNA isolated from the mutant. This information localized the transposon insertion to a gene whose predicted product shares significant similarity with FhuG of Bacillus subtilis. DNA sequence information was then used to clone a larger fragment of DNA surrounding the fhuG gene, and this resulted in the identification of an operon of three genes, fhuCBG, all of which show significant similarities to ferric hydroxamate uptake (fhu) genes in B. subtilis. FhuB and FhuG are highly hydrophobic, suggesting that they are embedded within the cytoplasmic membrane, while FhuC shares significant homology with ATP-binding proteins. Given this, the S. aureus FhuCBG proteins were predicted to be part of a binding protein-dependent transport system for ferric hydroxamates. Exogenous iron levels were shown to regulate ferric hydroxamate uptake in S. aureus. This regulation is attributable to Fur in S. aureus because a strain containing an insertionally inactivated fur gene showed maximal levels of ferric hydroxamate uptake even when the cells were grown under iron-replete conditions. By using the Fur titration assay, it was shown that the Fur box sequences upstream of fhuCBG are recognized by the Escherichia coli Fur protein.

The N-terminal region of the Escherichia coli WecA (Rfe) protein, containing three predicted transmembrane helices, is required for function but not for membrane insertion

The correct site for translation initiation for Escherichia coli WecA (Rfe), presumably involved in catalyzing the transfer of N-acetylglucosamine 1-phosphate to undecaprenylphosphate, was determined by using its FLAG-tagged derivatives. The N-terminal region containing three predicted transmembrane helices was found to be necessary for function but not for membrane localization of this protein.

The rfaE gene from Escherichia coli encodes a bifunctional protein involved in biosynthesis of the lipopolysaccharide core precursor ADP-L-glycero-D-manno-heptose

The intermediate steps in the biosynthesis of the ADP-L-glycero-D-manno-heptose precursor of inner core lipopolysaccharide (LPS) are not yet elucidated. We isolated a mini-Tn10 insertion that confers a heptoseless LPS phenotype in the chromosome of Escherichia coli K-12. The mutation was in a gene homologous to the previously reported rfaE gene from Haemophilus influenzae. The E. coli rfaE gene was cloned into an expression vector, and an in vitro transcription-translation experiment revealed a polypeptide of approximately 55 kDa in mass. Comparisons of the predicted amino acid sequence with other proteins in the database showed the presence of two clearly separate domains. Domain I (amino acids 1 to 318) shared structural features with members of the ribokinase family, while Domain II (amino acids 344 to 477) had conserved features of the cytidylyltransferase superfamily that includes the aut gene product of Ralstonia eutrophus. Each domain was expressed individually, demonstrating that only Domain I could complement the rfaE::Tn10 mutation in E. coli, as well as the rfaE543 mutation of Salmonella enterica SL1102. DNA sequencing of the rfaE543 gene revealed that Domain I had one amino acid substitution and a 12-bp in-frame deletion resulting in the loss of four amino acids, while Domain II remained intact. We also demonstrated that the aut::Tn5 mutation in R. eutrophus is associated with heptoseless LPS, and this phenotype was restored following the introduction of a plasmid expressing the E. coli Domain II. Thus, both domains of rfaE are functionally different and genetically separable confirming that the encoded protein is bifunctional. We propose that Domain I is involved in the synthesis of D-glycero-D-manno-heptose 1-phosphate, whereas Domain II catalyzes the ADP transfer to form ADP-D-glycero-D-manno-heptose.

Genetic organization of the O7-specific lipopolysaccharide biosynthesis cluster of Escherichia coli VW187 (O7:K1)

In previous studies the authors cloned and characterized the DNA sequence of the regions at both ends of the O7-specific lipopolysaccharide (LPS) biosynthesis cluster of Escherichia coli VW187 (O7:K1), and identified the biosynthetic genes for dTDP-rhamnose and GDP-mannose, as well as one of the candidate glycosyltransferases. In this work the complete DNA sequence of a 6.9 kb intervening region is presented. Seven new ORFs were identified. All the functions required for the synthesis and transfer of the O7 LPS were assigned on the basis of complementation experiments of transposon insertion mutants, and amino acid sequence homology to proteins involved in LPS synthesis of other bacteria. Of the seven ORFs, two encoded membrane proteins that were homologous to the O-antigen translocase (Wzx) and polymerase (Wxy), two were involved in the biosynthesis of dTDP-N-acetylviosamine, and the remaining three showed homologies to sugar transferases. The O antigen chain length regulator gene wzz was also identified in the vicinity of the O7 polysaccharide cluster. O7-specific DNA primers were designed and tested for serotyping of O7 E. coli strains.

An iron- and fur-repressed Legionella pneumophila gene that promotes intracellular infection and encodes a protein with similarity to the Escherichia coli aerobactin synthetases

Legionella pneumophila, a parasite of alveolar macrophages, requires iron for intra- and extracellular growth. Although its mechanisms for iron assimilation are poorly understood, this bacterium produces Fur, a protein that can repress gene transcription in response to iron concentration. Because iron- and Fur-regulated genes are important for infection in other bacteria, the identification of similar genes in L. pneumophila was undertaken. A wild-type strain of L. pneumophila was randomly mutated with a mini-Tn10' lacZ transposon, and the resulting gene fusions were tested for iron regulation by assessing beta-galactosidase production in the presence and absence of iron chelators. Of the initial six mutants with iron-repressed lacZ fusions, two strains, NU229 and NU232, possessed fusions that were stably iron regulated. To assay for Fur regulation, the levels of beta-galactosidase were measured in strains no longer producing Fur. As in a number of pathogenic bacteria, L. pneumophila fur could not be insertionally inactivated, but spontaneous Fur- derivatives were generated by selecting for manganese resistance. Strain NU229 contained a Fur-repressed fusion based on derepression of lacZ expression in its manganese-resistant derivative. Extracellular growth of NU229 in bacteriological media was similar to that of wild-type strain 130b. To assess the role of an iron- and Fur-regulated (frgA) gene in intracellular infection, the ability of NU229 to grow within U937 cell monolayers was tested. Quantitative infection assays demonstrated that intracellular growth of NU229 was impaired as much as 80-fold. Reconstruction of the mutant by allelic exchange proved that the infectivity defect in NU229 was due to the inactivation of frgA and not to a second-site mutation. Subsequently, complementation of the interrupted gene by an intact plasmid-encoded gene demonstrated that the infectivity defect was due to the loss of frgA and not to a polar effect. Nucleotide sequence analysis revealed that the 63-kDa FrgA protein has homology with the aerobactin synthetases IucA and IucC of Escherichia coli, raising the possibility that L. pneumophila encodes a siderophore which is required for optimal intracellular replication. Southern hybridization analysis determined that frgA is specific to L. pneumophila.

Genetic analysis of the dTDP-rhamnose biosynthesis region of the Escherichia coli VW187 (O7:K1) rfb gene cluster: identification of functional homologs of rfbB and rfbA in the rff cluster and correct location of the rffE gene

The O-repeating unit of the Escherichia coli O7-specific lipopolysaccharide is made of galactose, mannose, rhamnose, 4-acetamido-4,6-dideoxyglucose, and N-acetyglucosamine. We have recently characterized the genes involved in the biosynthesis of the sugar precursor GDP-mannose occurring in the E. coli O7:K1 strain VW187 (C. L. Marolda and M. A. Valvano, J. Bacteriol. 175:148-158, 1993). In the present study, we identified and sequenced the rfbBDAC genes encoding the enzymes for the biosynthesis of another precursor, dTDP-rhamnose. These genes are localized on the upstream end of the rfbEcO7 region, and they are strongly conserved compared with similar genes found in various enteric and nonenteric bacteria. Upstream of rfbB we identified a DNA segment containing the rfb promoter and a highly conserved untranslated leader sequence also present in the promoter regions of other surface polysaccharide gene clusters. Also, we have determined that rfbB and rfbA have homologs, rffG (o355) and rffH (o292), respectively, located on the rff cluster, which is involved in the synthesis of enterobacterial common antigen. We provide biochemical evidence that rffG and rffH encode dTDP-glucose dehydratase and glucose-1-phosphate thymidylyltransferase activities, respectively, and we also show that rffG complemented the rfbB defect in the O7+ cosmid pJHCV32. We also demonstrate that rffG is distinct from rffE and map the rffE gene to the second gene of the rff cluster.

Role of the rfe gene in the biosynthesis of the Escherichia coli O7-specific lipopolysaccharide and other O-specific polysaccharides containing N-acetylglucosamine

We report that rfe mutants of wild-type strains of Escherichia coli O7, O18, O75, and O111 did not express O-specific polysaccharide unless the rfe mutation was complemented by a cloned rfe gene supplied in a plasmid. The O polysaccharides in these strains are known to have N-acetylglucosamine (GlcNAc) in their O repeats. In addition, in vitro transferase assays with bacterial membranes from either the O7 wild-type strain or its isogenic rfe mutant showed that GlcNAc is the first carbohydrate added onto the lipid acceptor in the assembly of the O7 repeat and that this function is inhibited by tunicamycin. Our results indicate that the rfe gene product is a general requirement for the synthesis of O polysaccharides containing GlcNAc.

Genetic analysis of the O-specific lipopolysaccharide biosynthesis region (rfb) of Escherichia coli K-12 W3110: identification of genes that confer group 6 specificity to Shigella flexneri serotypes Y and 4a

We recently reported a novel genetic locus located in the sbcB-his region of the chromosomal map of Escherichia coli K-12 which directs the expression of group 6-positive phenotype in Shigella flexneri lipopolysaccharide, presumably due to the transfer of O-acetyl groups onto rhamnose residues of the S. flexneri O-specific polysaccharide (Z. Yao, H. Liu, and M. A. Valvano, J. Bacteriol. 174:7500-7508, 1992). In this study, we identified the genetic region encoding group 6 specificity as part of the rfb gene cluster of E. coli K-12 strain W3110 and established the DNA sequence of most of this cluster. The rfbBDACX block of genes, located in the upstream region of the rfb cluster, was found to be strongly conserved in comparison with the corresponding region in Shigella dysenteriae type 1 and Salmonella enterica. Six other genes, four of which were shown to be essential for the expression of group 6 reactivity in S. flexneri serotypes Y and 4a, were identified downstream of rfbX. One of the remaining two genes showed similarities with rfc (O-antigen polymerase) of S. enterica serovar typhimurium, whereas the other, located in the downstream end of the cluster next to gnd (gluconate-6-phosphate dehydrogenase), had an IS5 insertion. Recently, it has been reported that the IS5 insertion mutation (rfb-50) can be complemented, resulting in the formation of O16-specific polysaccharide by E. coli K-12 (D. Liu and P. R. Reeves, Microbiology 140:49-57, 1994). We present immunochemical evidence suggesting that S. flexneri rfb genes also complement the rfb-50 mutation; in the presence of rfb genes of E. coli K-12, S. flexneri isolates express O16-specific polysaccharide which is also acetylated in its rhamnose residues, thereby eliciting group 6 specificity.

Immunological variants of the aerobactin-cloacin DF13 outer membrane protein receptor IutA among enteric bacteria

Mouse monoclonal antibodies (MAbs) were generated against a 76-kDa IutA receptor of pathogenic avian Escherichia coli 15972. Six of the eight IutA-specific MAbs isolated (AB1 to AB6) were shown to be directed toward membrane-exposed conformational epitopes, although they did not interfere with the uptake of ferric aerobactin and cloacin DF13 as assessed by competition experiments with purified ligands. The two remaining IutA MAbs (AB9 and AB10) recognized linear epitopes buried in the IutA molecule. The panel of IutA MAbs was used to characterize IutA variants occurring in strains of E. coli, Klebsiella pneumoniae, Enterobacter spp., and Shigella spp., resulting in the identification of four immunological groups of IutAs. MAb AB9 defined an epitope conserved in all IutA variants. In addition, the panel of IutA MAbs served to identify the presence of IutA in wild-type bacteria grown in the presence of diphenylamine to reduce the expression of O-specific polysaccharide.

Cloning and Characterization of Aerobactin Biosynthesis Genes of the Biological Control Agent Enterobacter cloacae

Five strains of Enterobacter cloacae that are biological control agents of Pythium damping-off diseases produced the hydroxamate siderophore aerobactin under iron-limiting conditions. Genes determining aerobactin biosynthesis of the biocontrol strain E. cloacae EcCT-501 were localized to a 12.3-kb region, which conferred aerobactin production to Escherichia coli DH5α. The aerobactin biosynthesis genes of E. cloacae hybridized to those of the pColV-K30 plasmid of E. coli , but restriction patterns of the aerobactin regions of pColV-K30 and E. cloacae differed. A derivative strain with a deletion in the aerobactin biosynthesis locus was as effective as strain EcCT-501 in biological control of Pythium damping-off of cucumber. Thus, aerobactin production did not contribute significantly to the biological control activity of EcCT-501 under the conditions of this study.

Role of iron in regulation of virulence genes

The abilities of bacterial pathogens to adapt to the environment within the host are essential to their virulence. Microorganisms have adapted to the iron limitation present in mammalian hosts by evolving diverse mechanisms for the assimilation of iron sufficient for growth. In addition, many bacterial pathogens have used the low concentration of iron present in the host as an important signal to enhance the expression of a wide variety of bacterial toxins and other virulence determinants. The molecular basis of coordinate regulation by iron has been most thoroughly studied in Escherichia coli. In this organism, coordinate regulation of gene expression by iron depends on the regulatory gene, fur. Regulation of gene expression by iron in a number of pathogenic organisms is coordinated by proteins homologous to the Fur protein of E. coli. Additional regulatory proteins may be superimposed on the Fur repressor to provide the fine-tuning necessary for the precise regulation of individual virulence genes in response to iron and other environmental signals. Studies of the mechanisms of regulation of iron acquisition systems and virulence determinants by iron should lead to a better understanding of the adaptive response of bacteria to the low-iron environment of the host and its importance in virulence.

Construction and biochemical characterization of recombinant cytoplasmic forms of the IucD protein (lysine:N6-hydroxylase) encoded by the pColV-K30 aerobactin gene cluster

The aerobactin gene cluster in pColV-K30 consists of five genes (iucABCD iutA); four of these (iucABCD) are involved in aerobactin biosynthesis, whereas the fifth one (iutA) encodes the ferriaerobactin outer membrane receptor. iucD encodes lysine:N6-hydroxylase, which catalyzes the first step in aerobactin biosynthesis. Regardless of the method used for cell rupture, we have consistently found that IucD remains membrane bound, and repeated efforts to achieve a purified and active soluble form of the enzyme have been unsuccessful. To circumvent this problem, we have constructed recombinant IucD proteins with modified amino termini by creating three in-frame gene fusions of IucD to the amino-terminal amino acids of the cytoplasmic enzyme beta-galactosidase. Two of these constructs resulted in the addition to the iucD coding region of a hydrophilic leader sequence of 13 and 30 amino acids. The other construct involved the deletion of the first 47 amino acids of the IucD amino terminus and the addition of 19 amino acids of the amino terminus of beta-galactosidase. Cells expressing any of the three recombinant IucD forms were found to produce soluble N6-hydroxylysine. One of these proteins, IucD439, was purified to homogeneity from the soluble fraction of the cell lysates, and it was capable of participating in the biosynthesis of aerobactin, as determined in vitro by a cell-free system and in vivo by a cross-feeding bioassay. A medium ionic strength of 0.25 (250 mM NaCl) or higher was required to maintain the protein in a catalytically functional, tetrameric state.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification, expression, and DNA sequence of the GDP-mannose biosynthesis genes encoded by the O7 rfb gene cluster of strain VW187 (Escherichia coli O7:K1)

The O7-specific lipopolysaccharide (LPS) in strains of Escherichia coli consists of a repeating unit made of galactose, mannose, rhamnose, 4-acetamido-2,6-dideoxyglucose, and N-acetylglucosamine. We have recently cloned and characterized genetically the O7-specific LPS biosynthesis region (rfbEcO7) of the E. coli O7:K1 strain VW187 (C. L. Marolda, J. Welsh, L. Dafoe, and M. A. Valvano, J. Bacteriol. 172:3590-3599, 1990). In this study, we localized the gnd gene encoding gluconate-6-phosphate dehydrogenase at one end of the rfbEcO7 gene cluster and sequenced that end of the cluster. Three open reading frames (ORF) encoding polypeptides of 275, 464, and 453 amino acids were identified upstream of gndEcO7, all transcribed toward the gnd gene. ORF275 had 45% similarity at the protein level with ORF16.5, which occupies a similar position in the Salmonella enterica LT2 rfb region, and presumably encodes a nucleotide sugar transferase. The polypeptides encoded by ORFs 464 and 453 were expressed under the control of the ptac promoter and visualized in Coomassie blue-stained sodium dodecyl sulfate-polyacrylamide gels and by maxicell analysis. ORF464 expressed GDP-mannose pyrophosphorylase and ORF453 encoded a phosphomannomutase, the enzymes for the biosynthesis pathway of GDP-mannose, one of the nucleotide sugar precursors for the formation of the O7 repeating unit. They were designated rfbMEcO7 and rfbKEcO7, respectively. The RfbMEcO7 polypeptide was homologous to the corresponding protein in S. enterica LT2, XanB of Xanthomonas campestris, and AlgA of Pseudomonas aeruginosa, all GDP-mannose pyrophosphorylases. RfbKEcO7 was very similar to CpsG of S. enterica LT2, an enzyme presumably involved in the biosynthesis of the capsular polysaccharide colanic acid, but quite different from the corresponding RfbK protein of S. enterica LT2.

Role of tol genes in cloacin DF13 susceptibility of Escherichia coli K-12 strains expressing the cloacin DF13-aerobactin receptor IutA

IutA is the outer membrane protein receptor for ferric aerobactin and the bacteriocin cloacin DF13. Although the same receptor is shared, ferric aerobactin transport across the outer membrane in Escherichia coli is TonB dependent, whereas cloacin DF13 transport is not. We have recently observed that tolQ is required for cloacin DF13 susceptibility (J.A. Thomas and M.A. Valvano, FEMS Microbiol. Lett. 91:107-112, 1992). In this study, we demonstrate that the genes tolQ, tolR, and tolA, but not tolB, tolC, and ompF, are required for the internalization of cloacin DF13 and they are not involved in the transport of ferric aerobactin.

Acetylation of O-specific lipopolysaccharides from Shigella flexneri 3a and 2a occurs in Escherichia coli K-12 carrying cloned S. flexneri 3a and 2a rfb genes

Most of the Shigella flexneri O-specific serotypes result from O-acetyl and/or glucosyl groups added to a common O-repeating unit of the lipopolysaccharide (LPS) molecule. The genes involved in acetylation and/or glucosylation of S. flexneri LPS are physically located on lysogenic bacteriophages, whereas the rfb cluster contains the biosynthesis genes for the common O-repeating unit (D.A.R. Simmons and E. Romanowska, J. Med. Microbiol. 23:289-302, 1987). Using a cosmid cloning strategy, we have cloned the rfb regions from S. flexneri 3a and 2a. Escherichia coli K-12 containing plasmids pYS1-5 (derived from S. flexneri 3a) and pEY5 (derived from S. flexneri 2a) expressed O-specific LPS which reacted immunologically with S. flexneri polyvalent O antiserum. However, O-specific LPS expressed in E. coli K-12 also reacted with group 6 antiserum, indicating the presence of O-acetyl groups attached to one of the rhamnose components of the O-repeating unit. This was confirmed by measuring the amounts of acetate released from purified LPS samples and also by the chemical removal of O-acetyl groups, which abolished group 6 reactivity. The O-acetylation phenotype was absent in an E. coli strain with an sbcB-his-rfb chromosomal deletion and could be restored upon conjugation of F' 129, which carries sequences corresponding to a portion of the deleted region. Our data demonstrate that E. coli K-12 strains possess a novel locus which directs the O acetylation of LPS and is located in the sbcB-rfb region of the chromosomal map.

Identification of aerobactin genes in clinical isolates of E. coli using a non-radioactive DNA probe

A digoxigenin-labelled gene probe was used for the identification of aerobactin genes in 21 E. coli strains from clinical isolates by means of a colony hybridization test. The results were compared with those obtained by previous hybridization experiments with radiolabelled DNA probes as well as with a crossfeeding-assay. It could be demonstrated that only after an additional proteinase K treatment of the filters before hybridization and after a repeated blocking during the immunological detection procedure, specific hybridization signals could be observed.

High-affinity iron uptake systems present in Erwinia carotovora subsp. carotovora include the hydroxamate siderophore aerobactin

The phytopathogenic bacterium Erwinia carotovora subsp. carotovora W3C105 produced the hydroxamate siderophore aerobactin under iron-limiting conditions. A survey of 22 diverse strains of E. carotovora revealed that strain W3C105 alone produced aerobactin. The ferric-aerobactin receptor of strain W3C105 was an 80-kDa protein, identified by immunoblots of Sarkosyl-soluble proteins obtained from E. carotovora cells grown in iron-depleted medium and probed with antiserum raised against the 74-kDa ferric-aerobactin receptor encoded by the pColV-K30 plasmid of Escherichia coli. Genes determining aerobactin biosynthesis and uptake were localized to an 11.3-kb EcoRI-HindIII chromosomal fragment of strain W3C105. A 10-kb subclone of the fragment conferred on E. coli DH5 alpha both aerobactin biosynthesis and uptake, determined by cloacin DF13 sensitivity, the presence of the 80-kDa receptor protein, and iron-independent growth of E. coli clones. The aerobactin biosynthesis genes of E. carotovora W3C105 hybridized to those of the pColV-K30 plasmid of E. coli, but the restriction patterns of the aerobactin regions of E. coli and E. carotovora differed. Although the aerobactin region of enteric bacteria is commonly flanked by IS1-like sequences, IS1 sequences were not detected in the genomic DNA or the cloned aerobactin region of E. carotovora. E. coli DH5 alpha cells harboring cloned aerobactin biosynthesis genes from E. carotovora W3C105 produced greater quantities of aerobactin and the 80-kDa ferric-aerobactin receptor when grown in iron-limited than in iron-replete medium. Strain W3C105 grew on an iron-limited medium, whereas derivatives that lacked a functional aerobactin iron acquisition system did not grow on the medium. These results provide evidence for the occurrence and heterogeneity of aerobactin as a high-affinity iron uptake system of both clinical and phytopathogenic species of the Enterobacteriaceae. Although future studies may reveal a role for aerobactin in the virulence or ecology of strain W3C105, a functional aerobactin iron acquisition system is not necessary for the pathogenicity of E. carotovora.

The rcsA gene of Escherichia coli O9:K30:H12 is involved in the expression of the serotype-specific group I K (capsular) antigen

Escherichia coli produces two distinct types of capsular polysaccharide (designated groups I and II), which are distinguished by chemical, physical, and genetic characteristics. The K30 capsular antigen is a member of the group I, or heat-stable, capsules. We have cloned rcsA from E. coli O9:K30 and determined the nucleotide sequence. The rcsAK30 sequence is virtually identical to the rcsAK-12 sequence (V. Stout, A. Torres-Cabassa, M. R. Maurizi, D. Gutnick, and S. Gottesman, J. Bacteriol. 173:1738-1747, 1991). RcsAK-12 is a transcriptional activator involved in expression of the extracellular polysaccharide colanic acid in E. coli K-12. rcsAK30 complemented an rcsAK-12 mutation and activated colanic acid synthesis in E. coli K-12 strains. However, in E. coli K30, increasing the levels of RcsA by introducing multicopy rcsAK30 or a Lon mutation resulted in elevated synthesis of the K30 capsular polysaccharide; no colanic acid was detected. E. coli K-12 strains in which the chromosomal his region was replaced by that from E. coli K30 were able to synthesize K30 capsular polysaccharide. These K-12/K30 hybrid strains did not produce colanic acid, suggesting that the genes for synthesis of colanic acid and the K30 capsular polysaccharide may be allelic. rcsA sequences were also detected in the group II strains E. coli K1 and K5. Introduction of rcsAK30 into group II strains resulted in activation of colanic acid biosynthesis rather than the group II capsule. Given the role of RcsA in other members of the family Enterobacteriaceae, our results provide further evidence that this protein may be a relatively widespread regulatory component for the synthesis of enterobacterial extracellular polysaccharides.

Relatedness of O-specific lipopolysaccharide side chain genes from strains of Shigella boydii type 12 belonging to two clonal groups and from Escherichia coli O7:K1

The O-specific lipopolysaccharide side chains of Escherichia coli O7 and Shigella boydii type 12 possess similar but not identical chemical structures. We investigated the genetic relatedness between the O-specific side chain genes in members of these two species. Examination of outer membrane protein and lipopolysaccharide (LPS) banding patterns demonstrated that five strains which had been identified as S. boydii type 12 fell into two clonal groups, SB1 and SB2. Hybridizations with O7-specific radiolabeled probes derived from the chromosomal DNA of an E. coli O7 strain detected identical fragments among the three SB1 strains of S. boydii type 12 and the two E. coli O7 reference isolates. The two other S. boydii type 12 strains, which belonged to the SB2 clone, did not show homologies with the O7 probe under high-stringency conditions of hybridization. The homology between the O7 and type 12 LPS gene regions from the SB1 strains was further confirmed by the construction of O-specific side chain-deficient mutations in these strains by homologous recombination of a suicide plasmid containing O7-specific DNA sequences. Immunoblot experiments with O7 antiserum gave a weak cross-reaction with LPS purified from the SB2 strains but a very strong cross-reaction with the LPS from SB1 isolates. Antiserum raised to one of the SB2 strains cross-reacted only with S. boydii type 12 LPS from the SB1 clone but failed to react with O7 LPS.

Virulence factors associated with cytotoxic necrotizing factor type two in bovine diarrheic and septicemic strains of Escherichia coli

Forty-three bovine isolates of Escherichia coli producing a second type of cytotoxic necrotizing factor (CNF2) and three K-12 strains carrying different Vir plasmids coding for CNF2 were tested for the presence of several virulence factors. Most of the strains were serum resistant (79%), produced an aerobactin (70%), and adhered to calf villi (53%); some of them produced a colicin (32%) and a hemolysin (9%). These strains were also tested by a colony hybridization assay with gene probes for six toxins (classical heat-stable [STaP and STb] and heat-labile [LT-I and LT-IIa] enterotoxins and Shiga-like toxins [SLT-I and SLT-II]) and five adhesion factors (K99, K88, 987P, F17, and F41). Only two gene probes, LT-IIa (9%) and F17A (53%), hybridized with the CNF2 strains. However, antibodies raised against F17 fimbriae did not agglutinate the strains hybridizing with the F17A probe. In contrast, all except one of these strains adhered to calf villi. Interestingly, these two properties, F17A positivity and adherence to calf villi, were the only ones expressed by the K-12 strains carrying different Vir plasmids. In conclusion, this study confirmed that CNF2-producing strains are unrelated to previously described toxigenic E. coli strains and also demonstrated that in half of the strains the production of CNF2 was associated with an adhesion factor genetically related to, but different from, F17, which is more than likely encoded by Vir plasmids.

Colicin V virulence plasmids

ColV plasmids are a heterogeneous group of IncFI plasmids which encode virulence-related properties such as the aerobactin iron uptake system, increased serum survival, and resistance to phagocytosis. These plasmids have been found in invasive strains of Escherichia coli which infect vertebrate hosts including humans and livestock. Colicin V was the first colicin to be identified, in 1925, but not until the field experienced a renewed interest has the mechanism of colicin V activity been explored. As encoded by ColV plasmid pColV-K30, the aerobactin iron uptake system has been extensively investigated, but other ColV-encoded phenotypes remain largely uncharacterized. Restriction enzyme mapping of the 144-kb pColV-K30 and of the 80-kb pColV-B188 has facilitated systematic study, so that questions can be addressed by a molecular and comparative approach regarding the contributions of individual factors and plasmids to the virulence of host E. coli in model systems. The family of large ColV plasmids could be analogous to other families of large virulence plasmids, and insights gained from studying these plasmids should contribute to our understanding of cross-genetic interactions and the role of large plasmids in bacterial pathogenesis.

Polymorphism in the aerobactin-cloacin DF13 receptor genes from an enteroinvasive strain of Escherichia coli and pColV-K30 is associated only with a decrease in cloacin susceptibility

We have cloned chromosomal genes mediating the aerobactin iron transport system from the enteroinvasive strain Escherichia coli 978-77. The physical map of the region spanning the siderophore biosynthesis genes and the upstream portion of the receptor gene in strain 978-77-derived clones was identical to the corresponding regions in pColV-K30, while the downstream portion was different. Recombinant plasmids derived from strain 978-77 encoded a 76-kDa outer membrane protein, in contrast to the 74-kDa polypeptide encoded by similar clones derived from pColV-K30. No differences were found in the uptake of ferric aerobactin mediated by either the 76-kDa- or the 74-kDa-encoding plasmids. In contrast, cells containing the 76-kDa-encoding plasmids showed a 16-fold decrease in susceptibility to cloacin compared with cells harboring the 74-kDa-encoding plasmids. Two classes of chimeric aerobactin receptor genes were constructed by exchanging sequences corresponding to the downstream portion from the aerobactin receptor gene of both systems. The pColV-K30-978-77 chimeric gene encoded a 76-kDa outer membrane protein which mediated a low level of cloacin susceptibility, whereas the 978-77-pColV-K30 type encoded a protein of 74 kDa determining a level of cloacin susceptibility identical to that mediated by pColV-K30.

Virulence factors in Escherichia coli urinary tract infection

Uropathogenic strains of Escherichia coli are characterized by the expression of distinctive bacterial properties, products, or structures referred to as virulence factors because they help the organism overcome host defenses and colonize or invade the urinary tract. Virulence factors of recognized importance in the pathogenesis of urinary tract infection (UTI) include adhesins (P fimbriae, certain other mannose-resistant adhesins, and type 1 fimbriae), the aerobactin system, hemolysin, K capsule, and resistance to serum killing. This review summarizes the virtual explosion of information regarding the epidemiology, biochemistry, mechanisms of action, and genetic basis of these urovirulence factors that has occurred in the past decade and identifies areas in need of further study. Virulence factor expression is more common among certain genetically related groups of E. coli which constitute virulent clones within the larger E. coli population. In general, the more virulence factors a strain expresses, the more severe an infection it is able to cause. Certain virulence factors specifically favor the development of pyelonephritis, others favor cystitis, and others favor asymptomatic bacteriuria. The currently defined virulence factors clearly contribute to the virulence of wild-type strains but are usually insufficient in themselves to transform an avirulent organism into a pathogen, demonstrating that other as-yet-undefined virulence properties await discovery. Virulence factor testing is a useful epidemiological and research tool but as yet has no defined clinical role. Immunological and biochemical anti-virulence factor interventions are effective in animal models of UTI and hold promise for the prevention of UTI in humans.

Genetic analysis of the O7-polysaccharide biosynthesis region from the Escherichia coli O7:K1 strain VW187

We recently cloned biosynthesis genes for the O7-lipopolysaccharide (O7-LPS) side chain from the Escherichia coli K-1 strain VW187 (M. A. Valvano, and J. H. Crosa, Infect. Immun. 57:937-943, 1989). To characterize the O7-LPS region, the recombinant cosmids pJHCV31 and pJHCV32 were mutagenized by transposon mutagenesis with Tn3HoHo1, which carries a promoterless lac operon and can therefore generate lacZ transcriptional fusions with target DNA sequences. Cells containing mutated plasmids were examined for their ability to react by coagglutination with O7 antiserum. The LPS pattern profiles of the insertion mutants were also investigated by electrophoresis of cell envelope fractions, followed by silver staining and immunoblotting analysis. These experiments identified three phenotypic classes of mutants and defined a region in the cloned DNA of about 14 kilobase pairs that is essential for O7-LPS expression. Analysis of beta-galactosidase production by cells carrying plasmids with transposon insertions indicated that transcription occurs in only one direction along the O7-LPS region. In vitro transcription-translation experiments revealed that the O7-LPS region encodes at least 16 polypeptides with molecular masses ranging from 20 to 48 kilodaltons. Also, the O7-LPS region in VW187 was mutagenized by homologous recombination with subsets of the cloned O7-LPS genes subcloned into a suicide plasmid vector. O7-LPS-deficient mutants of VW187 were complemented with pJHCV31 and pJHCV32, confirming that these cosmids contain genetic information that is essential for the expression of the O7 polysaccharide.

Characterization of bovine septicemic, bovine diarrheal, and human enteroinvasive Escherichia coli that hybridize with K88 and F41 accessory gene probes but do not express these adhesins

Certain DNA probes derived from accessory genes of cloned K88 and F41 determinants hybridize with Escherichia coli strains that express K88 or F41 and with certain other E. coli strains that do not express these antigens. We found that these probes hybridized with human enteroinvasive E. coli, and with bovine E. coli isolates which produced a fatal septicemia in experimentally infected piglets. These strains did not hybridize with probes derived from the structural subunit genes encoding the K88 and F41 antigens. E. coli strains isolated from turkeys with septicemia, Shigella and Salmonella strains did not hybridize to the K88 and F41 accessory gene probes. The K88 and F41 accessory genes probes hybridized with a 200 kb plasmid which is required for invasion by human enteroinvasive E. coli. The K88 and F41 accessory gene homology in the bovine isolates was located on a 150 kb transmissible plasmid but was unrelated to plasmids encoding aerobactin, Vir, or colicin V, which are suspected virulence factors in septicemic E. coli. A common plasmid-encoded antigen was associated with bovine isolates that hybridized with the K88 and F41 accessory gene probes. This included strains which express CS31A, a surface antigen associated with bovine septicemic E. coli, which also hybridized with the K88 and F4 accessory gene probes. The results suggest that the K88 and F41 accessory gene probes hybridized with sequences that may be associated with a common mechanism of pilus expression in distinct groups of E. coli pathogens.

Role of bacterial and host factors in the pathogenesis of Shigella septicemia

To determine the role of bacterial and host factors in the pathogenesis of shigellemia, blood and fecal Shigella isolates were compared for serum resistance and siderophore production, and shigellemic patients were examined for decreased serum bactericidal activity or increased serum transferrin saturation compared to control patients with non-bacteremic shigellosis. The majority of both blood (36/38) and fecal (36/48) Shigella isolates were sensitive to normal serum (greater than 2 log kill/60 min). Shigella dysenteriae type 1 strains were the most sensitive, and Shigella sonnei strains were the most resistant. Siderophore production was species- and serotype-dependent. Shigella dysenteriae type 1 produced only enterochelin; most Shigella flexneri expressed only aerobactin, and Shigella sonnei secreted both types of siderophores. No difference in siderophores was noted between blood and fecal strains within a given serotype. In contrast, shigellemic patients exhibited a 43-fold decrease in serum bactericidal activity and a two-fold increase in transferrin saturation compared to well-nourished control patients. These results indicate that the pathogenesis of Shigella septicemia is not related to serum resistance or siderophore production but may involve serum abnormalities associated with malnutrition.

Genetics and molecular biology of siderophore-mediated iron transport in bacteria

The possession of specialized iron transport systems may be crucial for bacteria to override the iron limitation imposed by the host or the environment. One of the most commonly found strategies evolved by microorganisms is the production of siderophores, low-molecular-weight iron chelators that have very high constants of association for their complexes with iron. Thus, siderophores act as extracellular solubilizing agents for iron from minerals or organic compounds, such as transferrin and lactoferrin in the host vertebrate, under conditions of iron limitation. Transport of iron into the cell cytosol is mediated by specific membrane receptor and transport systems which recognize the iron-siderophore complexes. In this review I have analyzed in detail three siderophore-mediated iron uptake systems: the plasmid-encoded anguibactin system of Vibrio anguillarum, the aerobactin-mediated iron assimilation system present in the pColV-K30 plasmid and in the chromosomes of many enteric bacteria, and the chromosomally encoded enterobactin iron uptake system, found in Escherichia coli, Shigella spp., Salmonella spp., and other members of the family Enterobacteriaceae. The siderophore systems encoded by Pseudomonas aeruginosa, namely, pyochelin and pyoverdin, as well as the siderophore amonabactin, specified by Aeromonas hydrophila, are also discussed. The potential role of siderophore-mediated systems as virulence determinants in the specific host-bacteria interaction leading to disease is also analyzed with respect to the influence of these systems in the expression of other factors, such as toxins, in the bacterial virulence repertoire.

CoIV plasmids pCoIV-B188 and pCoIV-K30: genetic maps according to restriction enzyme sites and landmark phenotypic characteristics

The CoIV plasmids are large virulence plasmids of the incompatibility group IncFI. We have obtained the genetic maps of two of the most studied CoIV plasmids, pCoIV-B188 and pCoIV-K30, according to restriction enzyme sites and landmark phenotypic characteristics such as colicin V, the aerobactin iron uptake system, the transfer region, replication regions, and repeated sequences. Although the two plasmids differ in size (pCoIV-B188 is 80 kb and pCoIV-K30 is 144 kb), the maps reveal many regions which are apparently identical or very similar.

Immune response to the iron-deprivation-induced proteins of Salmonella typhi in typhoid fever

Iron starvation conditions limited the growth of Salmonella typhi, as evidenced by an increase in the lag phase of a culture and a decrease in the number of bacteria reached in the stationary phase. The analysis of the outer membrane of bacteria grown under these conditions identified new protein components with apparent molecular weights of 83,000, 78,000, and 69,000. The extent of induction of these proteins was regulated by increased iron deprivation. Immunoblot analysis showed that the serum of patients with typhoid fever exhibited an immunoglobulin G response to these iron-deprivation-induced proteins. The results of bioassays and DNA-DNA hybridization experiments indicated that pathogenic strains of S. typhi produced enterochelin but not aerobactin. Immunodetection with an anti-FepA antiserum confirmed that one of the induced proteins is the S. typhi analog of the Escherichia coli fepA gene product. These studies suggest a role for iron uptake in the pathogenesis of typhoid fever and confirm the immunogenicity of some of the outer membrane proteins of this pathogen.

Molecular cloning and expression in Escherichia coli K-12 of chromosomal genes determining the O7 lipopolysaccharide antigen of a human invasive strain of E. coli O7:K1

We have cloned and studied the expression in Escherichia coli K-12 of chromosomal rfb genes determining the biosynthesis of the O7 lipopolysaccharide (LPS) antigen from E. coli K1 strain VW187. Two E. coli K-12 strains carrying recombinant cosmids gave positive coagglutination reactions with protein A-rich staphylococcal particles bearing an O7-specific rabbit polyclonal antiserum. Silver-stained polyacrylamide gels of total membranes extracted with hot phenol showed O side chain material which had O7 specificity as determined by immunoblotting experiments. However, the amount of O7 LPS expressed in E. coli K-12 was considerably lower than that produced by the wild-type strain VW187. Deletion and transposition experiments identified a region of about 17 kilobase pairs which is essential for the expression of O7 LPS. The existence of homologies between the O7 LPS genes and other E. coli O side chain genes was investigated by Southern blot hybridization experiments. An O7-specific probe fragment of 15 kilobase pairs did not hybridize to genomic DNA digests of E. coli strains belonging to several different O types, demonstrating that the O7 LPS genes are unique.

Molecular cloning, expression, and regulation in Escherichia coli K-12 of a chromosome-mediated aerobactin iron transport system from a human invasive isolate of E. coli K1

We have cloned chromosomal genes determining the aerobactin iron transport system from the Escherichia coli K1 strain VW187. Mapping and hybridization experiments showed that the VW187 aerobactin region was identical to that of the plasmid ColV-K30. However, in the E. coli K-12 background, the biosynthesis of both siderophore and ferric aerobactin receptor encoded by the VW187-derived recombinant plasmids was not repressed by iron to the same extent found when a recombinant plasmid derived from pColV-K30 was used. RNA-DNA dot-blot hybridization experiments demonstrated that the aerobactin-specific mRNA synthesized by the VW187-derived clones was not iron regulated in E. coli K-12. In contrast, the synthesis of aerobactin and its receptor in strain VW187 was completely repressed by iron regardless of whether the recombinant plasmids originated from VW187 or pColV-K30. Similar results were obtained with gene fusions in which a promoterless lac operon was placed under the control of aerobactin promoter regions of either chromosome- or plasmid-mediated aerobactin systems. DNA sequencing of the chromosomal aerobactin promoter region showed changes in bases located immediately upstream to the -35 region compared with the corresponding region in pColV-K30, which is known to be part of the binding site for the Fur repressor protein.

New aerobactin-mediated iron uptake system in a septicemia-causing strain of Enterobacter cloacae

Unlike the great majority of the aerobactin-producing enteric bacteria documented in the literature, Enterobacter cloacae EK33, isolated from a case of human neonatal meningitis, did not show any homology at the DNA level with the prototype aerobactin system encoded by the ColV-K30 plasmid. However, both the nuclear magnetic resonance spectrum and fast-atom bombardment mass spectrometry of the siderophore purified from EK33 confirmed its identity with aerobactin. Bioassay screening of a gene library of total DNA of EK33 led to the isolation of several aerobactin-positive clones. Under conditions of iron limitation, these clones expressed in Escherichia coli a protein of 72 kilodaltons that reacted with antiserum raised against the pColV-K30 74-kilodalton aerobactin receptor, while the original E. cloacae strain synthesized an 85-kilodalton protein which also cross-reacted with the antiserum. Restriction endonuclease analysis of the cloned DNA confirmed the structural differences between the two aerobactin genetic systems.

Divergence of the aerobactin iron uptake systems encoded by plasmids pColV-K30 in Escherichia coli K-12 and pSMN1 in Aerobacter aerogenes 62-1

Although the aerobactin-mediated iron uptake system has been characterized genetically in Escherichia coli, the siderophore aerobactin was chemically characterized after purification from culture supernatants of Aerobacter aerogenes 62-1, a member of the Klebsielleae. We have cloned and mapped the genes encoding the aerobactin system genes of A. aerogenes 62-1 and begun characterization of the relevant proteins and enzymatic activities of this plasmid-mediated aerobactin system. Published chemical data indicate that the siderophore aerobactin of E. coli is the same molecule as the aerobactin of Aerobacter aerogenes 62-1, but we have found that both the genes and the complement of proteins making up the biosynthetic enzymes in the two systems have diverged. In contrast, the outer membrane receptors for ferric aerobactin of the two systems showed immunologic cross-reactivity, were of the same molecular size (74 kilodaltons), and were encoded by homologous DNA sequences.

Aerobactin production as a virulence factor: a reevaluation

Iron starvation is one of the major barriers that virulent bacteria must overcome in order to proliferate in the host. Virtually all microorganisms possess high affinity iron (III) transport systems mediated by low molecular weight iron specific chelators called siderophores, the synthesis of which is activated under iron-limiting conditions. Siderophore aerobactin is frequently produced by enterobacteria which cause various types of infections in humans and animals. The status of aerobactin production as a virulence factor is evaluated both from data derived from experimental infection systems and the actual presence of this siderophore in clinical isolates. Aerobactin appears to be an important contributor to extracellular pathogenesis (mostly, that of Escherichia coli strains causing septicaemia and urinary tract infections) and to the extracellular stages of growth of intracellular pathogens like Shigella. When invasive bacteria actually enter target cells, acquisition of iron seems to occur independently of siderophore production. The feasibility of an antimicrobial therapy aimed at interfering with siderophore functioning is discussed.