Bacteria have aggressive acquisition processes for iron, an essential nutrient. Siderophores are small iron chelators that facilitate cellular iron transport. The siderophore enterobactin is a triscatechol derivative of a cyclic triserine lactone. Studies of the chemistry, regulation, synthesis, recognition, and transport of enterobactin make it perhaps the best understood of the siderophore-mediated iron uptake systems, displaying a lot of function packed into this small molecule. However, recent surprises include the isolation of corynebactin, a closely related trithreonine triscatechol derivative lactone first found in Gram-positive bacteria, and the crystal structure of a ferric enterobactin complex of a protein identified as an antibacterial component of the human innate immune system.

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Enterobactin: an archetype for microbial iron transport.

Catheter-associated urinary tract infections (CAUTIs) represent the most common type of nosocomial infection and are a major health concern due to the complications and frequent recurrence. These infections are often caused by Escherichia coli and Proteus mirabilis. Gram-negative bacterial species that cause CAUTIs express a number of virulence factors associated with adhesion, motility, biofilm formation, immunoavoidance, and nutrient acquisition as well as factors that cause damage to the host. These infections can be reduced by limiting catheter usage and ensuring that health care professionals correctly use closed-system Foley catheters. A number of novel approaches such as condom and suprapubic catheters, intermittent catheterization, new surfaces, catheters with antimicrobial agents, and probiotics have thus far met with limited success. While the diagnosis of symptomatic versus asymptomatic CAUTIs may be a contentious issue, it is generally agreed that once a catheterized patient is believed to have a symptomatic urinary tract infection, the catheter is removed if possible due to the high rate of relapse. Research focusing on the pathogenesis of CAUTIs will lead to a better understanding of the disease process and will subsequently lead to the development of new diagnosis, prevention, and treatment options.

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Complicated Catheter-Associated Urinary Tract Infections Due to Escherichia coli and Proteus mirabilis

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.

Role of iron in regulation of virulence genes.

Vibrio vulnificus is a halophilic estuarine bacterium that causes fatal septicemia and necrotizing wound infections in humans. Virulent V. vulnificus isolates produce a catechol siderophore called vulnibactin, made up of one residue of 2, 3-dihydroxybenzoic acid (2, 3-DHBA) and two residues of salicylic acid (SA). Vulnibactin biosynthetic genes (VV2_0828 to VV2_0844) are clustered at one locus of chromosome 2, expression of which is significantly up-regulated in vivo. In the present study, we decipher the biosynthetic network of vulnibactin, focusing specifically on genes around SA and 2, 3-DHBA biosynthetic steps. Deletion mutant of isochorismate pyruvate lyase (VV2_0839) or 2,3-dihydroxybenzoate-2,3-dehydrogenase (VV2_0834) showed retarded growth under iron-limited conditions though the latter showed more significant growth defect than the former, suggesting a dominant role of 2, 3-DHBA in the vulnibactin biosynthesis. A double deletion mutant of VV2_0839 and VV2_0834 manifested additional growth defect under iron limitation. Though the growth defect of respective single deletion mutants could be restored by exogenous SA or 2, 3-DHBA, only 2, 3-DHBA could rescue the double mutant when supplied alone. However, double mutant could be rescued with SA only when hydrogen peroxide was supplied exogenously, suggesting a chemical conversion of SA to 2, 3-DHBA. Assembly of two SA and one 2, 3-DHBA into vulnibactin was mediated by two AMP ligase genes (VV2_0836 and VV2_0840). VV2_0836 deletion mutant showed more significant growth defect under iron limitation, suggesting its dominant function. In conclusion, using molecular genetic analytical tools, we confirm that vulnibactin is assembled of both 2, 3-DHBA and SA. However, conversion of SA to 2, 3-DHBA in presence of hydrogen peroxide and growth profile of AMP ligase mutants suggest a plausible existence of yet unidentified alternative siderophore that may be composed solely of 2, 3-DHBA.

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Molecular characterization of vulnibactin biosynthesis in Vibrio vulnificus indicates the existence of an alternative siderophore

Vibrio vulnificus is an opportunistic human pathogen that is highly lethal and is responsible for the overwhelming majority of reported seafood-related deaths in the United States ([30][1], [117][2]). This bacterium is a part of the natural flora of coastal marine environments worldwide and has been

Vibrio vulnificus: Disease and Pathogenesis

Infections with Vibrio vulnificus resulting in septicemia and high mortality have been correlated with pre-existing liver disease and hemochromatosis. As these conditions are associated with impaired iron metabolism and as iron availability in the host has been implicated in the pathogenicity of a number of bacterial infections, the role of iron as a possible factor in the pathogenesis of V. vulnificus was examined. Injection of mice with iron resulted in a lowering of the 50% lethal dose from 10(6) to 1.1 cells and in a reduction in the time of death postinfection. Elevated serum iron levels were also produced by damaging livers with injections of CCl4. The inoculum size required to kill these mice was directly correlated with serum iron levels. Since the portal of infection of this organism may be ingestion of contaminated seafood, the effects of iron upon orally induced infection were also studied. The effects of adding iron, transferrin, or Desferal (an iron chelate) upon the growth of V. vulnificus in human and rabbit sera were also examined. Iron appeared to be the limiting factor in the ability of this organism to survive or grow in mammalian sera. These results, both in vitro and in vivo, provided strong evidence that iron may play a major role in the pathogenesis of V. vulnificus.

Role of iron in the pathogenesis of Vibrio vulnificus infections.

Translucent, avirulent spontaneous phase variants of Vibrio vulnificus MO6-24 reverted back to the original opaque, encapsulated phenotype under both in vivo and in vitro conditions. Two translucent, acapsular mutants, which did not show phase variation, were constructed by using the transposon Tn5 IS50L::phoA (TnphoA). Loss of capsule was accompanied by decreases in virulence, hydrophilicity, and serum resistance. The ability to utilize transferrin-bound iron for growth was lost in only one of the two unencapsulated mutants. Our data emphasize the apparent importance of capsule in the virulence of V. vulnificus and indicate that utilization of transferrin-bound iron is independent of encapsulation. Images

https://iai.asm.org/content/iai/58/6/1769.full.pdf

Phenotypic evaluation of acapsular transposon mutants of Vibrio vulnificus.

Although Vibrio cholerae O139 synonym Bengal strains, from the current epidemics in India and Bangladesh, are closely related to seventh-pandemic strains, as shown by multilocus enzyme electrophoresis, Bengal strains are encapsulated and portions of the O1 antigen biosynthetic complex genes found in O1 strains are altered or lacking. Encapsulated Bengal strains showed resistance to killing by normal human serum. The presence of the capsule suggests the potential for bloodstream invasion in susceptible hosts and has profound implications for vaccine development.

Vibrio cholerae O139 synonym bengal is closely related to Vibrio cholerae El Tor but has important differences.

The ca. 51-kDa extracellular cytolysin of Vibrio vulnificus has been proposed as a virulence factor. We inactivated the structural gene for cytolysin in fully virulent, clinical V. vulnificus strains by both transposon mutagenesis and marker exchange techniques. Inactivation of the cytolysin did not affect virulence in our mouse models. The 50% lethal dose of cytolysin-negative strains was comparable to that of the cytolysin-positive parent strains after intraperitoneal inoculation with and without iron loading. Similar results were obtained after intradermal injection: cytolysin-positive and -negative strains had the same 50% lethal dose and caused comparable tissue damage. While we cannot say that the cytolysin has no effect on the pathogenesis of V. vulnificus infections, its role appears to be of much less importance than are other factors, such as encapsulation.

The extracellular cytolysin of Vibrio vulnificus: inactivation and relationship to virulence in mice.

We screened 44 lactose-positive Vibrio strains isolated from the marine environment for homology with a 3.2-kilobase DNA fragment encoding the Vibrio vulnificus cytotoxin-hemolysin gene. All 29 marine isolates identified as V. vulnificus on the basis of numerical taxonomy and DNA-DNA hybridization studies hybridized with the cytotoxin gene probe, as did all V. vulnificus reference strains. Homologous gene sequences were identified in no other lactose-positive marine vibrio isolates nor in 10 other Vibrio species.

A C Wright

Papers

Role of iron in the pathogenesis of Vibrio vulnificus infections.

Phenotypic evaluation of acapsular transposon mutants of Vibrio vulnificus.

Vibrio cholerae O139 synonym bengal is closely related to Vibrio cholerae El Tor but has important differences.

The extracellular cytolysin of Vibrio vulnificus: inactivation and relationship to virulence in mice.

Identification of environmental Vibrio vulnificus isolates with a DNA probe for the cytotoxin-hemolysin gene.