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Vibrio anguillarum is a marine pathogen that causes vibriosis, a hemorrhagic septicemia in aquatic invertebrate as well as vertebrate animals. The siderophore anguibactin system is one of the most important virulence factors of this bacterium. Most of the anguibactin biosynthesis and transport genes are located in the 65-kb pJM1 virulence plasmid although some of them are found in the chromosome of this fish pathogen. Over 30 years of research unveiled the role numerous chromosomal and pJM1 genes play in the synthesis of anguibactin and the transport of cognate ferric complexes into the bacterial cell. Furthermore, these studies showed that pJM1-carrying strains might be originated from pJM1-less strains producing the chromosome-mediated siderophore vanchrobactin. Additionally, we recently identified a chromosome-mediated anguibactin system in V. harveyi suggesting the possible evolutional origin of the V. anguillarum anguibactin system. In this review, we present our current understanding of the mechanisms and evolution hypothesis of the anguibactin system that might have occurred in these pathogenic vibrios.

Acinetobacter baumannii is a gram-negative bacterium that causes serious infections in compromised patients. More recently, it has emerged as the causative agent of severe infections in military personnel wounded in Iraq and Afghanistan. This pathogen grows under a wide range of conditions including iron-limiting conditions imposed by natural and synthetic iron chelators. Initial studies using the type strain 19606 showed that the iron proficiency of this pathogen depends on the expression of the acinetobactin-mediated iron acquisition system. More recently, we have observed that hemin but not human hemoglobin serves as an iron source when 19606 isogenic derivatives affected in acinetobactin transport and biosynthesis were cultured under iron-limiting conditions. This finding is in agreement with the observation that the genome of the strain 17978 has a gene cluster coding for putative hemin-acquisition functions, which include genes coding for putative hemin utilization functions and a TonBExbBD energy transducing system. This system restored enterobactin biosynthesis in an E. coli ExbBD deficient strain but not when introduced into a TonB mutant. PCR and Southern blot analyses showed that this hemin-utilization gene cluster is also present in the 19606 strain. Analysis of the 17978 genome also showed that this strain harbors genes required for acinetobactin synthesis and transport as well as a gene cluster that could code for additional iron acquisition functions. This hypothesis is in agreement with the fact that the inactivation of the basD acinetobactin biosynthetic gene did not affect the growth of A. baumannii 17978 cells under iron-chelated conditions. Interestingly, this second iron uptake gene cluster is flanked by perfect inverted repeats and includes transposase genes that are expressed transcriptionally. Also interesting is the observation that this additional cluster could not be detected in the type strain 19606, an observation that suggests some significant differences in the iron uptake capacity between these two A. baumannii strains. Transposome mutagenesis of the strain 19606 resulted in the isolation of a derivative unable to grow under iron-chelated conditions. Gene mapping and protein analysis together with complementation assays showed that a protein related to SecA, which is a component of the Sec protein secretion system in a wide range of bacteria, is needed at least for the production of the BauA acinetobactin outer membrane receptor. Furthermore, this derivative was unable to use hemin as an iron source under limiting conditions. Taken together, these results indicate that A. baumannii expresses siderophore-mediated and hemin acquisition functions, although different isolates differ in their iron acquisition capacity. Unexpectedly, the ability of this pathogen to acquire iron depends on the expression of a SecA protein secretion function, which has not been associated with iron acquisition in bacteria.

Actinobacillus actinomycetemcomitans requires iron to grow under limiting conditions imposed by synthetic and natural chelators. Although none of the strains tested used hemoglobin, lactoferrin or transferrin, all of them used FeCl3 and hemin as iron sources under chelated conditions. Dot-blot binding assays showed that all strains bind lactoferrin, hemoglobin, and hemin but not transferrin. When compared with smooth strains, the rough isolates showed higher hemin binding activity, which was sensitive to proteinase K treatment. A. actinomycetemcomitans harbors the Fur-regulated afeABCD locus coding for iron acquisition in isogenic and non-isogenic cell backgrounds. The genome of this oral pathogen also harbors several other predicted iron uptake genes including the hitABC locus, which restored iron acquisition in the E. coli 1017 ent mutant. However, the disruption of this locus in the parental strain did not affect iron acquisition as drastically as the inactivation of AfeABCD, suggesting that the latter system could be more involved in iron transport than the HitABC system. The genome of this oral pathogen also harbors an active copy of the exbBexbDtonB operon, which could provide the energy needed for hemin acquisition. However, inactivation of each coding region of this operon did not affect the hemin and iron acquisition phenotypes of isogenic derivatives. This observation suggests that the function of these proteins could be replaced by those coded for by tolQ, tolR and tolA as it was described for other bacterial transport systems. Interruption of a hasR homolog, an actively transcribed gene that is predicted to code for an outer membrane hemophore receptor protein, did not affect the ability of an isogenic derivative to bind and use hemin under chelated conditions. This result also indicates that A. actinomycetemcomitans could produce more than one outer membrane hemin receptor as it was described in other human pathogens. All strains tested formed biofilms on plastic under iron-rich and iron-chelated conditions. However, smooth strains attached poorly and formed weaker biofilms when compared with rough isolates. The incubation of rough cells in the presence of FeCl3 or hemin resulted in an increased number of smaller aggregates and microcolonies as compared to the fewer but larger aggregates formed when cells were grown in the presence of dipyridyl.