Identification and characterization of genes required for utilization of desferri-ferrichrome and aerobactin in Vibrio parahaemolyticus

The analysis of complete genome sequence and comparative genomics of Vibrio parahaemolyticus LF1113 in Hainan

Vibrio parahaemolyticus is a Gram-negative, halophilic and polymorphic coccobacillus. It is world-widely distributed and has resulted in great economic losses since its first appearance. In this study, a pathogenic strain was isolated from diseased pearl gentian grouper and identified as V. parahaemolyticus based on the sequencing results of 16S rDNA gene. In order to gain a comprehensive understanding of this isolation, the whole genome sequencing was conducted. Phylogenetic analysis of the complete genomes of 16 Vibrio species showed that LF1113, ATCC17802, ATCC33787, 2210633, FORC 004, and 160807 were the most closely related. Animal experiments demonstrated that the isolated LF1113 strain was pathogenic in a fish model. This study is the first study to describe the complete genome sequence of a V. parahaemolyticus isolate, which infected pearl gentian grouper from an outbreak in a fish factory farm in Hainan. The results will expand our understanding of genetic characteristics, pathogenesis, diagnostics and disease prevention of V. parahaemolyticus, and lay the foundation for further study.

Molecular Mechanism of Iron Transport Systems in Vibrio

The ability to acquire iron from the environment is often an important virulence factor for pathogenic bacteria and Vibrios are no exception to this. Vibrios are reported mainly from marine habitats and most of the species are pathogenic. Among those, the pathogenic vibrios eg. V cholerae, V. parahaemolyticus, V. vulnificus causes foodborne illnesses. Vibrios are capable of producing all different classes of siderophores like hydroxamate (aerobactin), catecholate (vibriobactin, fluvibactin), carboxylate (vibrioferrin), and amphiphilic (amphibactin). Every different species of vibrios are capable of utilizing some endogenous or xenosiderophores. Being Gram-negative bacteria, Vibrios import iron siderophore via TonB dependent transport system and unlike other Gamma proteobacteria these usually possess two or even three partially redundant TonB systems for iron siderophore transport. Other than selected few iron siderophores, most pathogenic Vibrios are known to be able to utilize heme as the sole iron source, while some species are capable of importing free iron from the environment. As per the present knowledge, the spectrum of iron compound transport and utilization in Vibrios is better understood than the siderophore biosynthetic capability of individual species.

Vibrio fischeri siderophore production drives competitive exclusion during dual-species growth

When two or more bacterial species inhabit a shared niche, often, they must compete for limited nutrients. Iron is an essential nutrient that is especially scarce in the marine environment. Bacteria can use the production, release, and re-uptake of siderophores, small molecule iron chelators, to scavenge iron. Siderophores provide fitness advantages to species that employ them by enhancing iron acquisition, and moreover, by denying iron to competitors incapable of using the siderophore-iron complex. Here, we show that cell-free culture fluids from the marine bacterium Vibrio fischeri ES114 prevent the growth of other vibrio species. Mutagenesis reveals the aerobactin siderophore as the inhibitor. Our analysis reveals a gene, that we name aerE, encodes the aerobactin exporter, and LuxT is a transcriptional activator of aerobactin production. In co-culture, under iron-limiting conditions, aerobactin production allows V. fischeri ES114 to competitively exclude Vibrio harveyi, which does not possess aerobactin production and uptake genes. In contrast, V. fischeri ES114 mutants incapable of aerobactin production lose in competition with V. harveyi. Introduction of iutA, encoding the aerobactin receptor, together with fhuCDB, encoding the aerobactin importer are sufficient to convert V. harveyi into an "aerobactin cheater."

Molecular mechanisms of Vibrio parahaemolyticus pathogenesis

Vibrio parahaemolyticus is a Gram-negative halophilic bacterium that is mainly distributed in the seafood such as fish, shrimps and shellfish throughout the world. V. parahaemolyticus can cause diseases in marine aquaculture, leading to huge economic losses to the aquaculture industry. More importantly, it is also the leading cause of seafood-borne diarrheal disease in humans worldwide. With the development of animal model, next-generation sequencing as well as biochemical and cell biological technologies, deeper understanding of the virulence factors and pathogenic mechanisms of V. parahaemolyticus has been gained. As a globally transmitted pathogen, the pathogenicity of V. parahaemolyticus is closely related to a variety of virulence factors. This article comprehensively reviewed the molecular mechanisms of eight types of virulence factors: hemolysin, type III secretion system, type VI secretion system, adhesion factor, iron uptake system, lipopolysaccharide, protease and outer membrane proteins. This review comprehensively summarized our current understanding of the virulence factors in V. parahaemolyticus, which are potentially new targets for the development of therapeutic and preventive strategies.

Transcriptional regulation of the ferric aerobactin receptor gene by a GntR-like repressor IutR in Vibrio furnissii

We found that Vibrio furnissii can utilize aerobactin (AERO) as a xenosiderophore. A homology search of its genome revealed that this bacterium possesses genes encoding an AERO-mediated iron acquisition system similar to that of V. vulnificus. The system consists of the ABC transporter gene vatCDB, the GntR-type transcriptional repressor gene iutR, and the outer membrane receptor gene iutA. The functions of the vatCDB operon and iutA in V. furnissii were confirmed by the inability of the corresponding deletion mutants to utilize AERO. Reverse transcription-quantitative PCR revealed that iutA transcription under iron-limiting conditions was extensively activated by the addition of AERO to the growth medium; therefore, we focused on elucidating this phenomenon. Electrophoretic mobility shift and DNase I footprinting assays revealed that glutathione S-transferase-fused IutR (GST-IutR) bound directly to a specific palindromic sequence in the iutA promoter region. However, GST-IutR did not bind to this sequence when either AERO or ferric AERO was present in the assay mixture. These in vitro findings suggest that, under iron-limiting conditions, iutA transcription in V. furnissii is artfully regulated both by IutR, acting as a direct repressor of iutA, and by AERO, acting as an effector for IutR, leading to the derepression of iutA transcription.

Distribution of siderophore gene systems on a Vibrionaceae phylogeny: Database searches, phylogenetic analyses and evolutionary perspectives

Siderophores are small molecules synthesized and secreted by bacteria and fungi to scavenge iron. Extracellular ferri-siderohores are recognized by cognate receptors on the cell surface for transport over membranes. Several siderophore systems from Vibrionaceae representatives are known and well understood, e.g., the molecular structure of the siderophore, the biosynthesis gene cluster and pathway, and the gene expression pattern. Less is known about how these systems are distributed among the ~140 Vibrionaceae species, and which evolutionary processes contributed to the present-day distribution. In this work, we compiled existing knowledge on siderophore biosynthesis systems and siderophore receptors from Vibrionaceae and used phylogenetic analyses to investigate their organization, distribution, origin and evolution. Through literature searches, we identified nine different siderophore biosynthesis systems and thirteen siderophore receptors in Vibrionaceae. Homologs were identified by BLAST searches, and the results were mapped onto a Vibrionaceae phylogeny. We identified 81 biosynthetic systems distributed in 45 Vibrionaceae species and 16 unclassified Vibrionaceae strains, and 409 receptors in 89 Vibrionaceae species and 49 unclassified Vibrionaceae strains. The majority of taxa are associated with at least one type of siderophore biosynthesis system, some (e.g., aerobactin and vibrioferrin) of which are widely distributed in the family, whereas others (i.e., bisucaberin and vibriobactin) are found in one lineage. Cognate receptors are found more widespread. Phylogenetic analysis of three siderophore systems (piscibactin, vibrioferrin and aerobactin) show that their present-day distribution can be explained by an old insertion into Vibrionaceae, followed mainly by stable vertical evolution and extensive loss, and some cases of horizontal gene transfers. The present work provides an up to date overview of the distribution of siderophore-based iron acquisition systems in Vibrionaceae, and presents phylogenetic analysis of these systems. Our results suggest that the present-day distribution is a result of several evolutionary processes, such as old and new gene acquisitions, gene loss, and both vertical and horizontal gene transfers.

Iron Acquisition Strategies of Vibrio anguillarum

The hemorrhagic septicemic disease vibriosis caused by Vibrio anguillarum shows noticeable similarities to invasive septicemia in humans, and in this case, the V. anguillarum–host system has the potential to serve as a model for understanding native eukaryotic host–pathogen interactions. Iron acquisition, as a fierce battle occurring between pathogenic V. anguillarum and the fish host, is a pivotal step for virulence. In this article, advances in defining the roles of iron uptake pathways in growth and virulence of V. anguillarum have been summarized, divided into five aspects, including siderophore biosynthesis and secretion, iron uptake, iron release, and regulation of iron uptake. Understanding the molecular mechanisms of iron acquisition will have important implications for the pathogenicity of this organism.

Regulation of the Expression of Iron-acquisition System Genes in Pathogenic Vibrio Species

The genus Vibrio includes >70 species, of which roughly a dozen cause vibriosis such as gastroenteritis, wound infections, and septicemia. Most bacteria, including Vibrio species, require iron for survival and growth. However, the bioavailability of iron is extremely low because it is usually present as an insoluble ferric complex in an aerobic environment or is bound to iron-binding proteins in mammalian hosts. Therefore many bacteria have developed iron acquisition systems, including biosynthesis and secretion of low-molecular-mass iron-chelating compounds called siderophores, and uptake of iron-bound siderophores into bacterial cells through specific active transport systems. Vibrio parahaemolyticus, a major pathogenic Vibrio species, contains multiple iron-acquisition systems mediated by its own siderophore vibrioferrin and several xenosiderophores produced by other microorganisms. In this review, I have focused on the transcriptional and posttranscriptional regulation of genes encoding iron acquisition systems in V. parahaemolyticus. All genes involved in its iron acquisition systems are repressed by Fur, which acts as a ferrous-dependent transcriptional repressor. Furthermore, the stability of polycistronic mRNA involved in vibrioferrin biosynthesis is positively regulated by a small RNA, RyhB, which is repressed by Fur. Expression of PeuA receptor required for utilization of a xenosiderophore, enterobactin, occurs under iron-limiting conditions at alkaline pH. PeuA expression is induced by a two-component regulatory system, PeuRS, which enhances expression of an alternative peuA transcript without an intrinsic translation-inhibitory structure in response to changes in alkaline pH.

Vibrio Iron Transport: Evolutionary Adaptation to Life in Multiple Environments

Iron is an essential element for Vibrio spp., but the acquisition of iron is complicated by its tendency to form insoluble ferric complexes in nature and its association with high-affinity iron-binding proteins in the host. Vibrios occupy a variety of different niches, and each of these niches presents particular challenges for acquiring sufficient iron. Vibrio species have evolved a wide array of iron transport systems that allow the bacteria to compete for this essential element in each of its habitats. These systems include the secretion and uptake of high-affinity iron-binding compounds (siderophores) as well as transport systems for iron bound to host complexes. Transporters for ferric and ferrous iron not complexed to siderophores are also common to Vibrio species. Some of the genes encoding these systems show evidence of horizontal transmission, and the ability to acquire and incorporate additional iron transport systems may have allowed Vibrio species to more rapidly adapt to new environmental niches. While too little iron prevents growth of the bacteria, too much can be lethal. The appropriate balance is maintained in vibrios through complex regulatory networks involving transcriptional repressors and activators and small RNAs (sRNAs) that act posttranscriptionally. Examination of the number and variety of iron transport systems found in Vibrio spp. offers insights into how this group of bacteria has adapted to such a wide range of habitats.

Strategies of Vibrio parahaemolyticus to acquire nutritional iron during host colonization

Iron is an essential element for the growth and development of virtually all living organisms. As iron acquisition is critical for the pathogenesis, a host defense strategy during infection is to sequester iron to restrict the growth of invading pathogens. To counteract this strategy, bacteria such as Vibrio parahaemolyticus have adapted to such an environment by developing mechanisms to obtain iron from human hosts. This review focuses on the multiple strategies employed by V. parahaemolyticus to obtain nutritional iron from host sources. In these strategies are included the use of siderophores and xenosiderophores, proteases and iron-protein receptor. The host sources used by V. parahaemolyticus are the iron-containing proteins transferrin, hemoglobin, and hemin. The implications of iron acquisition systems in the virulence of V. parahaemolyticus are also discussed.

Regulation of the expression of the Vibrio parahaemolyticus peuA gene encoding an alternative ferric enterobactin receptor

A pvsB-vctA-irgA triple deletion mutant of Vibrio parahaemolyticus can utilize enterobactin under iron-limiting conditions by inducing a previously undescribed receptor, PeuA (VPA0150), in response to extracellular alkaline pH and enterobactin. In silico analyses revealed the existence of a two-component regulatory system operon, peuRS, immediately upstream of peuA, which constitutes an operon with the TonB2 system genes. Both the peuRS and peuA-tonB2 operons were found to be upregulated under iron-limiting conditions in a ferric uptake regulator (Fur)-dependent manner. The involvement of peuA and peuRS in enterobactin utilization was analyzed by complementation experiments using deletion mutants. Primer extension analysis indicated that, under iron-limiting conditions, the transcription of peuA was initiated from the +1 site at pH 7.0 and from both the +1 and +39 sites at pH 8.0 in the presence of enterobactin. The +39 transcript was absent from the peuRS deletion mutant. Secondary structure prediction of their 5′-untranslated regions suggested that translation initiation is blocked in the +1 transcript, but not in the +39 transcript. Consistent with this, in vitro translation analysis demonstrated that production of PeuA was determined only by the +39 transcript. These studies establish a novel gene regulation mechanism in which the two-component regulatory system PeuRS enhances expression of the alternative +39 transcript that possesses non-inhibitory structure, allowing the peuA expression to be regulated at the translation stage.

The Vibrio parahaemolyticus small RNA RyhB promotes production of the siderophore vibrioferrin by stabilizing the polycistronic mRNA

High-affinity iron acquisition in Vibrio parahaemolyticus is mediated by the cognate siderophore vibrioferrin. We have previously reported that the vibrioferrin biosynthesis operon (pvsOp) is regulated at the transcriptional level by the iron-responsive repressor Fur (T. Tanabe, T. Funahashi, H. Nakao, S. Miyoshi, S. Shinoda, and S. Yamamoto, J. Bacteriol. 185:6938-6949, 2003). In this study, we identified the Fur-regulated small RNA RyhB and the RNA chaperone Hfq protein as additional regulatory proteins of vibrioferrin biosynthesis. We found that vibrioferrin production was greatly impaired in both the ryhB and hfq deletion mutants, and a TargetRNA search (http://snowwhite.wellesley.edu/targetRNA/index2.html) revealed that the 5'-untranslated region of pvsOp mRNA (pvsOp 5'-UTR) contains a potential base-pairing region required for the formation of the RyhB-pvsOp 5'-UTR duplex. An electrophoresis mobility shift assay indicated that RyhB can directly bind to the pvsOp 5'-UTR with the aid of Hfq. Rifampin chase experiments indicated that the half-life of pvsOp mRNA in the ryhB and hfq mutants was approximately 3-fold shorter than that in the parental strain, suggesting that both RyhB and Hfq are engaged in the stabilization of pvsOp mRNA. Chrome azurol S assays followed by electrophoresis mobility shift assays and rifampin chase experiments carried out for mutant strains indicated that base pairing between RyhB and the pvsOp 5'-UTR results in an increase in the stability of pvsOp mRNA, thereby leading to the promotion of vibrioferrin production. It is unprecedented that RyhB confers increased stability on a polycistronic mRNA involved in siderophore biosynthesis as a direct target.

Identification of genes, desR and desA, required for utilization of desferrioxamine B as a xenosiderophore in Vibrio furnissii

We found that Vibrio (V.) furnissii ATCC35016 can gain iron through a xenosiderophore desferrioxamine B (DFOB) for its growth under iron-limiting conditions, concurrent with the expression of the 79-kDa iron-repressible outer membrane protein (IROMP) in response to the presence of DFOB. Based on the sequence of the ferrioxamine B (an iron-bound form of DFOB) receptor gene in V. vulnificus, two V. furnissii genes, termed desA and desR, encoding the 79-kDa IROMP and AraC-type transcriptional regulator, respectively, were identified and cloned. Nucleotide sequences located in the promoter regions of both desR and desA predicted the presence of consensus ferric uptake regulation (Fur)-binding sequences. The transcription of both genes was negatively regulated by exogenous iron levels. Deletion of the desA gene abolished the ability of V. furnissii to utilize DFOB, and neither desA mRNA nor DesA was detected in the deletion mutant of desR regardless of the presence of DFOB. The functions of DesA and DesR as the ferrioxamine B receptor and transcriptional activator for desA, respectively, were confirmed by complementation of desA and desR deletion mutants.

Vibrio parahaemolyticus cell biology and pathogenicity determinants

Vibrio parahaemolyticus is a significant cause of gastroenteritis worldwide. Characterization of this pathogen has revealed a unique repertoire of virulence factors that allow for colonization of the human host and disease. The following describes the known pathogenicity determinants while establishing the need for continued research.

The TonB energy transduction systems in Vibrio species

Studying the organization and conservation of the TonB systems across the genus Vibrio, we can tease out trends in gene arrangement and function that lead to clues about the evolution and necessity of the proteins in multiple TonB systems. The TonB2 systems, with additional TtpC proteins, are in general more promiscuous regarding their interactions with many different TonB-dependent transporters in the outer membrane. Studies show that the TtpC protein spans the periplasmic space, suggesting that it can be the connection between the energy from the proton motive force and the outer membrane protein receptors, which the shorter TonB2 cannot provide. As an earlier system, the combination of the TtpC protein and a TonB2 system must have been necessary for the function of the smaller TonB2 protein and to transduce energy in a medium that can have osmotic challenges.