Molecular and genetic characterization of the TonB2-cluster TtpC protein in pathogenic vibrios

Two novel proteins, TtpB2 and TtpD2, are essential for iron transport in the TonB2 system of Vibrio vulnificus

In gram-negative bacteria, energy-dependent active transport of iron-bound substrates across the outer membrane is achieved through the TonB systems of proteins. Three TonB systems have been identified in the human pathogen Vibrio vulnificus. The TonB1 system contains three proteins: TonB1, ExbB1, and ExbD1. Both the TonB2 and TonB3 systems have been shown to also contain a fourth protein, TtpC2 and TtpC3, respectively. Here, we report and begin to characterize two additional proteins in the TonB2 and TonB3 systems: TtpB and TtpD. Both TtpB2 and TtpD2 are absolutely required for the function of the TonB2 system in V. vulnificus. However, although both TtpB3 and TtpD3 in the TonB3 system are related to the proteins in the TonB2 system, neither are active in iron transport. All six protein components of the TonB2 system-TonB2, ExbB2, ExbD2, TtpB2, TtpC2, and TtpD2-are essential for the uptake of both endogenously produced iron-bound siderophores and exogenous siderophores produced from other organisms. Through complementation, we have shown that V. vulnificus is capable of using different TtpD2 proteins from other Vibrio species to bring in multiple siderophores. In contrast, we also demonstrate that TtpB2 must come from V. vulnificus, and not other species within the genus, to complement mutations in the TonB2 system.

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.

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.

Application of relative real-time PCR to detect differential expression of virulence genes in Vibrio anguillarum under standard and stressed growth conditions

In this study, we aimed to understand whether abiotic factors affect the expression of virulence genes in Vibrio anguillarum. We observed the in vitro responses of two Mediterranean strains of V. anguillarum to temperature, NaCl and iron concentration changes. We monitored growth performance and gene transcription levels by comparing the results obtained under stressed conditions (temperatures of 5 °C, 15 °C and 37 °C; NaCl concentrations of 3% and 5%; and iron depletion and excess) with those obtained under standard growth conditions (25 °C, 1.5% NaCl and 0.6 μm of iron). The results showed that the strains respond differently. The strain 975/I was most strongly affected by conditions of 15 °C and iron depletion; these conditions induced increased transcription levels of empA, angR and fatA. Growth of the strain 17/I was inhibited at 15 °C and in iron depletion conditions; this strain also showed dramatic changes in the transcription levels of toxR and tonB2 under increased NaCl concentrations. These results demonstrate that environmental stress affects the expression of virulence genes in V. anguillarum that have implications for the competitiveness, stress tolerance and the ability of V. anguillarum to cause infection.

The siderophore-interacting protein is involved in iron acquisition and virulence of Riemerella anatipestifer strain CH3

Riemerella anatipestifer causes epizootic infectious disease in poultry and serious economic losses especially to the duck industry. However, little is known regarding the molecular basis of its pathogenesis. The ability to acquire iron under low-iron conditions is related to the virulence of a variety of bacterial pathogens. In this study, a sip (Riean_1281) deletion mutant CH3Δsip was constructed and characterized for iron-limited growth, biofilm formation, and pathogenicity to ducklings. Results showed that siderophore-interacting protein (SIP) was involved in iron utilization and the sip deletion significantly reduced biofilm formation and adherence to and invasion of Vero cells. In addition, the sip gene was absent in 1 of 24 (4.17%) virulent strains and 2 of 3 (66.7%) avirulent strains of R. anatipestifer, and the sip gene from six R. anatipestifer strains, which belong to serotypes 1, 2, and 10, respectively, shared 100% amino acid identities to those of R. anatipestifer strains DSM15868 and RA-GD. These results suggested that siderophore-mediated iron acquisition may be an important iron-uptake pathway in R. anatipestifer. Animal experiments indicated that the median lethal dose of the CH3Δsip mutant in ducklings was about 35-fold higher than that of the wild-type CH3 strain. Thus, our results demonstrated that R. anatipestifer SIP was involved in iron acquisition and necessary for its optimal virulence.

Plasmid- and chromosome-encoded siderophore anguibactin systems found in marine vibrios: biosynthesis, transport and evolution

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.

The ttpC gene is contained in two of three TonB systems in the human pathogen Vibrio vulnificus, but only one is active in iron transport and virulence

The TonB system of proteins is required for the energy-dependent active transport of iron-bound substrates across the outer membrane of gram-negative bacteria. We have identified three TonB systems within the human pathogen Vibrio vulnificus. The TonB1 system contains the TonB1, ExbD1, and ExbB1 proteins, whereas both the TtpC2-TonB2 and TtpC3-TonB3 systems contain an additional fourth protein, TtpC. Here we report that TtpC3, although highly related to TtpC2, is inactive in iron transport, whereas TtpC2 is essential for the function of the TtpC2-TonB2 system in V. vulnificus. This protein, together with TonB2, is absolutely required for both the uptake of endogenously produced iron-bound siderophores as well as siderophores produced from other organisms. Through complementation we show that V. vulnificus is capable of using different TtpC2 proteins from other Vibrio species to drive the uptake of multiple siderophores. We have also determined that aerobactin, a common bacterial siderophore involved in virulence of enteric bacteria, can only be brought into the cell using the TtpC2-TonB2 system, indicating an important evolutionary adaptation of TtpC2 and TonB2. Furthermore, in the absence of TonB1, TtpC2 is essential for a fully virulent phenotype as demonstrated using 50% lethal dose (LD(50)) experiments in mice.

The Vibrio cholerae VctPDGC system transports catechol siderophores and a siderophore-free iron ligand

Vibrio cholerae, the causative agent of cholera, has an absolute requirement for iron. It transports the catechol siderophores vibriobactin, which it synthesizes and secretes, and enterobactin. These siderophores are transported across the inner membrane by one of two periplasmic binding protein-dependent ABC transporters, VctPDGC or ViuPDGC. We show here that one of these inner membrane transport systems, VctPDGC, also promotes iron acquisition in the absence of siderophores. Plasmids carrying the vctPDGC genes stimulated growth in both rich and minimal media of a Shigella flexneri mutant that produces no siderophores. vctPDGC also stimulated the growth of an Escherichia coli enterobactin biosynthetic mutant in low iron medium, and this effect did not require feoB, tonB or aroB. A tyrosine to phenylalanine substitution in the periplasmic binding protein VctP did not alter enterobactin transport, but eliminated growth stimulation in the absence of a siderophore. These data suggest that the VctPDGC system has the capacity to transport both catechol siderophores and a siderophore-free iron ligand. We also show that VctPDGC is the previously unidentified siderophore-independent iron transporter in V. cholerae, and this appears to complete the list of iron transport systems in V. cholerae.

Taking the Escherichia coli TonB transmembrane domain "offline"? Nonprotonatable Asn substitutes fully for TonB His20

The TonB system of Gram-negative bacteria uses the proton motive force (PMF) of the cytoplasmic membrane to energize active transport of nutrients across the outer membrane. The single transmembrane domain (TMD) anchor of TonB, the energy transducer, is essential. Within that TMD, His20 is the only TMD residue that is unable to withstand alanine replacement without a loss of activity. H20 is required for a PMF-dependent conformational change, suggesting that the importance of H20 lies in its ability to be reversibly protonated and deprotonated. Here all possible residues were substituted at position 20 (H20X substitutions). The His residue was also relocated throughout the TonB TMD. Surprisingly, Asn, a structurally similar but nonprotonatable residue, supported full activity at position 20; H20S was very weakly active. All the remaining substitutions, including H20K, H20R, H20E, and H20D, the obvious candidates to mimic a protonated state or support proton translocation, were inactive. A second-site suppressor, ExbB(A39E), indiscriminately reactivated the majority of H20 substitutions and relocations, including H20V, which cannot be made protonatable. These results suggested that the TonB TMD was not on a proton conductance pathway and thus only indirectly responds to PMF, probably via ExbD.

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.

Genetic Determinants of Virulence in the Marine Fish Pathogen Vibrio anguillarum

One of the most studied fish pathogens is Vibrio anguillarum. Development of the genetics and biochemistry of the mechanisms of virulence in this fish pathogen together with clinical and ecologic studies has permitted the intensive development of microbiology in fish diseases. It is the intention of this review to compile the exhaustive knowledge accumulated on this bacterium and its interaction with the host fish by reporting a complete analysis of the V. anguillarum virulence factors and the genetics of their complexity.