The ZnuABC operon is important for Yersinia ruckeri infections of rainbow trout, Oncorhynchus mykiss (Walbaum)

Virulence regulation of Zn2+ uptake system znuABC on mesophilic Aeromonas salmonicida SRW-OG1

Psychrophilic Aeromonas salmonicida could not grow above 25°C and therefore thought unable to infect mammals and humans. In our previous study, a mesophilic A. salmonicida SRW-OG1 was isolated from Epinephelus coioides with furunculosis. Through the analysis of preliminary RNA-seq, it was found that the Zn2+ uptake related genes znuA, znuB and znuC might be involved in the virulence regulation of A. salmonicida SRW-OG1. Therefore, the purpose of this study was to explore the effect of znuABC silencing on the virulence regulation of A. salmonicida SRW-OG1. The results showed that the growth of the znuA-RNAi, znuB-RNAi, and znuC-RNAi strains was severely restricted under the Fe2+ starvation, but surprisingly there was no significant difference under the Zn2+ restriction. In the absence of Zn2+ and Fe2+, the expression level of znuABC was significantly increased. The motility, biofilm formation, adhesion and hemolysis of the znuA-RNAi, znuB-RNAi, and znuC-RNAi strains were significantly reduced. We also detected the expression of znuABC under different growth periods, temperatures, pH, as well as Cu2+ and Pb2+ stresses. The results showed that znuABC was significantly up-regulated in the logarithmic phase and the decline phase of A. salmonicida. Interestingly, the trend of expression levels of the znuABC at 18, 28, and 37°C was reversed to another Zn2+ uptake related gene zupT. Taken together, these indicated that the znuABC was necessary for A. salmonicida SRW-OG1 pathogenicity and environmental adaptability, and was cross regulated by iron starvation, but it was not irreplaceable for A. salmonicida SRW-OG1 Zn2+ uptake in the host.

Pan-genome survey of the fish pathogen Yersinia ruckeri links accessory- and amplified genes to virulence

While both virulent and putatively avirulent Yersinia ruckeri strains exist in aquaculture environments, the relationship between the distribution of virulence-associated factors and de facto pathogenicity in fish remains poorly understood. Pan-genome analysis of 18 complete genomes, representing established virulent and putatively avirulent lineages of Y. ruckeri, revealed the presence of a number of accessory genetic determinants. Further investigation of 68 draft genome assemblies revealed that the distribution of certain putative virulence factors correlated well with virulence and host-specificity. The inverse-autotransporter invasin locus yrIlm was, however, the only gene present in all virulent strains, while absent in lineages regarded as avirulent. Strains known to be associated with significant mortalities in salmonid aquaculture display a combination of serotype O1-LPS and yrIlm, with the well-documented highly virulent lineages, represented by MLVA clonal complexes 1 and 2, displaying duplication of the yrIlm locus. Duplication of the yrIlm locus was further found to have evolved over time in clonal complex 1, where some modern, highly virulent isolates display up to three copies.

Dual RNA-Seq of Trunk Kidneys Extracted From Channel Catfish Infected With Yersinia ruckeri Reveals Novel Insights Into Host-Pathogen Interactions

Host-pathogen intectarions are complex, involving large dynamic changes in gene expression through the process of infection. These interactions are essential for understanding anti-infective immunity as well as pathogenesis. In this study, the host-pathogen interaction was analyzed using a model of acute infection where channel catfish were infected with Yersinia ruckeri. The infected fish showed signs of body surface hyperemia as well as hyperemia and swelling in the trunk kidney. Double RNA sequencing was performed on trunk kidneys extracted from infected channel catfish and transcriptome data was compared with data from uninfected trunk kidneys. Results revealed that the host-pathogen interaction was dynamically regulated and that the host-pathogen transcriptome fluctuated during infection. More specifically, these data revealed that the expression levels of immune genes involved in Cytokine-cytokine receptor interactions, the NF-kappa B signaling pathway, the JAK-STAT signaling pathway, Toll-like receptor signaling and other immune-related pathways were significantly upregulated. Y. ruckeri mainly promote pathogenesis through the flagellum gene fliC in channel catfish. The weighted gene co-expression network analysis (WGCNA) R package was used to reveal that the infection of catfish is closely related to metabolic pathways. This study contributes to the understanding of the host-pathogen interaction between channel catfish and Y. ruckeri, more specifically how catfish respond to infection through a transcriptional perspective and how this infection leads to enteric red mouth disease (ERM) in these fish.

The transcriptional regulator Zur regulates the expression of ZnuABC and T6SS4 in response to stresses in Yersinia pseudotuberculosis

Zinc homeostasis is crucial for the development and stress resistance of bacteria in the environment. Serial zinc sensing transcriptional regulators, zinc transporters and zinc binding proteins were found to maintain the zinc homeostasis in bacteria. Zur is a zinc uptake regulator that is widely distributed in species, and ZnuABC, as well as the Type VI Secretion System (T6SS4) function in zinc acquisition. Here, we report that the regulator Zur inhibits the expression of the ZnuABC which inhibition could be eliminated at low zinc level, and upregulates the T6SS4 operon in Yersinia pseudotuberculosis to facilitate Zn²⁺ uptake and oxidative stress resistance. Zur regulates the expression of ZnuABC and T6SS4 by directly binding to their promoter regions. Zur senses the Zn²⁺ concentration and represses ZnuABC in a Zn²⁺-containing environment. Zur works as an auxiliary regular activator of T6SS4, facilitating oxidative stress resistance. This study revealed the dual function of regulator Zur on ZnuABC and T6SS4, and enriched the knowledge of Zn²⁺ homeostasis maintenance in Y. pseudotuberculosis.

The Zinc Nutritional Immunity of Epinephelus coioides Contributes to the Importance of znuC During Pseudomonas plecoglossicida Infection

Previously, the dual RNA-seq was carried out in a Pseudomonas plecoglossicida- Epinephelus coioides infection model to investigate the dynamics of pathogen-host interplay in vivo. ZnuC, a member of ZnuCBA Zn importer, was found transcriptionally up-regulated during infection. Thus, this study aimed to assess its role during the trade-off for Zn between host and P. plecoglossicida. ICP-MS analysis and fluorescent staining showed that Zn was withheld from serum and accumulated in the spleen, with increased Zn uptake in the Golgi apparatus of macrophages after infection. Additionally, growth assay, macrophage infection and animal infection after gene knockout / silencing revealed that znuC was necessary for growth in Zn-limiting conditions, colonization, intracellular viability, immune escape and virulence of P. plecoglossicida. Further analysis with dual RNA-seq revealed associations of host's Zn nutritional immunity genes with bacterial Zn assimilation genes. IL6 and ZIP4 played key roles in this network, and markedly affected znuB expression, intracellular viability and immune escape, as revealed by gene silencing. Moreover, EMSA and GFP reporter gene analysis showed that Fur sensed changes in Fe concentration to regulate znuCBA in P. plecoglossicida. Jointly, these findings suggest a trade-off for Zn between host and P. plecoglossicida, while ZnuC is important for P. plecoglossicida Zn acquisition.

Zur: Zinc-Sensing Transcriptional Regulator in a Diverse Set of Bacterial Species

Zinc (Zn) is the quintessential d block metal, needed for survival in all living organisms. While Zn is an essential element, its excess is deleterious, therefore, maintenance of its intracellular concentrations is needed for survival. The living organisms, during the course of evolution, developed proteins that can track the limitation or excess of necessary metal ions, thus providing survival benefits under variable environmental conditions. Zinc uptake regulator (Zur) is a regulatory transcriptional factor of the FUR superfamily of proteins, abundant among the bacterial species and known for its intracellular Zn sensing ability. In this study, we highlight the roles played by Zur in maintaining the Zn levels in various bacterial species as well as the fact that in recent years Zur has emerged not only as a Zn homeostatic regulator but also as a protein involved directly or indirectly in virulence of some pathogens. This functional aspect of Zur could be exploited in the ventures for the identification of newer antimicrobial targets. Despite extensive research on Zur, the insights into its overall regulon and its moonlighting functions in various pathogens yet remain to be explored. Here in this review, we aim to summarise the disparate functional aspects of Zur proteins present in various bacterial species.

Yersinia ruckeri strain SC09 disrupts proinflammatory activation via Toll/IL-1 receptor-containing protein STIR-3

Virulent pathogenic microorganisms often enhance their infectivity through immune evasion mechanisms. Our research on the integrative and conjugative element (ICE(r2)) of the virulent fish pathogen Yersinia ruckeri SC09 led to the identification of genes related to immune evasion (designated stir-1, stir-2, stir-3 and stir-4), among which stir-1 and stir-2 were determined as the key contributors to bacterial toxicity and immune evasion. Here, we further examined the ability of stir-3 to mediate immune evasion based on detailed bioinformatic analysis of ICE(r2) from Y. ruckeri SC09. Interactions among the translated STIR-1, STIR-2, STIR-3 and STIR-4 proteins in the secretory process were additionally explored. STIR-3 was positively correlated with bacterial toxicity and inhibited host toll-like receptor (TLR) signaling by interacting with MyD88, thereby facilitating bacterial survival in host cells. Importantly, our data showed co-secretion of STIR-1, STIR-2 and STIR-3 as a complex, with secretion failure occurring in the absence of any one of these proteins. While stir-1, stir-2, stir-3 and stir-4 genes werespecific to Y. ruckeri SC09, the ICE(r2) region where these genes were located is a mobile component widely distributed in bacteria. Therefore, the potential transmission risk of these immune evasion genes requires further research attention.

Modulation of Symbiotic Compatibility by Rhizobial Zinc Starvation Machinery

The rhizobium-legume symbiosis contributes around 65% of biological nitrogen fixation in agriculture systems and is critical for sustainable agriculture by reducing the amount of chemical nitrogen fertilizer being used. Rhizobial inocula have been commercialized for more than 100 years, but the efficiency of inoculation can vary among legume cultivars, field sites, and years. These long-lasting challenging problems impede the establishment of a sustainable agriculture, particularly in developing countries. Here, we report that rhizobial zinc starvation machinery containing a conserved high-affinity zinc transporter and accessory components makes cumulative contributions to modulating rhizobial symbiotic compatibility. This work highlights a critical role of largely unexplored nutritional immunity in the rhizobium-legume symbiosis, which makes zinc starvation machinery an attractive target for improving rhizobial symbiotic compatibility.

Pathogenic bacteria need high-affinity zinc uptake systems to counteract the nutritional immunity exerted by infected hosts. However, our understanding of zinc homeostasis in mutualistic systems such as the rhizobium-legume symbiosis is limited. Here, we show that the conserved high-affinity zinc transporter ZnuABC and accessory transporter proteins (Zip1, Zip2, and c06450) made cumulative contributions to nodulation of the broad-host-range strain Sinorhizobium fredii CCBAU45436. Zur acted as a zinc-dependent repressor for the znuC-znuB-zur operon, znuA, and c06450 by binding to the associated Zur box, but did not regulate zip1 and zip2. ZnuABC was the major zinc transporter. Combined mutants lacking znuA and one of the three accessory genes had more severe defects in nodulation and growth under zinc starvation conditions than the znuA mutant, though rhizoplane colonization by these mutants was not impaired. In contrast to the elite strain CCBAU45436, more drastic symbiotic defects were observed for the znuA mutants of other Sinorhizobium strains, which lack at least one of the three accessory genes in their genomes and are characterized by their limited host range and geographical distribution. The znu-derived mutants showed a higher expression level of nod genes involved in Nod factor biosynthesis and a reduced expression of genes encoding a type three secretion system and its effector NopP, which can interfere with the host immune system. Application of exogenous zinc restored the nodulation ability of these znu-derived mutants. Therefore, the conserved ZnuABC and accessory components in the zinc starvation machinery play an important role in modulating symbiotic compatibility.

Overcoming Fish Defences: The Virulence Factors of Yersinia ruckeri

Yersinia ruckeri is the causative agent of enteric redmouth disease, a bacterial infection of marine and freshwater fish. The disease mainly affects salmonids, and outbreaks have significant economic impact on fish farms all over the world. Vaccination routines are in place against the major serotypes of Y. ruckeri but are not effective in all cases. Despite the economic importance of enteric redmouth disease, a detailed molecular understanding of the disease is lacking. A considerable number of mostly omics-based studies have been performed in recent years to identify genes related to Y. ruckeri virulence. This review summarizes the knowledge on Y. ruckeri virulence factors. Understanding the molecular pathogenicity of Y. ruckeri will aid in developing more efficient vaccines and antimicrobial compounds directed against enteric redmouth disease.

Zinc transporters YbtX and ZnuABC are required for the virulence of Yersinia pestis in bubonic and pneumonic plague in mice

A number of bacterial pathogens require the ZnuABC Zinc (Zn²⁺) transporter and/or a second Zn²⁺ transport system to overcome Zn²⁺ sequestration by mammalian hosts. Previously we have shown that in addition to ZnuABC, Yersinia pestis possesses a second Zn²⁺ transporter that involves components of the yersiniabactin (Ybt), siderophore-dependent iron transport system. Synthesis of the Ybt siderophore and YbtX, a member of the major facilitator superfamily, are both critical components of the second Zn²⁺ transport system. Here we demonstrate that a ybtX znu double mutant is essentially avirulent in mouse models of bubonic and pneumonic plague while a ybtX mutant retains high virulence in both plague models. While sequestration of host Zn is a key nutritional immunity factor, excess Zn appears to have a significant antimicrobial role in controlling intracellular bacterial survival. Here, we demonstrate that ZntA, a Zn²⁺ exporter, plays a role in resistance to Zn toxicity in vitro, but that a zntA zur double mutant retains high virulence in both pneumonic and bubonic plague models and survival in macrophages. We also confirm that Ybt does not directly bind Zn²⁺in vitro under the conditions tested. However, we detect a significant increase in Zn²⁺-binding ability of filtered supernatants from a Ybt⁺ strain compared to those from a strain unable to produce the siderophore, supporting our previously published data that Ybt biosynthetic genes are involved in the production of a secreted Zn-binding molecule (zincophore). Our data suggest that Ybt or a modified Ybt participate in or promote Zn-binding activity in culture supernatants and is involved in Zn acquisition in Y. pestis.

The Infection Process of Yersinia ruckeri: Reviewing the Pieces of the Jigsaw Puzzle

Finding the keys to understanding the infectious process of Yersinia ruckeri was not a priority for many years due to the prompt development of an effective biotype 1 vaccine which was used mainly in Europe and USA. However, the gradual emergence of outbreaks in vaccinated fish, which have been reported since 2003, has awakened interest in the mechanism of virulence in this pathogen. Thus, during the last two decades, a large number of studies have considerably enriched our knowledge of many aspects of the pathogen and its interaction with the host. By means of both conventional and a variety of novel strategies, such as cell GFP tagging, bioluminescence imaging and optical projection tomography, it has been possible to determine three putative Y. ruckeri infection routes, the main point of entry for the bacterium being the gill lamellae. Moreover, a wide range of potential virulence factors have been highlighted by specific gene mutagenesis strategies or genome-wide transposon/plasmid insertion-based screening approaches, such us in vivo expression technology (IVET) and signature tagged mutagenesis (STM). Finally, recent proteomic and whole genomic analyses have allowed many of the genes and systems that are potentially implicated in the organism's pathogenicity and its adaptation to the host environmental conditions to be elucidated. Altogether, these studies contribute to a better understanding of the infectious process of Y. ruckeri in fish, which is crucial for the development of more effective strategies for preventing or treating enteric redmouth disease (ERM).

Genome Sequence of the Fish Pathogen Yersinia ruckeri SC09 Provides Insights into Niche Adaptation and Pathogenic Mechanism

Yersinia ruckeri is the etiologic agent of enteric red mouth disease (ERM), a severe fish disease prevailing in worldwide aquaculture industries. Here we report for the first time the complete genome of Y. ruckeri (Yersinia ruckeri) SC09, a highly virulent strain isolated from Ictalurus punctatus with severe septicemia. SC09 possesses a single chromosome of 3,923,491 base pairs, which contains 3651 predicted protein coding sequences (CDS), 19 rRNA genes, and 79 tRNA genes. Among the CDS, we have identified a Ysa locus containing genes encoding all the components of a type III secretion system (T3SS). Comparative analysis suggest that SC09-Ysa share extensive similarity in sequence, gene content, and gene arrangement with Salmonella enterica pathogenicity island 1 (SPI1) and chromosome-encoded T3SS from Yersinia enterocolitica biotype 1B. Furthermore, phylogenetic analysis shown that SC09-Ysa and SPI1-T3SS belong on the same branch of the phylogenetic tree. These results suggest that SC09-Ysa and SPI1-T3SS appear to mediate biological function to adapt to specific hosts with a similar niche, and both of them are likely to facilitate the development of an intracellular niche. In addition, our analysis also indicated that a substantial part of the SC09 genome might contribute to adaption in the intestinal microenvironment, including a number of proteins associated with aerobic or anaerobic respiration, signal transduction, and various stress reactions. Genomic analysis of the bacterium offered insights into the pathogenic mechanism associated with intracellular infection and intestinal survivability, which constitutes an important first step in understanding the pathogenesis of Y. ruckeri.

Manganese uptake and streptococcal virulence

Streptococcal solute-binding proteins (SBPs) associated with ATP-binding cassette transporters gained widespread attention first as ostensible adhesins, next as virulence determinants, and finally as metal ion transporters. In this mini-review, we will examine our current understanding of the cellular roles of these proteins, their contribution to metal ion homeostasis, and their crucial involvement in mediating streptococcal virulence. There are now more than 35 studies that have collected structural, biochemical and/or physiological data on the functions of SBPs across a broad range of bacteria. This offers a wealth of data to clarify the formerly puzzling and contentious findings regarding the metal specificity amongst this group of essential bacterial transporters. In particular we will focus on recent findings related to biological roles for manganese in streptococci. These advances will inform efforts aimed at exploiting the importance of manganese and manganese acquisition for the design of new approaches to combat serious streptococcal diseases.

Metal ion homeostasis in Listeria monocytogenes and importance in host-pathogen interactions

Listeria monocytogenes is responsible for one of the most life-threatening food-borne infections and the leading cause of food-poisoning associated deaths in the UK. Infection may be of the unborn/newly born infant where disease may manifest as listeric abortion, stillbirth or late-onset neonatal listeriosis, while in adults, infection usually affects the central nervous system causing meningitis. Crucial to the survival of L. monocytogenes, both inside and outside the host, is its ability to acquire metals which act as cofactors for a broad range of its cellular proteins. However, L. monocytogenes must also protect itself against the innate toxicity of metals. The importance of metals in host-pathogen interactions is illustrated by the restriction of metals (including zinc and iron) in vertebrates in response to infection and the use of high levels of metals (copper and zinc) as part of the antimicrobial defences within host phagocytes. As such, L. monocytogenes is equipped with various mechanisms to tightly control its cellular metal pools and avoid metal poisoning. These include multiple DNA-binding metal-responsive transcription factors, metal-acquisition, metal-detoxification and metal-storage systems, some of which represent key L. monocytogenes virulence determinants. This review discusses current knowledge of the role of metals in L. monocytogenes infections, with a focus on the mechanisms that contribute to zinc and copper homeostasis in this organism. The requirement to precisely control cellular metal levels may impose a vulnerability to L. monocytogenes which can be exploited in antimicrobials and therapeutics.

Deletion of the znuA virulence factor attenuates Actinobacillus pleuropneumoniae and confers protection against homologous or heterologous strain challenge

The znuA gene is known to be important for growth and survival in Escherichia coli, Haemophilus spp., Neisseria gonorrhoeae, and Pasteurella multocida under low Zn(2+) conditions. This gene is also present in Actinobacillus pleuropneumoniae serotype 1; therefore, the aim of this study was to investigate the existence of a similar role for the znuA gene in the growth and virulence of this organism. A precisely defined ΔznuA deletion mutant of A. pleuropneumoniae was constructed based on the sequence of the wild-type SLW01 using transconjugation and counterselection. This mutation was found to be lethal in low-Zn(2+) medium. Furthermore, the ΔznuA mutant strain exhibited attenuated virulence (≥22-fold) as well as reduced mortality and morbidity in a murine (Balb/C) model of infection. The majority of the bacteria were cleared from the lungs within 2 weeks. The ΔznuA mutant strain caused no adverse effects in pigs at doses of up to 1.0×10(9) CFU/mL. The ΔznuA mutant strain induced a significant immune response and conferred 80% and 100% protection on immunised pigs against challenge with A. pleuropneumoniae strains belonging to homologous or heterologous serovars, respectively, compared to the blank controls. The data obtained in this study indicate the potential of the mutant ΔznuA strain for development as a live vaccine capable of inducing reliable cross-serovar protection following intratracheal immunisation.

The heat sensitive factor (HSF) of Yersinia ruckeri is produced by an alkyl sulphatase involved in sodium dodecyl sulphate (SDS) degradation but not in virulence

BACKGROUND

The heat sensitive factor (HSF) of the fish pathogen Yersinia ruckeri was previously identified as an unusual band on SDS-PAGE. According to this, Y. ruckeri strains were classified in HSF+ and HSF - in terms of the presence/absence of the factor. Experiments carried out by injection challenge with HSF + strains caused high mortalities in rainbow trout. In contrast, HSF - strains did not cause mortality. In conclusion, HSF appeared to be a relevant virulence factor in Y. ruckeri.

RESULTS

We report here the identification and study of the gene coding for the enzyme involved in the production of HSF. Culture medium containing SDS and Coomassie brilliant blue dye was used to screen a mini-Tn5 Km2 mutant library of Y. ruckeri 150. Blue colonies lacking a surrounding creamy deposit, a phenotype described in former studies as HSF - , were identified. DNA sequence analysis of a selected mutant revealed that this had a transposon interruption in a chromosome-located gene which codes for a heat sensitive alkyl sulphatase of 78.7 kDa (YraS; Yersinia ruckeri alkyl sulphatase) which is able to degrade SDS to 1-dodecanol. As it was expected, the introduction of the yraS gene into an HSF - strain turned this into HSF + . Surprisingly, although the protein allows Y. ruckeri to degrade SDS, the bacterium could not use this compound as the sole carbon source. Moreover, the yraS mutant showed a similar level of SDS resistance to the parental strain. It was the interruption of the acrA gene which made Y. ruckeri sensitive to this compound. LD50 experiments showed a similar virulence of the yraS mutant and parental strain.

CONCLUSIONS

The HSF of Y. ruckeri is the product of the alkyl sulphatase YraS, able to degrade SDS to 1-dodecanol. This degradation is not linked to the utilization of SDS as a carbon source and surprisingly, the enzyme is not involved in bacterial virulence or in the high SDS resistance displayed by the bacterium. This role is played by the AcrAB-TolC system.

The Yersinia pestis siderophore, yersiniabactin, and the ZnuABC system both contribute to zinc acquisition and the development of lethal septicaemic plague in mice

Bacterial pathogens must overcome host sequestration of zinc (Zn(2+) ), an essential micronutrient, during the infectious disease process. While the mechanisms to acquire chelated Zn(2+) by bacteria are largely undefined, many pathogens rely upon the ZnuABC family of ABC transporters. Here we show that in Yersinia pestis, irp2, a gene encoding the synthetase (HMWP2) for the siderophore yersiniabactin (Ybt) is required for growth under Zn(2+) -deficient conditions in a strain lacking ZnuABC. Moreover, growth stimulation with exogenous, purified apo-Ybt provides evidence that Ybt may serve as a zincophore for Zn(2+) acquisition. Studies with the Zn(2+) -dependent transcriptional reporter znuA::lacZ indicate that the ability to synthesize Ybt affects the levels of intracellular Zn(2+) . However, the outer membrane receptor Psn and TonB as well as the inner membrane (IM) ABC transporter YbtPQ, which are required for Fe(3+) acquisition by Ybt, are not needed for Ybt-dependent Zn(2+) uptake. In contrast, the predicted IM protein YbtX, a member of the Major Facilitator Superfamily, was essential for Ybt-dependent Zn(2+) uptake. Finally, we show that the ZnuABC system and the Ybt synthetase HMWP2, presumably by Ybt synthesis, both contribute to the development of a lethal infection in a septicaemic plague mouse model.

The yrpAB operon of Yersinia ruckeri encoding two putative U32 peptidases is involved in virulence and induced under microaerobic conditions

In an attempt to dissect the virulence mechanisms of Yersinia ruckeri two adjacent genes, yrpA and yrpB, encoding putative peptidases belonging to the U32 family, were analyzed. Similar genes, with the same genetic organization were identified in genomic analysis of human-pathogenic yersiniae. RT-PCR studies indicated that these genes form an operon in Y. ruckeri. Transcriptional studies using an yrpB::lacZY fusion showed high levels of expression of these genes in the presence of peptone in the culture medium, as well as under oxygen-limited conditions. These two factors had a synergic effect on gene induction when both were present simultaneously during bacterial incubation, which indicates the important role that environmental conditions in the fish gut can play in the regulation of specific genes. LD 50 experiments using an yrpA insertional mutant strain demonstrated the participation of this gene in the virulence of Y. ruckeri.

Competition for zinc binding in the host-pathogen interaction

Due to its favorable chemical properties, zinc is used as a structural or catalytic cofactor in a very large number of proteins. Despite the apparent abundance of this metal in all cell types, the intracellular pool of loosely bound zinc ions available for biological exchanges is in the picomolar range and nearly all zinc is tightly bound to proteins. In addition, to limit bacterial growth, some zinc-sequestering proteins are produced by eukaryotic hosts in response to infections. Therefore, to grow and multiply in the infected host, bacterial pathogens must produce high affinity zinc importers, such as the ZnuABC transporter which is present in most Gram-negative bacteria. Studies carried in different bacterial species have established that disruption of ZnuABC is usually associated with a remarkable loss of pathogenicity. The critical involvement of zinc in a plethora of metabolic and virulence pathways and the presence of very low number of zinc importers in most bacterial species mark zinc homeostasis as a very promising target for the development of novel antimicrobial strategies.

Iron, copper, zinc, and manganese transport and regulation in pathogenic Enterobacteria: correlations between strains, site of infection and the relative importance of the different metal transport systems for virulence

For all microorganisms, acquisition of metal ions is essential for survival in the environment or in their infected host. Metal ions are required in many biological processes as components of metalloproteins and serve as cofactors or structural elements for enzymes. However, it is critical for bacteria to ensure that metal uptake and availability is in accordance with physiological needs, as an imbalance in bacterial metal homeostasis is deleterious. Indeed, host defense strategies against infection either consist of metal starvation by sequestration or toxicity by the highly concentrated release of metals. To overcome these host strategies, bacteria employ a variety of metal uptake and export systems and finely regulate metal homeostasis by numerous transcriptional regulators, allowing them to adapt to changing environmental conditions. As a consequence, iron, zinc, manganese, and copper uptake systems significantly contribute to the virulence of many pathogenic bacteria. However, during the course of our experiments on the role of iron and manganese transporters in extraintestinal Escherichia coli (ExPEC) virulence, we observed that depending on the strain tested, the importance of tested systems in virulence may be different. This could be due to the different set of systems present in these strains, but literature also suggests that as each pathogen must adapt to the particular microenvironment of its site of infection, the role of each acquisition system in virulence can differ from a particular strain to another. In this review, we present the systems involved in metal transport by Enterobacteria and the main regulators responsible for their controlled expression. We also discuss the relative role of these systems depending on the pathogen and the tissues they infect.

In vivo monitoring of Yersinia ruckeri in fish tissues: progression and virulence gene expression

In this study, the utilization of bioluminescence imaging (BLI) allowed us to define the progression of Yersinia ruckeri during the infection of rainbow trout. A luminescent Y. ruckeri 150 strain was engineered using the pCS26-Pac plasmid containing the lux operon from Photorhabdus luminescens. Two different models of infection of rainbow trout were defined depending on the route in which bacteria were administered, being the gut the major organ affected following bath immersion. This indicates that this organ is important for bacterial dissemination inside the fish and the establishment of the infection. Moreover, the expression of three previously selected operons by in vivo expression technology (IVET) was analysed, the yhlBA involved in the production of a haemolysin, the cdsAB related to the uptake of cysteine and the yctCBA implicated in citrate uptake. Apart from these factors, the expression of yrp1 encoding a serralysin metalloprotease involved in pathogenesis was also analysed. The results indicated that all of the assayed promoters were expressed during infection of rainbow trout. In addition to these findings, the methodology described in this work constitutes a useful model for studying the infection process in other fish pathogenic bacteria.

A novel cdsAB operon is involved in the uptake of L-cysteine and participates in the pathogenesis of Yersinia ruckeri

Application of in vivo expression technology (IVET) to Yersinia ruckeri, an important fish pathogen, allowed the identification of two adjacent genes that represent a novel bacterial system involved in the uptake and degradation of l-cysteine. Analysis of the translational products of both genes showed permease domains (open reading frame 1 [ORF1]) and amino acid position identities (ORF2) with the l-cysteine desulfidase from Methanocaldococcus jannaschii, a new type of enzyme involved in the breakdown of l-cysteine. The operon was named cdsAB (cysteine desulfidase) and is found widely in anaerobic and facultative bacteria. cdsAB promoter analysis using lacZY gene fusion showed highest induction in the presence of l-cysteine. Two cdsA and cdsB mutant strains were generated. The limited toxic effect and the low utilization of l-cysteine observed in the cdsA mutant, together with radiolabeled experiments, strongly suggested that CdsA is an l-cysteine permease. Fifty percent lethal dose (LD(50)) and competence index experiments showed that both the cdsA and cdsB loci were involved in the pathogenesis of the bacteria. In conclusion, this study has shown for the first time in bacteria the existence of an l-cysteine uptake system that together with an additional l-cysteine desulfidase-encoding gene constitutes a novel operon involved in bacterial virulence.

Role of a Zn-independent DksA in Zn homeostasis and stringent response

DksA is a global transcriptional regulator that directly interacts with RNA polymerase (RNAP) and, in conjunction with an alarmone ppGpp, alters transcription initiation at target promoters. DksA proteins studied to date contain a canonical Cys-4 Zn-finger motif thought to be essential for their proper folding and thus activity. In addition to the canonical DksA protein, the Pseudomonas aeruginosa genome encodes a closely related paralogue DksA2 that lacks the Zn-finger motif. Here, we report that DksA2 can functionally substitute for the canonical DksA in vivo in Escherichia coli and P. aeruginosa. We also demonstrate that DksA2 affects transcription by the E. coli RNAP in vitro similarly to DksA. The dksA2 gene is positioned downstream of a putative Zur binding site. Accordingly, we show that dksA2 expression is repressed by the presence of exogenous Zn, deletion of Zur results in constitutive expression of dksA2, and Zur binds specifically to the promoter region of dksA2. We also found that deletion of dksA2 confers a growth defect in the absence of Zn. Our data suggest that DksA2 plays a role in Zn homeostasis and serves as a back-up copy of the canonical Zn-dependent DksA in Zn-poor environments.

The yctCBA operon of Yersinia ruckeri, involved in in vivo citrate uptake, is not required for virulence

A three-gene operon, named yctCBA (Yersinia citrate transporter), induced by citrate and repressed by glucose was identified from a previously selected in vivo-induced (ivi) clone in the fish pathogen Yersinia ruckeri. Interestingly, despite being an ivi clone, the drastic growth reduction of the yctC mutant in the presence of citrate, and the relatively high content of this compound in rainbow trout serum, the operon was not required for virulence.

Znu is the predominant zinc importer in Yersinia pestis during in vitro growth but is not essential for virulence

Little is known about Zn homeostasis in Yersinia pestis, the plague bacillus. The Znu ABC transporter is essential for zinc (Zn) uptake and virulence in a number of bacterial pathogens. Bioinformatics analysis identified ZnuABC as the only apparent high-affinity Zn uptake system in Y. pestis. Mutation of znuACB caused a growth defect in Chelex-100-treated PMH2 growth medium, which was alleviated by supplementation with submicromolar concentrations of Zn. Use of transcriptional reporters confirmed that Zur mediated Zn-dependent repression and that it can repress gene expression in response to Zn even in the absence of Znu. Virulence testing in mouse models of bubonic and pneumonic plague found only a modest increase in survival in low-dose infections by the znuACB mutant. Previous studies of cluster 9 (C9) transporters suggested that Yfe, a well-characterized C9 importer for manganese (Mn) and iron in Y. pestis, might function as a second, high-affinity Zn uptake system. Isothermal titration calorimetry revealed that YfeA, the solute-binding protein component of Yfe, binds Mn and Zn with comparably high affinities (dissociation constants of 17.8 ± 4.4 nM and 6.6 ± 1.2 nM, respectively), although the complete Yfe transporter could not compensate for the loss of Znu in in vitro growth studies. Unexpectedly, overexpression of Yfe interfered with the znu mutant's ability to grow in low concentrations of Zn, while excess Zn interfered with the ability of Yfe to import iron at low concentrations; these results suggest that YfeA can bind Zn in the bacterial cell but that Yfe is incompetent for transport of the metal. In addition to Yfe, we have now eliminated MntH, FetMP, Efe, Feo, a substrate-binding protein, and a putative nickel transporter as the unidentified, secondary Zn transporter in Y. pestis. Unlike other bacterial pathogens, Y. pestis does not require Znu for high-level infectivity and virulence; instead, it appears to possess a novel class of transporter, which can satisfy the bacterium's Zn requirements under in vivo metal-limiting conditions. Our studies also underscore the need for bacterial cells to balance binding and transporter specificities within the periplasm in order to maintain transition metal homeostasis.

The UvrY response regulator of the BarA-UvrY two-component system contributes to Yersinia ruckeri infection of rainbow trout (Oncorhynchus mykiss)

To identify virulence-associated genes of a fish pathogen Yersinia ruckeri, we screened a total of 1056 mini-Tn5-Km2 signature-tagged mutants in rainbow trout by immersion challenge. Of 1056, 25 mutants were found survival-defective as they could not be re-isolated from fish kidney 7 days after infection. Mutated gene in F2-4 mutant, one of the 25 mutants, was homologous to uvrY that encodes UvrY response regulator of BarA-UvrY two-component system (TCS). Mutant F2-4 was significantly more sensitive (P < 0.05) to H2O2-mediated killing and was less able to infect Epithelioma papulosum cyprini cells. However, UvrY mutation did not affect survival of F2-4 mutant in the presence of non-immune fish serum and its ability to grow under iron starvation. In a time-course co-infection, mutant F2-4 had lower bacterial loads on day 1 itself, and by day 5 there was nearly a 1,000-fold difference in infection levels of the parent and mutant strains. The barA homolog of Y. ruckeri was PCR-amplified and sequence analyses identified four domains that were characteristic of hybrid histidine kinases. To conclude, the BarA-UvrY TCS contributes to the pathogenesis of Y. ruckeri in its natural host rainbow trout, possibly by regulating invasion of epithelial cells and sensitivity to oxidative stress induced by immune cells.

Yersinia ruckeri genes that attenuate survival in rainbow trout (Oncorhynchus mykiss) are identified using signature-tagged mutants

To identify genes that enable the enteric redmouth disease bacterium, Yersinia ruckeri, to persist in salmonid fish, 1056 signature-tagged mini-Tn5Km2 transposon mutants of a serotype 1 strain of Y. ruckeri, RS1154, were screened in rainbow trout by immersion infection. Two rounds of screening in fish identified 25 mutants that were not re-isolated from the kidney, 7 days post-infection. Six mutants were tested a third time in fish, in 1:1 competitive challenges with the parent strain; 4 failed to establish in kidney and 2 were present at low levels compared to the parent. Sequence analyses from the single transposon insertion sites in each of the 25 mutants identified genes with sequence homologies to genes for ZnuA, a periplasmic zinc-binding protein of ZnuABC transporter; the UvrY response regulator of BarA-UvrY two-component system; a PtrA protease of the insulin-degrading enzyme family; the RcpA protein of type IV bundle-forming pili; the ParA ATPase of a ParAB DNA-partitioning system; a Wzy polymerase; a polysaccharide deacetylase; a transporter belonging to the major facilitator superfamily and 7 hypothetical proteins of unknown function. The products of 5 of these mutated genes have predicted functions associated with cell surfaces or membranes, which could be important for survival of Y. ruckeri in rainbow trout, while other putative gene products could contribute to infection and invasion processes.