Urease is not involved in the virulence of Yersinia pseudotuberculosis in mice

Urease Expression in Pathogenic Yersinia enterocolitica Strains of Bio-Serotypes 2/O:9 and 1B/O:8 Is Differentially Regulated by the OmpR Regulator

Yersinia enterocolitica exhibits a dual lifestyle, existing as both a saprophyte and a pathogen colonizing different niches within a host organism. OmpR has been recognized as a regulator that controls the expression of genes involved in many different cellular processes and the virulence of pathogenic bacteria. Here, we have examined the influence of OmpR and varying temperature (26°C vs. 37°C) on the cytoplasmic proteome of Y. enterocolitica Ye9N (bio-serotype 2/O:9, low pathogenicity). Differential label-free quantitative proteomic analysis indicated that OmpR affects the cellular abundance of a number of proteins including subunits of urease, an enzyme that plays a significant role in acid tolerance and the pathogenicity of Y. enterocolitica. The impact of OmpR on the expression of urease under different growth conditions was studied in more detail by comparing urease activity and the transcription of ure genes in Y. enterocolitica strains Ye9N and Ye8N (highly pathogenic bio-serotype 1B/O:8). Urease expression was higher in strain Ye9N than in Ye8N and in cells grown at 26°C compared to 37°C. However, low pH, high osmolarity and the presence of urea did not have a clear effect on urease expression in either strain. Further analysis showed that OmpR participates in the positive regulation of three transcriptional units encoding the multi-subunit urease (ureABC, ureEF, and ureGD) in strain Ye9N, but this was not the case in strain Ye8N. Binding of OmpR to the ureABC and ureEF promoter regions was confirmed using an electrophoretic mobility shift assay, suggesting that this factor plays a direct role in regulating the transcription of these operons. In addition, we determined that OmpR modulates the expression of a ureR-like gene encoding a putative regulator of the ure gene cluster, but in the opposite manner, i.e., positively in Ye9N and negatively in Ye8N. These findings provide some novel insights into the function of OmpR in adaptation strategies of Y. enterocolitica.

RovM and CsrA Negatively Regulate Urease Expression in Yersinia pseudotuberculosis

Urease acts as an important acid resistance system and virulence factor that is widespread among microorganisms. RovM is a global regulator that regulates a series of genes and pathways including acid survival systems in the enteric bacterium Yersinia pseudotuberculosis (Yptb). However, whether RovM regulates the urease activity in Yptb was still unknown. In this study, by using qualitative and quantitative urease assays, we show that the urease expression responds to nutrient conditions and the RovM protein represses urease expression by binding to its promoter. A previously reported positive regulator OmpR activates urease activity but RovM plays a dominant role in different nutrient conditions. In addition, carbon storage regulator system A (CsrA), the upstream regulator of RovM, dramatically down-regulates urease activity possibly by its binding to the Shine-Dalgarno (SD) sequence of the mRNA encoding the urease. In conclusion, this study demonstrates that urease activity is strictly controlled by nutrient conditions and is down-regulated by the CsrA-RovM pathway.

Transcriptomic and Phenotypic Analysis Reveals New Functions for the Tat Pathway in Yersinia pseudotuberculosis

The twin-arginine translocation (Tat) system mediates the secretion of folded proteins that are identified via an N-terminal signal peptide in bacteria, plants, and archaea. Tat systems are associated with virulence in many bacterial pathogens, and our previous studies revealed that Tat-deficient Yersinia pseudotuberculosis was severely attenuated for virulence. Aiming to identify Tat-dependent pathways and phenotypes of relevance for in vivo infection, we analyzed the global transcriptome of parental and ΔtatC mutant strains of Y. pseudotuberculosis during exponential and stationary growth at 26°C and 37°C. The most significant changes in the transcriptome of the ΔtatC mutant were seen at 26°C during stationary-phase growth, and these included the altered expression of genes related to virulence, stress responses, and metabolism. Subsequent phenotypic analysis based on these transcriptome changes revealed several novel Tat-dependent phenotypes, including decreased YadA expression, impaired growth under iron-limited and high-copper conditions, as well as acidic pH and SDS. Several functionally related Tat substrates were also verified to contribute to these phenotypes. Interestingly, the phenotypic defects observed in the Tat-deficient strain were generally more pronounced than those in mutants lacking the Tat substrate predicted to contribute to that specific function. Altogether, this provides new insight into the impact of Tat deficiency on in vivo fitness and survival/replication of Y. pseudotuberculosis during infection.

IMPORTANCE

In addition to its established role in mediating the secretion of housekeeping enzymes, the Tat system has been recognized as being involved in infection. In some clinically relevant bacteria, such as Pseudomonas spp., several key virulence determinants can readily be identified among the Tat substrates. In enteropathogens, such as Yersinia spp., there are no obvious virulence determinants among the Tat substrates. Tat mutants show no growth defect in vitro but are highly attenuated in in vivo This makes Tat an attractive target for the development of novel antimicrobials. Therefore, it is important to establish the causes of the attenuation. Here, we show that the attenuation is likely due to synergistic effects of different Tat-dependent phenotypes that each contributes to lowered in vivo fitness.

Environmental Regulation of Yersinia Pathophysiology

Hallmarks of Yersinia pathogenesis include the ability to form biofilms on surfaces, the ability to establish close contact with eukaryotic target cells and the ability to hijack eukaryotic cell signaling and take over control of strategic cellular processes. Many of these virulence traits are already well-described. However, of equal importance is knowledge of both confined and global regulatory networks that collaborate together to dictate spatial and temporal control of virulence gene expression. This review has the purpose to incorporate historical observations with new discoveries to provide molecular insight into how some of these regulatory mechanisms respond rapidly to environmental flux to govern tight control of virulence gene expression by pathogenic Yersinia.

Silencing urease: a key evolutionary step that facilitated the adaptation of Yersinia pestis to the flea-borne transmission route

The arthropod-borne transmission route of Yersinia pestis, the bacterial agent of plague, is a recent evolutionary adaptation. Yersinia pseudotuberculosis, the closely related food-and water-borne enteric species from which Y. pestis diverged less than 6,400 y ago, exhibits significant oral toxicity to the flea vectors of plague, whereas Y. pestis does not. In this study, we identify the Yersinia urease enzyme as the responsible oral toxin. All Y. pestis strains, including those phylogenetically closest to the Y. pseudotuberculosis progenitor, contain a mutated ureD allele that eliminated urease activity. Restoration of a functional ureD was sufficient to make Y. pestis orally toxic to fleas. Conversely, deletion of the urease operon in Y. pseudotuberculosis rendered it nontoxic. Enzymatic activity was required for toxicity. Because urease-related mortality eliminates 30-40% of infective flea vectors, ureD mutation early in the evolution of Y. pestis was likely subject to strong positive selection because it significantly increased transmission potential.

Pathogenesis of Y. enterocolitica and Y. pseudotuberculosis in Human Yersiniosis

Yersiniosis is a food-borne illness that has become more prevalent in recent years due to human transmission via the fecal-oral route and prevalence in farm animals. Yersiniosis is primarily caused by Yersinia enterocolitica and less frequently by Yersinia pseudotuberculosis. Infection is usually characterized by a self-limiting acute infection beginning in the intestine and spreading to the mesenteric lymph nodes. However, more serious infections and chronic conditions can also occur, particularly in immunocompromised individuals. Y. enterocolitica and Y. pseudotuberculosis are both heterogeneous organisms that vary considerably in their degrees of pathogenicity, although some generalizations can be ascribed to pathogenic variants. Adhesion molecules and a type III secretion system are critical for the establishment and progression of infection. Additionally, host innate and adaptive immune responses are both required for yersiniae clearance. Despite the ubiquity of enteric Yersinia species and their association as important causes of food poisoning world-wide, few national enteric pathogen surveillance programs include the yersiniae as notifiable pathogens. Moreover, no standard exists whereby identification and reporting systems can be effectively compared and global trends developed. This review discusses yersinial virulence factors, mechanisms of infection, and host responses in addition to the current state of surveillance, detection, and prevention of yersiniosis.

Virulence determinants, drug resistance and mobile genetic elements of Laribacter hongkongensis: a genome-wide analysis

BACKGROUND

Laribacter hongkongensis is associated with community-acquired gastroenteritis and traveler's diarrhea. In this study, we performed an in-depth annotation of the genes in its genome related to the various steps in the infective process, drug resistance and mobile genetic elements.

RESULTS

For acid and bile resistance, L. hongkongensis possessed a urease gene cassette, two arc gene clusters and bile salt efflux systems. For intestinal colonization, it possessed a putative adhesin of the autotransporter family homologous to those of diffusely adherent Escherichia coli (E. coli) and enterotoxigenic E. coli. To evade from host defense, it possessed superoxide dismutase and catalases. For lipopolysaccharide biosynthesis, it possessed the same set of genes that encode enzymes for synthesizing lipid A, two Kdo units and heptose units as E. coli, but different genes for its symmetrical acylation pattern, and nine genes for polysaccharide side chains biosynthesis. It contained a number of CDSs that encode putative cell surface acting (RTX toxin and hemolysins) and intracellular cytotoxins (patatin-like proteins) and enzymes for invasion (outer membrane phospholipase A). It contained a broad variety of antibiotic resistance-related genes, including genes related to β-lactam (n = 10) and multidrug efflux (n = 54). It also contained eight prophages, 17 other phage-related CDSs and 26 CDSs for transposases.

CONCLUSIONS

The L. hongkongensis genome possessed genes for acid and bile resistance, intestinal mucosa colonization, evasion of host defense and cytotoxicity and invasion. A broad variety of antibiotic resistance or multidrug resistance genes, a high number of prophages, other phage-related CDSs and CDSs for transposases, were also identified.

Cra negatively regulates acid survival in Yersinia pseudotuberculosis

Survival in acidic environments is important for successful infection of gastrointestinal pathogens. Many bacteria have evolved elaborate mechanisms by inducing or repressing gene expression, which subsequently provide pH homeostasis and enable acid survival. In this study, we employed comparative proteomic analysis to identify the acid-responsive proteins of a food-borne enteric bacterium, Yersinia pseudotuberculosis. The expression level of eight proteins involved in carbohydrate metabolism was up- or downregulated over twofold at pH 4.5 compared with pH 7.0. The role of a global transcriptional regulator catabolite repressor/activator Cra was further studied in this acid survival process. lacZ-fusion analysis showed that expression of cra was repressed under acidic pH. Deletion of the cra gene increased acid survival by 10-fold, whereas complementation restored the wild-type phenotype. These results lead us to propose that, in response to acidic pH, the expression of cra gene is downregulated to increase acid survival. This is the first study to demonstrate the regulatory role of Cra in acid survival in an enteric bacterium.

Genes encoding specific nickel transport systems flank the chromosomal urease locus of pathogenic yersiniae

The transition metal nickel is an essential cofactor for a number of bacterial enzymes, one of which is urease. Prior to its incorporation into metalloenzyme active sites, nickel must be imported into the cell. Here, we report identification of two loci corresponding to nickel-specific transport systems in the gram-negative, ureolytic bacterium Yersinia pseudotuberculosis. The loci are located on each side of the chromosomal urease gene cluster ureABCEFGD and have the same orientation as the latter. The yntABCDE locus upstream of the ure genes encodes five predicted products with sequence homology to ATP-binding cassette nickel permeases present in several gram-negative bacteria. The ureH gene, located downstream of ure, encodes a single-component carrier which displays homology to polypeptides of the nickel-cobalt transporter family. Transporters with homology to these two classes are also present (again in proximity to the urease locus) in the other two pathogenic yersiniae, Y. pestis and Y. enterocolitica. An Escherichia coli nikA insertion mutant recovered nickel uptake ability following heterologous complementation with either the ynt or the ureH plasmid-borne gene of Y. pseudotuberculosis, demonstrating that each carrier is necessary and sufficient for nickel transport. Deletion of ynt in Y. pseudotuberculosis almost completely abolished bacterial urease activity, whereas deletion of ureH had no effect. Nevertheless, rates of nickel transport were significantly altered in both ynt and ureH mutants. Furthermore, the ynt ureH double mutant was totally devoid of nickel uptake ability, thus indicating that Ynt and UreH constitute the only routes for nickel entry. Both Ynt and UreH show selectivity for Ni(2+) ions. This is the first reported identification of genes coding for both kinds of nickel-specific permeases situated adjacent to the urease gene cluster in the genome of a microorganism.

The Yersinia pseudotuberculosis Yut protein, a new type of urea transporter homologous to eukaryotic channels and functionally interchangeable in vitro with the Helicobacter pylori UreI protein

Urea uptake in eukaryotes and prokaryotes occurs via diffusion or active transport across the cell membrane. Facilitated diffusion of urea in both types of organisms requires a single-component channel. In bacteria, these transport systems allow rapid access of urease to its substrate, resulting in ammonia production, which is needed either for resistance to acidity or as a nitrogen source. In Yersinia pseudotuberculosis, a ureolytic enteropathogenic bacterium, a gene of unknown function (yut) located near the urease locus was found to encode a putative membrane protein with weak homology to single-component eukaryotic urea transporters. When expressed in Xenopus oocytes, Yut greatly increases cellular permeability to urea. Inactivation of yut in Y. pseudotuberculosis results in diminished apparent urease activity and reduced resistance to acidity in vitro when urea is present in the medium. In the mouse model, bacterial colonization of the intestine mucosa is delayed with the Yut-deficient mutant. Although structurally unrelated, Yut and the Helicobacter pylori UreI urea channel were shown to be functionally interchangeable in vitro and are sufficient to allow urea uptake in both bacteria, thereby confirming their function in the respective parent organisms. Homologues of Yut were found in other yersiniae, Actinobacillus pleuropneumoniae, Brucella melitensis, Pseudomonas aeruginosa and Staphylococcus aureus. The Y. pseudotuberculosis Yut protein is therefore the first member of a novel class of bacterial urea permeases related to eukaryotic transporters.

Silencing and reactivation of urease in Yersinia pestis is determined by one G residue at a specific position in the ureD gene

Yersinia pestis, the plague agent, is a naturally nonureolytic microorganism, while all other Yersinia species display a potent urease activity. In this report we demonstrate that Y. pestis harbors a complete urease locus composed of three structural (ureABC) and four accessory (ureEFGD) genes. Absence of ureolytic activity is due to the presence of one additional G residue in a poly(G) stretch, which introduces a premature stop codon in ureD. The presence of the same additional G in eight other Y. pestis isolates indicates that this mutation is species specific. Spontaneous excision of the extra G occurs at a frequency of 10(-4) to 10(-5) and restores a ureolytic phenotype to Y. pestis. The virulence of two independent ureolytic clones of Y. pestis injected either intravenously, subcutaneously, or intragastrically did not differ from that of the parental strain in the mouse infection model. Coinfection experiments with an equal number of ureolytic and nonureolytic bacteria did not evidence any difference in the ability of the two variants to multiply in vivo and to cause a lethal infection. Altogether our results demonstrate that variation of one extra G residue in ureD determines the ureolytic activity of Y. pestis but does not affect its virulence for mice or its ability to multiply and disseminate.

Superantigen YPMa exacerbates the virulence of Yersinia pseudotuberculosis in mice

Yersinia pseudotuberculosis, a gram-negative bacterium responsible for enteric and systemic infection in humans, produces a superantigenic toxin designated YPMa (Y. pseudotuberculosis-derived mitogen). To assess the role of YPMa in the pathogenesis of Y. pseudotuberculosis, we constructed a superantigen-deficient mutant and compared its virulence in a mouse model of infection to the virulence of the wild-type strain. Determination of the survival rate after intravenous (i.v.) bacterial inoculation of OF1 mice clearly showed that inactivation of ypmA, encoding YPMa, reduced the virulence of Y. pseudotuberculosis. Mice infected i.v. with 10(4) and 10(5) wild-type bacteria died within 9 days, whereas mice infected with the ypmA mutant survived 12 and 3 days longer, respectively. This decreased virulence of the ypmA mutant strain was not due to an impaired colonization of the spleen, liver, or lungs. In contrast to i.v. challenge, bacterial inoculation by the intragastric (i.g.) route did not reveal any difference in virulence between wild-type Y. pseudotuberculosis and the ypmA mutant since the 50% lethal doses were identical for both strains. Moreover, inactivation of ypmA gene did not affect the bacterial growth of Y. pseudotuberculosis in Peyer's patches, mesenteric lymph nodes (MLNs), and spleen after oral infection. Histological studies of spleen, liver, lungs, heart, Peyer's patches, and MLNs after i.v. or i.g. challenge with the wild type or the ypmA mutant did not reveal any feature that can be specifically related to YPMa. Our data show that the superantigenic toxin YPMa contributes to the virulence of Y. pseudotuberculosis in systemic infection in mice.

Construction of urease-negative mutants of Yersinia enterocolitica serotypes O:3 and o:8: role of urease in virulence and arthritogenicity

Yersinia enterocolitica serotype O:3 and O:8 urease-negative mutants unable to express the 19-kDa beta subunit of urease were constructed and tested for virulence and arthritogenicity. Our results indicate that urease is needed for full virulence in oral infections and that it is not an arthritogenic factor in the rat model.