Characterization of common virulence plasmids in Yersinia species and their role in the expression of outer membrane proteins

The polyadenylase PAPI is required for virulence plasmid maintenance in pathogenic bacteria

Many species of pathogenic bacteria harbor critical plasmid-encoded virulence factors, and yet the regulation of plasmid replication is often poorly understood despite playing a critical role in plasmid-encoded gene expression. Human pathogenic Yersinia , including the plague agent Y. pestis and its close relative Y. pseudotuberculosis , require the type III secretion system (T3SS) virulence factor to subvert host defense mechanisms and colonize host tissues. The Yersinia T3SS is encoded on the IncFII p lasmid for Y ersinia v irulence (pYV). Several layers of gene regulation enables a large increase in expression of Yersinia T3SS genes at mammalian body temperature. Surprisingly, T3SS expression is also controlled at the level of gene dosage. The number of pYV molecules relative to the number of chromosomes per cell, referred to as plasmid copy number, increases with temperature. The ability to increase and maintain elevated pYV plasmid copy number, and therefore T3SS gene dosage, at 37°C is important for Yersinia virulence. In addition, pYV is highly stable in Yersinia at all temperatures, despite being dispensable for growth outside the host. Yet how Yersinia reinforces elevated plasmid replication and plasmid stability remains unclear. In this study, we show that the chromosomal gene pcnB encoding the polyadenylase PAP I is required for regulation of pYV plasmid copy number (PCN), maintenance of pYV in the bacterial population outside the host, robust T3SS activity, and Yersinia virulence in a mouse infection model. Likewise, pcnB /PAP I is also required for robust expression of the Shigella flexneri virulence plasmid-encoded T3SS. Furthermore, Yersinia and Shigella pcnB /PAP I is required for maintaining normal PCN of model antimicrobial resistance (AMR) plasmids whose replication is regulated by sRNA, thereby increasing antibiotic resistance by ten-fold. These data suggest that pcnB /PAP I contributes to the spread and stabilization of virulence and AMR plasmids in bacterial pathogens, and is essential in maintaining the gene dosage required to mediate plasmid-encoded traits. Importantly pcnB /PAP I has been bioinformatically identified in many species of bacteria despite being studied in only a few species to date. Our work highlights the potential importance of pcnB /PAP I in antibiotic resistance, and shows for the first time that pcnB /PAP I reinforces PCN and virulence plasmid stability in natural pathogenic hosts with a direct impact on bacterial virulence.

Author Summary

Many pathogens carry extrachromosomal DNA elements known as plasmids, which encode genes that confer bacterial virulence or antimicrobial resistance (AMR). Acquisition of these plasmids by bacteria can lead to the emergence of new pathogenic traits and the spread of AMR, yet the mechanisms by which plasmids are retained in bacterial populations particularly in the absence of selective pressure remain incompletely understood. Here we show that the major bacterial polyadenylase enzyme PAP I, encoded by the pcnB gene, is critical for the human pathogen Yersinia pseudotuberculosis to maintain its native virulence plasmid as well as AMR plasmids. Very little is known about the process of polyadenylation in bacteria, or the post-transcriptional addition of adenosine residues to the 3' end of transcripts. This study represents the first demonstration that PAP I-mediated polyadenylation contributes to bacterial pathogenesis.

Calcium-responsive plasmid copy number regulation is dependent on discrete YopD domains in Yersinia pseudotuberculosis

Yersinia pathogenicity depends mainly on a Type III Secretion System (T3SS) responsible for translocating effector proteins into the eukaryotic target cell cytosol. The T3SS is encoded on a 70 kb, low copy number virulence plasmid, pYV. A key T3SS regulator, YopD, is a multifunctional protein and consists of discrete modular domains that are essential for pore formation and translocation of Yop effectors. In Y. pseudotuberculosis, the temperature-dependent plasmid copy number increase that is essential for elevated T3SS gene dosage and virulence is also affected by YopD. Here, we found that the presence of intracellular YopD results in increased levels of the CopA-RNA and CopB, two inhibitors of plasmid replication. Secretion of YopD leads to decreased expression of copA and copB, resulting in increased plasmid copy number. Moreover, using a systematic mutagenesis of YopD mutants, we demonstrated that the same discrete modular domains important for YopD translocation are also necessary for both the regulation of plasmid copy number as well as copA and copB expression. Hence, Yersinia has evolved a mechanism coupling active secretion of a plasmid-encoded component of the T3SS, YopD, to the regulation of plasmid replication. Our work provides evidence for the cross-talk between plasmid-encoded functions with the IncFII replicon.

Inflammatory monocytes promote granuloma control of Yersinia infection

Granulomas are organized immune cell aggregates formed in response to chronic infection or antigen persistence. The bacterial pathogen Yersinia pseudotuberculosis (Yp) blocks innate inflammatory signalling and immune defence, inducing neutrophil-rich pyogranulomas (PGs) within lymphoid tissues. Here we uncover that Yp also triggers PG formation within the murine intestinal mucosa. Mice lacking circulating monocytes fail to form defined PGs, have defects in neutrophil activation and succumb to Yp infection. Yersinia lacking virulence factors that target actin polymerization to block phagocytosis and reactive oxygen burst do not induce PGs, indicating that intestinal PGs form in response to Yp disruption of cytoskeletal dynamics. Notably, mutation of the virulence factor YopH restores PG formation and control of Yp in mice lacking circulating monocytes, demonstrating that monocytes override YopH-dependent blockade of innate immune defence. This work reveals an unappreciated site of Yersinia intestinal invasion and defines host and pathogen drivers of intestinal granuloma formation.

Crystal structure of the Cys-NO modified YopH tyrosine phosphatase

Protein tyrosine phosphatases (PTPs) are key virulence factors in pathogenic bacteria, consequently, they have become important targets for new approaches against these pathogens, especially in the fight against antibiotic resistance. Among these targets of interest YopH (Yersinia outer protein H) from virulent species of Yersinia is an example. PTPs can be reversibly inhibited by nitric oxide (NO) since the oxidative modification of cysteine residues may influence the protein structure and catalytic activity. We therefore investigated the effects of NO on the structure and enzymatic activity of Yersinia enterocolitica YopH in vitro. Through phosphatase activity assays, we observe that in the presence of NO YopH activity was inhibited by 50%, and that this oxidative modification is partially reversible in the presence of DTT. Furthermore, YopH S-nitrosylation was clearly confirmed by a biotin switch assay, high resolution mass spectrometry (MS) and X-ray crystallography approaches. The crystal structure confirmed the S-nitrosylation of the catalytic cysteine residue, Cys403, while the MS data provide evidence that Cys221 and Cys234 might also be modified by NO. Interestingly, circular dichroism spectroscopy shows that the S-nitrosylation affects secondary structure of wild type YopH, though to a lesser extent on the catalytic cysteine to serine YopH mutant. The data obtained demonstrate that S-nitrosylation inhibits the catalytic activity of YopH, with effects beyond the catalytic cysteine. These findings are helpful for designing effective YopH inhibitors and potential therapeutic strategies to fight this pathogen or others that use similar mechanisms to interfere in the signal transduction pathways of their hosts.

Heightened Virulence of Yersinia Is Associated with Decreased Function of the YopJ Protein

Despite the maintenance of YopP/J alleles throughout the human-pathogenic Yersinia lineage, the benefit of YopP/J-induced phagocyte death for Yersinia pathogenesis in animals is not obvious. To determine how the sequence divergence of YopP/J has impacted Yersinia virulence, we examined protein polymorphisms in this type III secreted effector protein across 17 Yersinia species and tested the consequences of polymorphism in a murine model of subacute systemic yersiniosis. Our evolutionary analysis revealed that codon 177 has been subjected to positive selection; the Yersinia enterocolitica residue had been altered from a leucine to a phenylalanine in nearly all Yersinia pseudotuberculosis and Yersinia pestis strains examined. Despite this change being minor, as both leucine and phenylalanine have hydrophobic side chains, reversion of YopJ^F177 to the ancestral YopJ^L177 variant yielded a Y. pseudotuberculosis strain with enhanced cytotoxicity toward macrophages, consistent with previous findings. Surprisingly, expression of YopJ^F177L in the mildly attenuated ksgA^- background rendered the strain completely avirulent in mice. Consistent with this hypothesis that YopJ activity relates indirectly to Yersinia pathogenesis in vivo, ksgA^- strains lacking functional YopJ failed to kill macrophages but actually regained virulence in animals. Also, treatment with the antiapoptosis drug suramin prevented YopJ-mediated macrophage cytotoxicity and enhanced Y. pseudotuberculosis virulence in vivo. Our results demonstrate that Yersinia-induced cell death is detrimental for bacterial pathogenesis in this animal model of illness and indicate that positive selection has driven YopJ/P and Yersinia evolution toward diminished cytotoxicity and increased virulence, respectively.

The gatekeeper of Yersinia type III secretion is under RNA thermometer control

Many bacterial pathogens use a type III secretion system (T3SS) as molecular syringe to inject effector proteins into the host cell. In the foodborne pathogen Yersinia pseudotuberculosis, delivery of the secreted effector protein cocktail through the T3SS depends on YopN, a molecular gatekeeper that controls access to the secretion channel from the bacterial cytoplasm. Here, we show that several checkpoints adjust yopN expression to virulence conditions. A dominant cue is the host body temperature. A temperature of 37°C is known to induce the RNA thermometer (RNAT)-dependent synthesis of LcrF, a transcription factor that activates expression of the entire T3SS regulon. Here, we uncovered a second layer of temperature control. We show that another RNAT silences translation of the yopN mRNA at low environmental temperatures. The long and short 5’-untranslated region of both cellular yopN isoforms fold into a similar secondary structure that blocks ribosome binding. The hairpin structure with an internal loop melts at 37°C and thereby permits formation of the translation initiation complex as shown by mutational analysis, in vitro structure probing and toeprinting methods. Importantly, we demonstrate the physiological relevance of the RNAT in the faithful control of type III secretion by using a point-mutated thermostable RNAT variant with a trapped SD sequence. Abrogated YopN production in this strain led to unrestricted effector protein secretion into the medium, bacterial growth arrest and delayed translocation into eukaryotic host cells. Cumulatively, our results show that substrate delivery by the Yersinia T3SS is under hierarchical surveillance of two RNATs.

Temperature serves as reliable external cue for pathogenic bacteria to recognize the entry into or exit from a warm-blooded host. At the molecular level, a temperature of 37°C induces various virulence-related processes that manipulate host cell physiology. Here, we demonstrate the temperature-dependent synthesis of the secretion regulator YopN in the foodborne pathogen Yersinia pseudotuberculosis, a close relative of Yersinia pestis. YopN blocks secretion of effector proteins through the type III secretion system unless host cell contact is established. Temperature-specific regulation relies on an RNA structure in the 5’-untranslated region of the yopN mRNA, referred to as RNA thermometer, which allows ribosome binding and thus translation initiation only at an infection-relevant temperature of 37°C. A mutated variant of the thermosensor resulting in a closed conformation prevented synthesis of the molecular gatekeeper YopN and led to permanent secretion and defective translocation of virulence factors into host cells. We suggest that the RNA thermometer plays a critical role in adjusting the optimal cellular concentration of a surveillance factor that maintains the controlled translocation of virulence factors.

Molecular Characteristics of Escherichia coli Causing Bloodstream Infections During 2010-2015 in a Tertiary Hospital, Shanghai, China

BACKGROUND

The bloodstream infections (BSI) caused by Escherichia coli pose a serious threat to human health. To explore molecular characteristics of E. coli causing BSI, we collected E. coli isolates causing BSI in Huashan Hospital, Shanghai, China during 2010-2015.

METHODS

In all E. coli isolates causing BSI collected from this study, polymerase chain reaction (PCR) was used to detect ESBLs and carbapenemase genes, and minimum inhibitory concentrations (MICs) were determined with agar dilution method. Outer membrane proteins were examined by SDS-PAGE in carbapenem-resistant strains. The genetic background of bla KPC gene was investigated by combining next-generation sequencing with a PCR mapping approach. Conjugation and transformation experiments were performed to verify the mobilization of bla KPC. The transcription levels of the bla KPC gene were measured by RT-PCR.

RESULTS

During 2010-2015, a total of 207 E. coli BSI strains were isolated. The positive rates of β-lactamase resistant genes were 0.48% (bla KPC), 57% (bla TEM), 23.67% (bla CTX-M-1), 18.84% (bla CTX-M-9), and 1.93% (bla SHV). High rates of bla TEM, bla CTX-M-1, and bla CTX-M-9 were consistent with the poor activity of third-generation cephalosporins and aztreonam in vitro, except for carbapenem and β-lactamase inhibitor combinations. Low susceptibility rates were observed for piperacillin (25.1%) in contrast to the increased susceptibility when combined with β-lactamase inhibitors, namely piperacillin-tazobactam (90.8%). Only one KPC-producing E. coli strain was detected. Despite the combination of OmpC loss, the low expression level of KPC may be responsible for its lower resistance to carbapenems compared to E. coli DH5α (pKP12-100).

CONCLUSION

E. coli strains isolated from BSI were still highly susceptible to carbapenems and β-lactamase inhibitor combinations, and bla CTX-M was the dominant genotype of ESBLs. The low expression of bla KPC may be the reason for the low resistance to carbapenems.

Spatiotemporal Variations in Growth Rate and Virulence Plasmid Copy Number during Yersinia pseudotuberculosis Infection

Pathogenic Yersinia spp. depend on the activity of a potent virulence plasmid-encoded ysc/yop type 3 secretion system (T3SS) to colonize hosts and cause disease. It was recently shown that Yersinia pseudotuberculosis upregulates the virulence plasmid copy number (PCN) during infection and that the resulting elevated gene dose of plasmid-encoded T3SS genes is essential for virulence.

Pathogenic Yersinia spp. depend on the activity of a potent virulence plasmid-encoded ysc/yop type 3 secretion system (T3SS) to colonize hosts and cause disease. It was recently shown that Yersinia pseudotuberculosis upregulates the virulence plasmid copy number (PCN) during infection and that the resulting elevated gene dose of plasmid-encoded T3SS genes is essential for virulence. When and how this novel regulatory mechanism is deployed and regulates the replication of the virulence plasmid during infection is unknown. In the present study, we applied droplet digital PCR (ddPCR) to investigate the dynamics of Y. pseudotuberculosis virulence PCN variations and growth rates in infected mouse organs. We demonstrated that both PCN and growth varied in different tissues and over time throughout the course of infection, indicating that the bacteria adapted to discrete microenvironments during infection. The PCN was highest in Peyer’s patches and cecum during the clonal invasive phase of the infection, while the highest growth rates were found in the draining mesenteric lymph nodes. In deeper, systemic organs, the PCN was lower and more modest growth rates were recorded. Our study indicates that increased gene dosage of the plasmid-encoded T3SS genes is most important early in the infection during invasion of the host. The described ddPCR approach will greatly simplify analyses of PCN, growth dynamics, and bacterial loads in infected tissues and will be readily applicable to other infection models.

Complete Genome Assembly of Yersinia pseudotuberculosis IP2666pIB1

Yersinia pseudotuberculosis, closely related to Yersinia pestis, is a human pathogen and model organism for studying bacterial pathogenesis. To aid in genomic analysis and understanding bacterial virulence, we sequenced and assembled the complete genome of the human pathogen Yersinia pseudotuberculosis IP2666pIB1.

Yersinia pseudotuberculosis, closely related to Yersinia pestis, is a human pathogen and model organism for studying bacterial pathogenesis. To aid in genomic analysis and understanding bacterial virulence, we sequenced and assembled the complete genome of the human pathogen Yersinia pseudotuberculosis IP2666pIB1.

Bioluminescent tracing of a Yersinia pestis pCD1+-mutant and Yersinia pseudotuberculosis in subcutaneously infected mice

Yersinia pestis has evolved from Yersinia pseudotuberculosis serotype O:1b. A typical Y. pestis contains three plasmids: pCD1, pMT1 and pPCP1. However, some isolates only harbor pCD1 (pCD1⁺-mutant). Y. pestis and Y. pseudotuberculosis share a common plasmid (pCD1 or pYV), but little is known about whether Y. pseudotuberculosis exhibited plague-inducing potential before it was evolved into Y. pestis. Here, the luxCDABE::Tn5::kan was integrated into the chromosome of the pCD1⁺-mutant, Y. pseudotuberculosis or Escherichia coli K12 to construct stable bioluminescent strains for investigation of their dissemination in mice by bioluminescence imaging technology. After subcutaneous infection, the pCD1⁺-mutant entered the lymph nodes, followed by the liver and spleen, and, subsequently, the lungs, causing pathological changes in these organs. Y. pseudotuberculosis entered the lymph nodes, but not the liver, spleen and lungs. It also resided in the lymph nodes for several days, but did not cause lymphadenitis or pathological lesions. By contrast, E. coli K12-lux was not isolatable from mouse lymph nodes, liver, spleen and lungs. These results indicate that the pCD1⁺-mutant can cause typical bubonic and pneumonic plague-like diseases, and Y. pestis has inherited lymphoid tissue tropism from its ancestor rather than acquiring these properties independently.

Characterization of the Ruler Protein Interaction Interface on the Substrate Specificity Switch Protein in the Yersinia Type III Secretion System

Many pathogenic Gram-negative bacteria use the type III secretion system (T3SS) to deliver effector proteins into eukaryotic host cells. In Yersinia, the switch to secretion of effector proteins is induced first after intimate contact between the bacterium and its eukaryotic target cell has been established, and the T3SS proteins YscP and YscU play a central role in this process. Here we identify the molecular details of the YscP binding site on YscU by means of nuclear magnetic resonance (NMR) spectroscopy. The binding interface is centered on the C-terminal domain of YscU. Disrupting the YscU-YscP interaction by introducing point mutations at the interaction interface significantly reduced the secretion of effector proteins and HeLa cell cytotoxicity. Interestingly, the binding of YscP to the slowly self-cleaving YscU variant P264A conferred significant protection against autoproteolysis. The YscP-mediated inhibition of YscU autoproteolysis suggests that the cleavage event may act as a timing switch in the regulation of early versus late T3SS substrates. We also show that YscU_C binds to the inner rod protein YscI with a dissociation constant (K_d ) of 3.8 μm and with 1:1 stoichiometry. The significant similarity among different members of the YscU, YscP, and YscI families suggests that the protein-protein interactions discussed in this study are also relevant for other T3SS-containing Gram-negative bacteria.

'Add, stir and reduce': Yersinia spp. as model bacteria for pathogen evolution

Pathogenic species in the Yersinia genus have historically been targets for research aimed at understanding how bacteria evolve into mammalian pathogens. The advent of large-scale population genomic studies has greatly accelerated the progress in this field, and Yersinia pestis, Yersinia pseudotuberculosis and Yersinia enterocolitica have once again acted as model organisms to help shape our understanding of the evolutionary processes involved in pathogenesis. In this Review, we highlight the gene gain, gene loss and genome rearrangement events that have been identified by genomic studies in pathogenic Yersinia species, and we discuss how these findings are changing our understanding of pathogen evolution. Finally, as these traits are also found in the genomes of other species in the Enterobacteriaceae, we suggest that they provide a blueprint for the evolution of enteropathogenic bacteria.

Increased plasmid copy number is essential for Yersinia T3SS function and virulence

Pathogenic bacteria have evolved numerous virulence mechanisms that are essential for establishing infections. The enterobacterium Yersinia uses a type III secretion system (T3SS) encoded by a 70-kilobase, low-copy, IncFII-class virulence plasmid. We report a novel virulence strategy in Y. pseudotuberculosis in which this pathogen up-regulates the plasmid copy number during infection. We found that an increased dose of plasmid-encoded genes is indispensable for virulence and substantially elevates the expression and function of the T3SS. Remarkably, we observed direct, tight coupling between plasmid replication and T3SS function. This regulatory pathway provides a framework for further exploration of the environmental sensing mechanisms of pathogenic bacteria.

YopN and TyeA Hydrophobic Contacts Required for Regulating Ysc-Yop Type III Secretion Activity by Yersinia pseudotuberculosis

Yersinia bacteria target Yop effector toxins to the interior of host immune cells by the Ysc-Yop type III secretion system. A YopN-TyeA heterodimer is central to controlling Ysc-Yop targeting activity. A + 1 frameshift event in the 3-prime end of yopN can also produce a singular secreted YopN-TyeA polypeptide that retains some regulatory function even though the C-terminal coding sequence of this YopN differs greatly from wild type. Thus, this YopN C-terminal segment was analyzed for its role in type III secretion control. Bacteria producing YopN truncated after residue 278, or with altered sequence between residues 279 and 287, had lost type III secretion control and function. In contrast, YopN variants with manipulated sequence beyond residue 287 maintained full control and function. Scrutiny of the YopN-TyeA complex structure revealed that residue W₂₇₉ functioned as a likely hydrophobic contact site with TyeA. Indeed, a YopN_W279G mutant lost all ability to bind TyeA. The TyeA residue F₈ was also critical for reciprocal YopN binding. Thus, we conclude that specific hydrophobic contacts between opposing YopN and TyeA termini establishes a complex needed for regulating Ysc-Yop activity.

YscU/FlhB of Yersinia pseudotuberculosis Harbors a C-terminal Type III Secretion Signal

All type III secretion systems (T3SS) harbor a member of the YscU/FlhB family of proteins that is characterized by an auto-proteolytic process that occurs at a conserved cytoplasmic NPTH motif. We have previously demonstrated that YscUCC, the C-terminal peptide generated by auto-proteolysis of Yersinia pseudotuberculosis YscU, is secreted by the T3SS when bacteria are grown in Ca(2+)-depleted medium at 37 °C. Here, we investigated the secretion of this early T3S-substrate and showed that YscUCC encompasses a specific C-terminal T3S signal within the 15 last residues (U15). U15 promoted C-terminal secretion of reporter proteins like GST and YopE lacking its native secretion signal. Similar to the "classical" N-terminal secretion signal, U15 interacted with the ATPase YscN. Although U15 is critical for YscUCC secretion, deletion of the C-terminal secretion signal of YscUCC did neither affect Yop secretion nor Yop translocation. However, these deletions resulted in increased secretion of YscF, the needle subunit. Thus, these results suggest that YscU via its C-terminal secretion signal is involved in regulation of the YscF secretion.

A large family of antivirulence regulators modulates the effects of transcriptional activators in Gram-negative pathogenic bacteria

We have reported that transcription of a hypothetical small open reading frame (orf60) in enteroaggregative E. coli (EAEC) strain 042 is impaired after mutation of aggR, which encodes a global virulence activator. We have also reported that the cryptic orf60 locus was linked to protection against EAEC diarrhea in two epidemiologic studies. Here, we report that the orf60 product acts as a negative regulator of aggR itself. The orf60 protein product lacks homology to known repressors, but displays 44–100% similarity to at least fifty previously undescribed small (<10 kDa) hypothetical proteins found in many gram negative pathogen genomes. Expression of orf60 homologs from enterotoxigenic E. coli (ETEC) repressed the expression of the AraC-transcriptional ETEC regulator CfaD/Rns and its regulon in ETEC strain H10407. Complementation in trans of EAEC 042orf60 by orf60 homologs from ETEC and the mouse pathogen Citrobacter rodentium resulted in dramatic suppression of aggR. A C. rodentium orf60 homolog mutant showed increased levels of activator RegA and increased colonization of the adult mouse. We propose the name Aar (AggR-activated regulator) for the clinically and epidemiologically important orf60 product in EAEC, and postulate the existence of a large family of homologs among pathogenic Enterobacteriaceae and Pasteurellaceae. We propose the name ANR (AraC Negative Regulators) for this family.

We report here the identification and characterization of a new family of negative regulators in Gram-negative bacteria, including many pathotypes of diarrheagenic Enterobacteriaceae and members of the Pasteurellaceae. Members of this regulator family in enteroaggregative (EAEC) and enterotoxigenic E. coli (ETEC) and in Citrobacter rodentium downregulate the expression of positive regulator partners AggR, CfaD/Rns and RegA, respectively, all members of the AraC/XylS family of regulators. Accordingly, we propose the name ANR (AraC Negative Regulators) for this family. ANR members orf60 (termed Aar), orf02851 (Rnr), orf0450 and orf01070 (Cnr) from EAEC, C. rodentium and ETEC respectively were characterized in this study. Deletion of ANR homologs upregulated the expression of AggR and RegA in EAEC strain 042 and C. rodentium respectively; overexpression of orf60, orf02851, orf0450 and orf01070 in EAEC strain 042 down-regulated AggR. C. rodentium harboring a null mutation in orf02851 exhibited a significant increase in expression of the regA and RegA-regulated fimbriae. The orf02851 mutant showed higher levels of C. rodentium in feces and colonic contents, and greater weight loss compared to mice inoculated with the wild-type.

Synthetic chalcones and sulfonamides as new classes of Yersinia enterocolitica YopH tyrosine phosphatase inhibitors

YopH plays a relevant role in three pathogenic species of Yersinia. Due to its importance in the prevention of the inflammatory response of the host, this enzyme has become a valid target for the identification and development of new inhibitors. In this work, an in-house library of 283 synthetic compounds was assayed against recombinant YopH from Yersinia enterocolitica. From these, four chalcone derivatives and one sulfonamide were identified for the first time as competitive inhibitors of YopH with binding affinity in the low micromolar range. Molecular modeling investigations indicated that the new inhibitors showed similar binding modes, establishing polar and hydrophobic contacts with key residues of the YopH binding site.

Autoproteolysis and intramolecular dissociation of Yersinia YscU precedes secretion of its C-terminal polypeptide YscU(CC)

Type III secretion system mediated secretion and translocation of Yop-effector proteins across the eukaryotic target cell membrane by pathogenic Yersinia is highly organized and is dependent on a switching event from secretion of early structural substrates to late effector substrates (Yops). Substrate switching can be mimicked in vitro by modulating the calcium levels in the growth medium. YscU that is essential for regulation of this switch undergoes autoproteolysis at a conserved N↑PTH motif, resulting in a 10 kDa C-terminal polypeptide fragment denoted YscU(CC). Here we show that depletion of calcium induces intramolecular dissociation of YscU(CC) from YscU followed by secretion of the YscU(CC) polypeptide. Thus, YscU(CC) behaved in vivo as a Yop protein with respect to secretion properties. Further, destabilized yscU mutants displayed increased rates of dissociation of YscU(CC)in vitro resulting in enhanced Yop secretion in vivo at 30°C relative to the wild-type strain.These findings provide strong support to the relevance of YscU(CC) dissociation for Yop secretion. We propose that YscU(CC) orchestrates a block in the secretion channel that is eliminated by calcium depletion. Further, the striking homology between different members of the YscU/FlhB family suggests that this protein family possess regulatory functions also in other bacteria using comparable mechanisms.

Bacterial cell surface structures in Yersinia enterocolitica

Yersinia enterocolitica is a widespread member of the family of Enterobacteriaceae that contains both non-virulent and virulent isolates. Pathogenic Y. enterocolitica strains, especially belonging to serotypes O:3, O:5,27, O:8 and O:9 are etiologic agents of yersiniosis in animals and humans. Y. enterocolitica cell surface structures that play a significant role in virulence have been subject to many investigations. These include outer membrane (OM) glycolipids such as lipopolysaccharide (LPS) and enterobacterial common antigen (ECA) and several cell surface adhesion proteins present only in virulent Y. enterocolitica, i.e., Inv, YadA and Ail. While the yadA gene is located on the Yersinia virulence plasmid the Ail, Inv, LPS and ECA are chromosomally encoded. These structures ensure the correct architecture of the OM, provide adhesive properties as well as resistance to antimicrobial peptides and to host innate immune response mechanisms.

The C-terminal tail of Yersinia pseudotuberculosis YopM is critical for interacting with RSK1 and for virulence

Yersinia spp. undermine the immune responses of infected animals by translocating Yops directly into host cells with a type III secretion system. YopM, a leucine-rich repeat protein, is a critical virulence factor in infection. YopM localizes to both the nucleus and the cytoplasm in cultured cells, interacts with mammalian p90 ribosomal S6 kinase 1 (RSK1), and causes a decrease in NK cell populations in spleens. Little is known about the molecular interaction between YopM and RSK1 and its significance in pathogenesis. We performed a systematic deletion analysis of YopM in Yersinia pseudotuberculosis to determine which regions are required for RSK1 interactions, nuclear localization, virulence, and changes in immune cell populations during infection of mice. Full-length YopM associated with RSK1 in at least two protein complexes in infected cells, and deletion of its C-terminal tail abrogated all RSK1 interactions. The C-terminal tail was required for tissue colonization, as yopM mutants that failed to interact with RSK1 were as defective for tissue colonization as was a DeltayopM mutant; however, nuclear localization of YopM was not dependent on its RSK1 interaction. Mutants expressing YopM proteins which do not interact with RSK1 caused more pathology than did the DeltayopM mutant, suggesting that there are other RSK1-independent functions of YopM. Histopathological and flow cytometric analyses of spleens showed that infection with wild-type Y. pseudotuberculosis caused an influx of neutrophils, while mice infected with yopM mutants had increased numbers of macrophages. Decreases in NK cells after Y. pseudotuberculosis infection did not correlate with YopM expression. In conclusion, the C terminus of YopM is essential for RSK1 interactions and for virulence.

Yersinia pseudotuberculosis virulence determinants invasin, YopE, and YopT modulate RhoG activity and localization

The Yersinia pseudotuberculosis surface protein invasin binds to multiple beta1 integrins with high affinity, leading to misregulation of Rac1 activity. Upon host cell binding, alteration of Rho GTPase activity results from the action of several Yersinia outer proteins (Yops) that are translocated into the cytoplasm. We report here that three virulence determinants encoded by Y. pseudotuberculosis manipulate the Rho GTPase RhoG. Y. pseudotuberculosis binding to cells caused robust recruitment of RhoG to the site of attachment, which required high-affinity invasin-beta1 integrin association. Furthermore, inactivation of RhoG significantly reduced the efficiency of invasin-mediated bacterial internalization. To investigate the activation state of RhoG, a fluorescence resonance energy transfer-based activation biosensor was developed and used to show distinct spatial activation of RhoG at the site of bacterial attachment. The biosensor was also used to show efficient RhoG inactivation by Y. pseudotuberculosis YopE, a potent Rho GTPase activating protein. Additionally, RhoG mislocalization by the prenylcysteine endoprotease YopT was demonstrated by two independent assays. Functional bacterial uptake experiments demonstrated that RhoG activation can bypass a deficit in Rac1 activity. Interestingly, increasing the size of the particle gave results more consistent with a linear pathway, in which RhoG acts as an upstream activator of Rac1, indicating that increased surface area introduces constraints on the signaling pathways required for efficient internalization. Taken together, these data demonstrate the misregulation of RhoG by multiple Y. pseudotuberculosis virulence determinants. Since RhoG is imperative for proper neutrophil function, this misregulation may represent a unique mechanism by which Yersinia species dampen the immune response.

The role of relA and spoT in Yersinia pestis KIM5 pathogenicity

The ppGpp molecule is part of a highly conserved regulatory system for mediating the growth response to various environmental conditions. This mechanism may represent a common strategy whereby pathogens such as Yersinia pestis, the causative agent of plague, regulate the virulence gene programs required for invasion, survival and persistence within host cells to match the capacity for growth. The products of the relA and spoT genes carry out ppGpp synthesis. To investigate the role of ppGpp on growth, protein synthesis, gene expression and virulence, we constructed a ΔrelA ΔspoT Y. pestis mutant. The mutant was no longer able to synthesize ppGpp in response to amino acid or carbon starvation, as expected. We also found that it exhibited several novel phenotypes, including a reduced growth rate and autoaggregation at 26°C. In addition, there was a reduction in the level of secretion of key virulence proteins and the mutant was>1,000-fold less virulent than its wild-type parent strain. Mice vaccinated subcutaneously (s.c.) with 2.5×10⁴ CFU of the ΔrelA ΔspoT mutant developed high anti-Y. pestis serum IgG titers, were completely protected against s.c. challenge with 1.5×10⁵ CFU of virulent Y. pestis and partially protected (60% survival) against pulmonary challenge with 2.0×10⁴ CFU of virulent Y. pestis. Our results indicate that ppGpp represents an important virulence determinant in Y. pestis and the ΔrelA ΔspoT mutant strain is a promising vaccine candidate to provide protection against plague.

Attenuation and preserved immunogenic potential of Yersinia pseudotuberculosis mutant strains evidenced in oral pig model

Experimental oral infection of pigs with a parental Yersinia pseudotuberculosis strain pIB102, serotype O:3 and two mutant isogenic strains - pIB155,DeltayopK and pIB44,DeltaypkA has been carried out. Clinical findings, microbiological and immunological parameters were examined in dynamics from day 7 to day 60 post-infection (p.i.). All types of infections ran asymptomatically, without hyperthermia, loss of appetite, etc. Experiments on the blood parameters demonstrated a transient leucocytosis with lymphocytosis and monocytosis better expressed after yopK infection. Even though pig is usually known as a reservoir of yersiniae, bacterial colonization was found in mesenterial lymph nodes and tonsils on day 7, respectively 14 p.i. with parental strain, and only in tonsils on day 14 p.i. with both mutant strains. The augmented sensitivity of mutants to the bactericidal effect of leukocytes and blood sera is the characteristic feature of attenuation in their pathogenicity, compared to the parental strain. Comparative in vitro experiments on the immune response and immunostimulating capacity of Y. pseudotuberculosis mutant strains verify their preserved immunogenic potential, predominantly in case of yopK. Hyperplasia and strong activation of the lymph tissue of Peyer's patches, mesenterial lymph nodes, tonsils and spleen of pigs challenged with both mutant strains were proved as immunomorphological rearrangements. The results obtained give the reason to claim that the genetically constructed yopK null mutant strain is significantly attenuated but is still immunogenic and has the potential for a live vaccine carrier strain.

Reduced secretion of YopJ by Yersinia limits in vivo cell death but enhances bacterial virulence

Numerous microbial pathogens modulate or interfere with cell death pathways in cultured cells. However, the precise role of host cell death during in vivo infection remains poorly understood. Macrophages infected by pathogenic species of Yersinia typically undergo an apoptotic cell death. This is due to the activity of a Type III secreted effector protein, designated YopJ in Y. pseudotuberculosis and Y. pestis, and YopP in the closely related Y. enterocolitica. It has recently been reported that Y. enterocolitica YopP shows intrinsically greater capacity for being secreted than Y. pestis YopJ, and that this correlates with enhanced cytotoxicity observed for high virulence serotypes of Y. enterocolitica. The enzymatic activity and secretory capacity of YopP from different Y. enterocolitica serotypes have been shown to be variable. However, the underlying basis for differential secretion of YopJ/YopP, and whether reduced secretion of YopJ by Y. pestis plays a role in pathogenesis during in vivo infection, is not currently known. It has also been reported that similar to macrophages, Y. enterocolitica infection of dendritic cells leads to YopP-dependent cell death. We demonstrate here that in contrast to Y. enterocolitica, Y. pseudotuberculosis infection of bone marrow–derived dendritic cells does not lead to increased cell death. However, death of Y. pseudotuberculosis–infected dendritic cells is enhanced by ectopic expression of YopP in place of YopJ. We further show that polymorphisms at the N-terminus of the YopP/YopJ proteins are responsible for their differential secretion, translocation, and consequent cytotoxicity. Mutation of two amino acids in YopJ markedly enhanced both translocation and cytotoxicity. Surprisingly, expression of YopP or a hypersecreted mutant of YopJ in Y. pseudotuberculosis resulted in its attenuation in oral mouse infection. Complete absence of YopJ also resulted in attenuation of virulence, in accordance with previous observations. These findings suggest that control of cytotoxicity is an important virulence property for Y. pseudotuberculosis, and that intermediate levels of YopJ-mediated cytotoxicity are necessary for maximal systemic virulence of this bacterial pathogen.

The ability of bacterial pathogens to modulate death of infected host cells is an important virulence determinant. For pathogenic members of the genus Yersinia, the type III secreted effector protein YopJ/YopP is required for Yersinia-induced macrophage death. The YopJ protein is expressed by Y. pseudotuberculosis, while the ninety-four percent identical YopP protein is expressed by Y. enterocolitica. Y. enterocolitica infection also triggers YopP-dependent killing of dendritic cells, which are critical antigen presenting cells of the immune system. We demonstrate that in contrast to macrophages, dendritic cells are resistant to Y. pseudotuberculosis-mediated cytotoxicity. However, Y. pseudotuberculosis expressing YopP in place of YopJ was highly cytotoxic toward dendritic cells. This difference in cytotoxicity was attributable to a difference in the delivery of YopJ and YopP into mammalian cells. Furthermore, mutation of two amino acids at the N-terminus of YopJ enhanced its delivery and cytotoxicity. Remarkably, we found that enhancing the cytotoxicity of Y. pseudotuberculosis by expression of YopP led to its attenuation in a mouse model of Yersinia infection. This indicates that optimal virulence for a given pathogen requires careful regulation of virulence properties and highlights the potential evolutionary tradeoffs between cellular cytotoxicity and in vivo virulence.

Polynucleotide phosphorylase and the T3SS

Low temperatures as well as encounters with host phagocytes are two stresses that have been relatively well studied in many species of bacteria. The exoribonuclease polynucleotide phosphorylase (PNPase) has previously been shown to be required by several species of bacteria, including Yersinia, for low-temperature growth. We have shown that PNPase also enhances the ability of Yersinia to withstand the killing activities of murine macrophages. We have gone on to show that PNPase is required for the optimal functioning of Yersinia's type three secretion system (T3SS), an organelle that injects effector proteins directly into host cells. Surprisingly, the PNPase-mediated effect on T3SS activity is independent of PNPase's ribonuclease activity and instead requires only its S1 RNA-binding domain. In stark contrast, the catalytic activity of PNPase is strictly required for enhanced growth at low temperature. Preliminary experiments suggest that the RNA-binding interface of the S1 domain is critical for its T3SS-enhancing activity. Our findings indicate that PNPase plays versatile roles in promoting Yersinia's survival in response to stressful conditions.

Induction of the Yersinia type 3 secretion system as an all-or-none phenomenon

Pathogenic Yersinia spp. possess a protein secretion system, designated as type 3, that plays a clear role in promoting their survival vis-à-vis the macrophage. Inductive expression of the Yersinia type 3 secretion system (T3SS), triggered either by host cell contact, or, in the absence of host cells, by a reduction in extracellular calcium ion levels, is accompanied by a withdrawal from the bacterial division cycle. Here, we analyzed Ca(2+)-dependent induction of the T3SS at the single-cell level to understand how Yersinia coordinates pro-survival and growth-related activities. We utilized a novel high-throughput quantitative microscopy approach as well as flow cytometry to determine how Ca(2+) levels, T3SS expression, and cellular division are interrelated. Our analysis showed that there is a high degree of homogeneity in terms of T3SS expression levels among a population of Y. pseudotuberculosis cells following the removal of Ca(2+), and that T3SS expression appears to be independent of the cellular division cycle. Unexpectedly, our analysis showed that Ca(2+) levels are inversely related to the initiation of inductive T3SS expression, and not to the intensity of activation once initiated, thus providing a basis for the seemingly graded response of T3SS activation observed in bulk-level analyses. The properties of the system described here display both similarities to and differences from that of the lac operon first described 50 years ago by Novick and Weiner.

Serogroup-related escape of Yersinia enterocolitica YopE from degradation by the ubiquitin-proteasome pathway

Pathogenic Yersinia spp. employ a type III protein secretion system that translocates several Yersinia outer proteins (Yops) into the host cell to modify the host immune response. One strategy of the infected host cell to resist the bacterial attack is degradation and inactivation of injected bacterial virulence proteins through the ubiquitin-proteasome pathway. The cytotoxin YopE is a known target protein of this major proteolytic system in eukaryotic cells. Here, we investigated the sensitivity of YopE belonging to different enteropathogenic Yersinia enterocolitica serogroups to ubiquitination and proteasomal degradation. Analysis of the YopE protein levels in proteasome inhibitor-treated versus untreated cells revealed that YopE from the highly pathogenic Y. enterocolitica serotype O8 was subjected to proteasomal destabilization, whereas the YopE isotypes from serogroups O3 and O9 evaded degradation. Accumulation of YopE from serotypes O3 and O9 was accompanied by an enhanced cytotoxic effect. Using Yersinia strains that specifically produced YopE from either Y. enterocolitica O8 or O9, we found that only the YopE protein from serogroup O8 was modified by polyubiquitination, although both YopE isotypes were highly homologous. We determined two unique N-terminal lysines (K62 and K75) in serogroup O8 YopE, not present in serogroup O9 YopE, that served as polyubiquitin acceptor sites. Insertion of either lysine in serotype O9 YopE enabled its ubiquitination and destabilization. These results define a serotype-dependent difference in the stability and activity of the Yersinia effector protein YopE that could influence Y. enterocolitica pathogenesis.

The Ysa type 3 secretion system of Yersinia enterocolitica biovar 1B

Yersinia enterocolitica biovar 1B maintains two distinct and independently operating type 3 secretion (T3S) systems with the capacity to translocate toxic effector proteins into mammalian cells. Each of these T3S systems plays a role in the outcome of an infection by influencing different stages of infection. Recent investigations of the Ysa T3S system have revealed it is important for Y. enterocolitica survival during the gastrointestinal phase of infection. This sets this system apart from the Ysc T3S system which is important for systemic infections. Identification of the effector proteins has provided insight on how the Ysa T3S system modulates Y. enterocolitica interactions with the host. In part, the Ysa T3S system targets the innate immune response to suppress the ability of the host to rapidly clear an infection.

Oral inoculation with Type III secretion mutants of Yersinia pseudotuberculosis provides protection from oral, intraperitoneal, or intranasal challenge with virulent Yersinia

The enteric pathogen Yersinia pseudotuberculosis (Yptb) causes gastroenteritis, mesenteric lymphadenitis, and systemic infections in humans, livestock, and wild animals. Yptb Type III secretion system (pTTSS) mutants efficiently colonize lymphoid tissues, but not the gastrointestinal tract, spleen, or liver. Here, we show that a single oral inoculation of pTTSS mutants prevents morbidity in almost 100% of mice challenged intragastrically with virulent Yptb. In addition, a single oral inoculation of a pTTSS mutant protected 50% of mice challenged intraperitoneally or intranasally with virulent Yptb. In addition, the intranasally challenged mice that succumbed to infection lived significantly longer than non-immunized mice. Thus, pTTSS mutants can function as live attenuated vaccine when delivered orally. Potential uses for these attenuated strains include use as a livestock vaccine, a rodent plague control reagent in endemic areas around the world, and a vector for delivery of other antigens to the mesenteric lymph nodes.

The adaptor molecules LAT and SLP-76 are specifically targeted by Yersinia to inhibit T cell activation

T cell responses are critical to the survival of Yersinia-infected animals. Yersinia have the ability to directly suppress T lymphocyte activation through the virulence factor YopH, a tyrosine phosphatase. Using single cell video microscopy and FACS analysis, here we show that even an average of one Yersinia per T cell is sufficient to inhibit or alter T cell responses. This efficient inhibition is traced to specific targeting by YopH of the adaptor proteins, linker for activation of T cells (LAT) and SH2-domain–containing leukocyte protein of 76 kD (SLP-76), which are crucial for T cell antigen receptor (TCR) signaling. A catalytically inactive YopH translocated via the type III secretory pathway from the bacteria into T cells primarily binds to LAT and SLP-76. Furthermore, among the proteins of the TCR signaling pathway, the tyrosine phosphorylation levels of LAT and SLP-76 are the most affected in T cells exposed to low numbers of Yersinia pseudotuberculosis. This is the first example showing that a pathogen targets these adaptor proteins in the TCR signaling pathway, suggesting a novel mechanism by which pathogens may efficiently alter T cell–mediated immune responses.

Use of proteomics to identify novel virulence determinants that are required for Edwardsiella tarda pathogenesis

Edwardsiella tarda is an important cause of haemorrhagic septicaemia in fish and also of gastro- and extraintestinal infections in humans. Using a combination of comparative proteomics and TnphoA mutagenesis, we have identified five proteins that may contribute to E. tarda PPD130/91 pathogenesis. Lowered protein secretion, impaired autoaggregation and the absence of six proteins were observed only in three highly attenuated mutants when cultured in Dulbecco's modified eagle medium (DMEM). Five out of six proteins could be identified by their mass spectra. Three proteins were identified as putative effector proteins (EseB, EseC and EseD) that are homologous to SseB, SseC and SseD of a type III secretion system (TTSS) in Salmonella species. The other two were EvpA and EvpC, homologous to Eip20 and Eip18 in Edwardsiella ictaluri. The complete sequencing and homology studies of evpA-H indicate that similar gene clusters are widely distributed in other pathogens such as Escherichia, Salmonella, Vibrio and Yersinia species with unknown functions. Insertional inactivation and deletion of evpB or evpC led to lower replication rates in gourami phagocytes, and reduced protein secretion and virulence in blue gourami. Complementation of these deletion mutants showed partial recovery in the above three phenotypes, indicating that these genes are vital for E. tarda pathogenesis. The transport of the EvpC protein may not use the TTSS in E. tarda. The expression of EvpA and EvpC as well as EseB, EseC and EseD was temperature dependent (suppressed at 37 degrees C), and disruption of esrB affected their expression. The present study identifies two possible secretion systems (TTSS and Evp) that are vital for E. tarda pathogenesis.

Process of protein transport by the type III secretion system

The type III secretion system (TTSS) of gram-negative bacteria is responsible for delivering bacterial proteins, termed effectors, from the bacterial cytosol directly into the interior of host cells. The TTSS is expressed predominantly by pathogenic bacteria and is usually used to introduce deleterious effectors into host cells. While biochemical activities of effectors vary widely, the TTSS apparatus used to deliver these effectors is conserved and shows functional complementarity for secretion and translocation. This review focuses on proteins that constitute the TTSS apparatus and on mechanisms that guide effectors to the TTSS apparatus for transport. The TTSS apparatus includes predicted integral inner membrane proteins that are conserved widely across TTSSs and in the basal body of the bacterial flagellum. It also includes proteins that are specific to the TTSS and contribute to ring-like structures in the inner membrane and includes secretin family members that form ring-like structures in the outer membrane. Most prominently situated on these coaxial, membrane-embedded rings is a needle-like or pilus-like structure that is implicated as a conduit for effector translocation into host cells. A short region of mRNA sequence or protein sequence in effectors acts as a signal sequence, directing proteins for transport through the TTSS. Additionally, a number of effectors require the action of specific TTSS chaperones for efficient and physiologically meaningful translocation into host cells. Numerous models explaining how effectors are transported into host cells have been proposed, but understanding of this process is incomplete and this topic remains an active area of inquiry.

Process of protein transport by the type III secretion system

The type III secretion system (TTSS) of gram-negative bacteria is responsible for delivering bacterial proteins, termed effectors, from the bacterial cytosol directly into the interior of host cells. The TTSS is expressed predominantly by pathogenic bacteria and is usually used to introduce deleterious effectors into host cells. While biochemical activities of effectors vary widely, the TTSS apparatus used to deliver these effectors is conserved and shows functional complementarity for secretion and translocation. This review focuses on proteins that constitute the TTSS apparatus and on mechanisms that guide effectors to the TTSS apparatus for transport. The TTSS apparatus includes predicted integral inner membrane proteins that are conserved widely across TTSSs and in the basal body of the bacterial flagellum. It also includes proteins that are specific to the TTSS and contribute to ring-like structures in the inner membrane and includes secretin family members that form ring-like structures in the outer membrane. Most prominently situated on these coaxial, membrane-embedded rings is a needle-like or pilus-like structure that is implicated as a conduit for effector translocation into host cells. A short region of mRNA sequence or protein sequence in effectors acts as a signal sequence, directing proteins for transport through the TTSS. Additionally, a number of effectors require the action of specific TTSS chaperones for efficient and physiologically meaningful translocation into host cells. Numerous models explaining how effectors are transported into host cells have been proposed, but understanding of this process is incomplete and this topic remains an active area of inquiry.

Expression of a functional secreted YopN-TyeA hybrid protein in Yersinia pestis is the result of a +1 translational frameshift event

YopN is a secreted protein that prior to secretion directly interacts with the cytosolic SycN/YscB chaperone complex and TyeA. This study identifies a secreted YopN-TyeA hybrid protein that is expressed by Yersinia pestis, but not by Yersinia enterocolitica. DNA sequence analysis and site-directed mutagenesis studies demonstrate that the hybrid protein is the result of a +1 translational frameshift event.

Treatment of Salmonella enterica serovar Enteritidis with a sublethal concentration of trisodium phosphate or alkaline pH induces thermotolerance

The responses of Salmonella enterica serovar Enteritidis to a sublethal dose of trisodium phosphate (TSP) and its equivalent alkaline pH made with NaOH were examined. Pretreatment of S. enterica serovar Enteritidis cells with 1.5% TSP or pH 10.0 solutions resulted in a significant increase in thermotolerance, resistance to 2.5% TSP, resistance to high pH, and sensitivity to acid and H(2)O(2). Protein inhibition studies with chloramphenicol revealed that thermotolerance, unlike resistance to high pH, was dependent on de novo protein synthesis. Two-dimensional polyacrylamide gel electrophoresis (PAGE) of total cellular proteins from untreated control cells resolved as many as 232 proteins, of which 22 and 15% were absent in TSP- or alkaline pH-pretreated cells, respectively. More than 50% of the proteins that were either up- or down-regulated by TSP pretreatment were also up- or down-regulated by alkaline pH pretreatment. Sodium dodecyl sulfate-PAGE analysis of detergent-insoluble outer membrane proteins revealed the up-regulation of at least four proteins. Mass spectrometric analysis showed the up-regulated proteins to include those involved in the transport of small hydrophilic molecules across the cytoplasmic membrane and those that act as chaperones and aid in the export of newly synthesized proteins by keeping them in open conformation. Other up-regulated proteins included common housekeeping proteins like those involved in amino acid biosynthesis, nucleotide metabolism, and aminoacyl-tRNA biosynthesis. In addition to the differential expression of proteins following TSP or alkaline pH treatment, changes in membrane fatty acid composition were also observed. Alkaline pH- or TSP-pretreated cells showed a higher saturated and cyclic to unsaturated fatty acid ratio than did the untreated control cells. These results suggest that the cytoplasmic membrane could play a significant role in the induction of thermotolerance and resistance to other stresses following TSP or alkaline pH treatment.

Experimental infections with wild and mutant Yersinia pseudotuberculosis strains in rabbits

Experimental oral infections of rabbits with a wild-type Yersinia pseudotuberculosis strain (pIB102), and two null-mutants (yopK and ypkA) were carried out with the aim to explore the possibility to use mutant strains of Y. pseudotuberculosis as live carrier vaccine strains. The infectious process of the three strains proceed with passing hyperthermia, leucocytosis with granulocytosis, moderate monocytosis and a transient lymphopenia, better demonstrated at mutant strain infections. Short-term bacterial dissemination into the brain and viscera was observed at yopK infection. An augmented resistance to bactericidal activity of leucocytes at the initial phase of infection was followed by an increased sensitivity discovered earlier in case of yopK strain accompanied by at least 70- and 20-fold, respectively, for ypkA lower virulence for mice. The level of attenuation of yopK was accompanied by significant Yersinia specific IgG and IgM antibody response. Inflammatory foci were found by morphological examination in brain, lung and small intestines after infection with the wild-type strain, while such foci were only observed in brain and mesenterial lymph nodes after infection with the yopK mutant. After infection with the ypkA mutant foci were found in brain and spleen of the infected animals. Morphological changes in the lymphatic tissue of rabbits infected with mutant strains were consistent with induction of immunogenesis. The data suggest that genetically constructed yopK null-mutant exhibits characteristics that makes the strain suitable to be used as a live carrier vaccine to deliver heterologous antigens.

Carbapenem-resistant strain of Klebsiella oxytoca harboring carbapenem-hydrolyzing beta-lactamase KPC-2

We investigated a Klebsiella oxytoca isolate demonstrating resistance to imipenem, meropenem, extended-spectrum cephalosporins, and aztreonam. The MICs of both imipenem and meropenem were 32 microg/ml. The beta-lactamase activity against imipenem and meropenem was inhibited in the presence of clavulanic acid. Isoelectric focusing studies demonstrated five beta-lactamases with pIs of 8.2 (SHV-46), 6.7 (KPC-2), 6.5 (unknown), 6.4 (probable OXY-2), and 5.4 (TEM-1). The presence of the bla(SHV) and bla(TEM) genes was confirmed by specific PCR assays and DNA sequence analysis. Transformation and conjugation studies with Escherichia coli showed that the beta-lactamase with a pI of 6.7, Klebsiella pneumoniae carbapenemase-2 (KPC-2), was encoded on an approximately 70-kb conjugative plasmid that also carried SHV-46, TEM-1, and the beta-lactamase with a pI of 6.5. The bla(KPC-2) determinant was cloned in E. coli and conferred resistance to imipenem, meropenem, extended-spectrum cephalosporins, and aztreonam. The amino acid sequence of KPC-2 showed a single amino acid difference, S174G, when compared with KPC-1, another carbapenem-hydrolyzing beta-lactamase from K. pneumoniae 1534. Hydrolysis studies showed that purified KPC-2 hydrolyzed not only carbapenems but also penicillins, cephalosporins, and aztreonam. KPC-2 had the highest affinity for meropenem. The kinetic studies revealed that KPC-2 was inhibited by clavulanic acid and tazobactam. An examination of the outer membrane proteins of the parent K. oxytoca strain demonstrated that it expressed detectable levels of OmpK36 (the homolog of OmpC) and a higher-molecular-weight OmpK35 (the homolog of OmpF). Thus, carbapenem resistance in K. oxytoca 3127 is due to production of the Bush group 2f, class A, carbapenem-hydrolyzing beta-lactamase KPC-2. This beta-lactamase is likely located on a transposon that is part of a conjugative plasmid and thus has a very high potential for dissemination.

Aurintricarboxylic acid blocks in vitro and in vivo activity of YopH, an essential virulent factor of Yersinia pestis, the agent of plague

Yersinia are causative agents in human diseases ranging from gastrointestinal syndromes to Bubonic Plague. There is increasing risk of misuse of infectious agents, such as Yersinia pestis, as weapons of terror as well as instruments of warfare for mass destruction. YopH is an essential virulence factor whose protein-tyrosine phosphatase (PTP) activity is required for Yersinia pathogenicity. Consequently, there is considerable interest in developing potent and selective YopH inhibitors as novel anti-plague agents. We have screened a library of 720 structurally diverse commercially available carboxylic acids and identified 26 YopH inhibitors with IC50 values below 100 mum. The most potent and specific YopH inhibitor is aurintricarboxylic acid (ATA), which exhibits a Ki value of 5 nm for YopH and displays 6-120-fold selectivity in favor of YopH against a panel of mammalian PTPs. To determine whether ATA can block the activity of YopH in a cellular context, we have examined the effect of ATA on T-cell signaling in human Jurkat cells transfected with YopH. We show that YopH severely decreases the T-cell receptor-induced cellular tyrosine phosphorylation, ERK1/2 activity, and interleukin-2 transcriptional activity. We demonstrate that ATA can effectively block the inhibitory activity of YopH and restore normal T-cell function. These results provide a proof-of-concept for the hypothesis that small molecule inhibitors that selectively target YopH may be therapeutically useful. In addition, it is expected that potent and selective YopH inhibitors, such as ATA, should be useful reagents to delineate YopH's cellular targets in plague and other pathogenic conditions caused by Yersinia infection.

Crystal structure of the Yersinia protein-tyrosine phosphatase YopH complexed with a specific small molecule inhibitor

The pathogenic bacteria Yersinia are causative agents in human diseases ranging from gastrointestinal syndromes to bubonic plague. There is increasing risk of misuse of infectious agents, such as Yersinia pestis, as weapons of terror as well as instruments of warfare for mass destruction. Because the phosphatase activity of the Yersinia protein tyrosine phosphatase, YopH, is essential for virulence in the Yersinia pathogen, potent and selective YopH inhibitors are expected to serve as novel anti-plague agents. We have identified a specific YopH small molecule inhibitor, p-nitrocatechol sulfate (pNCS), which exhibits a Ki value of 25 microM for YopH and displays a 13-60-fold selectivity in favor of YopH against a panel of mammalian PTPs. To facilitate the understanding of the underlying molecular basis for tight binding and specificity, we have determined the crystal structure of YopH in complex with pNCS at a 2.0-A resolution. The structural data are corroborated by results from kinetic analyses of the interactions of YopH and its site-directed mutants with pNCS. The results show that while the interactions of the sulfuryl moiety and the phenyl ring with the YopH active site contribute to pNCS binding affinity, additional interactions of the hydroxyl and nitro groups in pNCS with Asp-356, Gln-357, Arg-404, and Gln-446 are responsible for the increased potency and selectivity. In particular, we note that residues Arg-404, Glu-290, Asp-356, and a bound water (WAT185) participate in a unique H-bonding network with the hydroxyl group ortho to the sulfuryl moiety, which may be exploited to design more potent and specific YopH inhibitors.

TyeA of Yersinia pseudotuberculosis is involved in regulation of Yop expression and is required for polarized translocation of Yop effectors

Type III secretion-dependent translocation of Yop (Yersinia outer proteins) effector proteins into host cells is an essential virulence mechanism common to the pathogenic Yersinia species. One unique feature of this mechanism is the polarized secretion of Yops, i.e. Yops are only secreted at the site of contact with the host cell and not to the surrounding medium. In vitro, secretion occurs in Ca2+-depleted media, a condition believed to somehow mimic cell contact. Three proteins, YopN, LcrG and TyeA have been suggested to control secretion and mutating any of these genes results in constitutive secretion. In addition, in Y. enterocolitica TyeA has been implied to be specifically required for delivery of a subset of Yop effectors into infected cells. In this work we have investigated the role of TyeA in secretion and translocation of Yop effectors by Y. pseudotuberculosis. An in frame deletion mutant of tyeA was found to be temperature-sensitive for growth and this phenotype correlated to a lowered expression of the negative regulatory element LcrQ. In medium containing Ca2+, Yop expression was somewhat elevated compared to the wild-type strain and low levels of Yop secretion was also seen. Somewhat surprisingly, expression and secretion of Yops was lower than for the wild-type strain when the tyeA mutant was grown in Ca2+-depleted medium. Translocation of YopE, YopH, YopJ and YopM into infected HeLa cells was significantly lower in comparison with the isogenic wild-type strain and Yop proteins could also be recovered in the tissue culture medium. This indicated that the tyeA mutant had lost the ability to translocate Yop proteins by a polarized mechanism. In order to exclude that the defect in translocation seen in the tyeA mutant was a result of lowered expression/secretion of Yops, a double lcrQ/tyeA mutant was constructed. This strain was de-repressed for Yop expression and secretion but was still impaired for translocation of both YopE and YopM. In addition, the low level of YopE translocation in the tyeA mutant was independent of the YopE chaperone YerA/SycE. TyeA was found to localize to the cytoplasm of the bacterium and we were unable to find any evidence that TyeA was secreted or surface located. From our studies in Y. pseudotuberculosis we conclude that TyeA is involved in regulation of Yop expression and required for polarized delivery of Yop effectors in general and is not as suggested in Y. enterocolitica directly required for translocation of a subset of Yop effectors.

Evidence for targeting of Yop effectors by the chromosomally encoded Ysa type III secretion system of Yersinia enterocolitica

Yersinia enterocolitica O:8 has two contact-dependent type III secretion systems (TTSSs). The Ysa TTSS is encoded by a set of genes located on the chromosome and exports Ysp proteins. The Ysc TTSS and the Yop effector proteins it exports are encoded by genes located on plasmid pYVe8081. In this study, secretion of YspG, YspH, and YspJ by the Ysa TTSS was shown to require pYVe8081. Furthermore, mutations that blocked the function of the Ysc TTSS did not affect YspG, YspH, and YspJ production. This indicated that YspG, YspH, and YspJ are encoded by genes located on pYVe8081 and that they may correspond to Yops. A comparison of Ysps with Yop effectors secreted by Y. enterocolitica indicated that YspG, YspH, and YspJ have apparent molecular masses similar to those of YopN, YopP, and YopE, respectively. Immunoblot analysis demonstrated that antibodies directed against YopN, YopP, and YopE recognized YspG, YspH, and YspJ. Furthermore, mutations in yopN, yopP, and yopE specifically blocked YopN, YopP, and YopE secretion by the Ysc TTSS and YspG, YspH, and YspJ secretion by the Ysa TTSS. These results indicate YspG, YspH, and YspJ are actually YopN, YopP, and YopE. Additional analysis demonstrated that YopP and YspH secretion was restored to yopP mutants by complementation in trans with a wild-type copy of the yopP gene. Examination of Y. enterocolitica-infected J774A.1 macrophages revealed that both the Ysc and Ysa TTSSs contribute to YopP-dependent suppression of tumor necrosis factor alpha production. This indicates that both the Ysa and Ysc TTSSs are capable of targeting YopP and that they influence Y. enterocolitica interactions with macrophages. Taken together, these results suggest that the Ysa and Ysc TTSSs contribute to Y. enterocolitica virulence by exporting both unique and common subsets of effectors.

Interactions among strategies associated with bacterial infection: pathogenicity, epidemicity, and antibiotic resistance

Infections have been the major cause of disease throughout the history of human populations. With the introduction of antibiotics, it was thought that this problem should disappear. However, bacteria have been able to evolve to become antibiotic resistant. Nowadays, a proficient pathogen must be virulent, epidemic, and resistant to antibiotics. Analysis of the interplay among these features of bacterial populations is needed to predict the future of infectious diseases. In this regard, we have reviewed the genetic linkage of antibiotic resistance and bacterial virulence in the same genetic determinants as well as the cross talk between antibiotic resistance and virulence regulatory circuits with the aim of understanding the effect of acquisition of resistance on bacterial virulence. We also discuss the possibility that antibiotic resistance and bacterial virulence might prevail as linked phenotypes in the future. The novel situation brought about by the worldwide use of antibiotics is undoubtedly changing bacterial populations. These changes might alter the properties of not only bacterial pathogens, but also the normal host microbiota. The evolutionary consequences of the release of antibiotics into the environment are largely unknown, but most probably restoration of the microbiota from the preantibiotic era is beyond our current abilities.

Structural and functional properties of Yersinia pestis Caf1 capsular antigen and their possible role in fulminant development of primary pneumonic plague

Yersinia pestis capsular antigen Caf1 is shown to be a beta-structural protein that in polymeric form possesses very high conformational stability. Different approaches show that a dimer is the minimal cooperative block of Caf1 adhesin. Caf1 dimer interacts effectively with IL-1 receptors of human macrophage and epithelial cells. The specificity of such interaction is confirmed by the inhibition of IL-1alpha binding by Caf1. The Caf1 role in pneumonic plague pathogenesis is discussed.

Role of fraction 1 antigen of Yersinia pestis in inhibition of phagocytosis

Yersinia pestis, the causative agent of plague, expresses a capsule-like antigen, fraction 1 (F1), at 37 degrees C. F1 is encoded by the caf1 gene located on the large 100-kb pFra plasmid, which is unique to Y. pestis. F1 is a surface polymer composed of a protein subunit, Caf1, with a molecular mass of 15.5 kDa. The secretion and assembly of F1 require the caf1M and caf1A genes, which are homologous to the chaperone and usher protein families required for biogenesis of pili. F1 has been implicated to be involved in the ability of Y. pestis to prevent uptake by macrophages. In this study we addressed the role of F1 antigen in inhibition of phagocytosis by the macrophage-like cell line J774. The Y. pestis strain EV76 was found to be highly resistant to uptake by J774 cells. An in-frame deletion of the caf1M gene of the Y. pestis strain EV76 was constructed and found to be unable to express F1 polymer on the bacterial surface. This strain had a somewhat lowered ability to prevent uptake by J774 cells. Strain EV76C, which is cured for the virulence plasmid common to the pathogenic Yersinia species, was, as expected, much reduced in its ability to resist uptake. A strain lacking both the virulence plasmid and caf1M was even further hampered in the ability to prevent uptake and, in this case, essentially all bacteria (95%) were phagocytosed. Thus, F1 and the virulence plasmid-encoded type III system act in concert to make Y. pestis highly resistant to uptake by phagocytes. In contrast to the type III effector proteins YopE and YopH, F1 did not have any influence on the general phagocytic ability of J774 cells. Expression of F1 also reduced the number of bacteria that interacted with the macrophages. This suggests that F1 prevents uptake by interfering at the level of receptor interaction in the phagocytosis process.