The genus Yersinia: biochemistry and genetics of virulence

The Podovirus ϕ80-18 Targets the Pathogenic American Biotype 1B Strains of Yersinia enterocolitica

We report here the complete genome sequence and characterization of Yersinia bacteriophage vB_YenP_ϕ80-18. ϕ80-18 was isolated in 1991 using a Y. enterocolitica serotype O:8 strain 8081 as a host from a sewage sample in Turku, Finland, and based on its morphological and genomic features is classified as a podovirus. The genome is 42 kb in size and has 325 bp direct terminal repeats characteristic for podoviruses. The genome contains 57 predicted genes, all encoded in the forward strand, of which 29 showed no similarity to any known genes. Phage particle proteome analysis identified altogether 24 phage particle-associated proteins (PPAPs) including those identified as structural proteins such as major capsid, scaffolding and tail component proteins. In addition, also the DNA helicase, DNA ligase, DNA polymerase, 5'-exonuclease, and the lytic glycosylase proteins were identified as PPAPs, suggesting that they might be injected together with the phage genome into the host cell to facilitate the take-over of the host metabolism. The phage-encoded RNA-polymerase and DNA-primase were not among the PPAPs. Promoter search predicted the presence of four phage and eleven host RNA polymerase -specific promoters in the genome, suggesting that early transcription of the phage is host RNA-polymerase dependent and that the phage RNA polymerase takes over later. The phage tolerates pH values between 2 and 12, and is stable at 50°C but is inactivated at 60°C. It grows slowly with a 50 min latent period and has apparently a low burst size. Electron microscopy revealed that the phage has a head diameter of about 60 nm, and a short tail of 20 nm. Whole-genome phylogenetic analysis confirmed that ϕ80-18 belongs to the Autographivirinae subfamily of the Podoviridae family, that it is 93.2% identical to Yersinia phage fHe-Yen3-01. Host range analysis showed that ϕ80-18 can infect in addition to Y. enterocolitica serotype O:8 strains also strains of serotypes O:4, O:4,32, O:20 and O:21, the latter ones representing similar to Y. enterocolitica serotype O:8, the American pathogenic biotype 1B strains. In conclusion, the phage ϕ80-18 is a promising candidate for the biocontrol of the American biotype 1B Y. enterocolitica.

Effect of natural polymorphism on structure and function of the Yersinia pestis outer membrane porin F (OmpF protein): a computational study

The Yersinia pestis outer membrane porin F (OmpF) is a transmembrane protein located in the outer membrane of this Gram-negative bacterium which is the causative agent of plague, where it plays a significant role in controlling the selective permeability of the membrane. The amino acid sequences of OmpF proteins from 48 Y. pestis strains representing all currently available phylogenetic groups of this Gram-negative bacterium were recently deduced. Comparison of these amino acid sequences revealed that the OmpF can be present in four isoforms, the pestis-pestis type, and the pestis-microtus types I, II, and III. OmpF of the most recent pestis-pestis type has an alanine residue at the position 148, where all the pestis-microtus types have threonine there (T148A polymorphism). The variability of different pestis-microtus types is caused by an additional polymorphism at the 193rd position, where the OmpFs of the pestis-microtus type II and type III have isoleucine-glycine (IG⁺₁₉₃) or isoleucine-glycine-isoleucine-glycine (IGIG⁺₁₉₃) insertions, respectively (IG⁺₁₉₃ and IGIG⁺₁₉₃ polymorphism). To investigate potential effects of these sequence polymorphisms on the structural properties of the OmpF protein, we conducted multi-level computational analysis of its isoforms. Analysis of the I-TASSER-generated 3D-models revealed that the Yersinia OmpF is very similar to other non-specific enterobacterial porins. The T148A polymorphism affected a loop located in the external vestibule of the OmpF channel, whereas IG⁺₁₉₃ and IGIG⁺₁₉₃ polymorphisms affected one of its β-strands. Our analysis also suggested that polymorphism has moderate effect on the predicted local intrinsic disorder predisposition of OmpF, but might have some functional implementations.

Disruption of the membrane nuclease gene (MBOVPG45_0215) of Mycoplasma bovis greatly reduces cellular nuclease activity

Although the complete genome sequences of three strains of Mycoplasma bovis are available, few studies have examined gene function in this important pathogen. Mycoplasmas lack the biosynthetic machinery for the de novo synthesis of nucleic acid precursors, so nucleases are likely to be essential for them to acquire nucleotide precursors. Three putative membrane nucleases have been annotated in the genome of M. bovis strain PG45, MBOVPG45_0089 and MBOVPG45_0310, both of which have the thermonuclease (TNASE_3) functional domain, and MBOVPG45_0215 (mnuA), which has an exonuclease/endonuclease/phosphatase domain. While previous studies have demonstrated the function of TNASE_3 domain nucleases in several mycoplasmas, quantitative comparisons of the contributions of different nucleases to cellular nuclease activity have been lacking. Mapping of a library of 319 transposon mutants of M. bovis PG45 by direct genome sequencing identified mutants with insertions in MBOVPG45_0310 (the Δ0310 mutant) and MBOVPG45_0215 (the Δ0215 mutant). In this study, the detection of the product of MBOVPG45_0215 in the Triton X-114 fraction of M. bovis cell lysates, its cell surface exposure, and its predicted signal peptide suggested that it is a surface-exposed lipoprotein nuclease. Comparison of a ΔmnuA mutant with wild-type M. bovis on native and denatured DNA gels and in digestion assays using double-stranded phage λ DNA and closed circular plasmid DNA demonstrated that inactivation of this gene abolishes most of the cellular exonuclease and endonuclease activity of M. bovis. This activity could be fully restored by complementation with the wild-type mnuA gene, demonstrating that MnuA is the major cellular nuclease of M. bovis.

IMPORTANCE

Nucleases are thought to be important contributors to virulence and crucial for the maintenance of a nutritional supply of nucleotides in mycoplasmas that are pathogenic in animals. This study demonstrates for the first time that of the three annotated cell surface nuclease genes in an important pathogenic mycoplasma, the homologue of the thermostable nuclease identified in Gram-positive bacteria is responsible for the majority of the nuclease activity detectable in vitro.

Roles of chaperone/usher pathways of Yersinia pestis in a murine model of plague and adhesion to host cells

Yersinia pestis and many other Gram-negative pathogenic bacteria use the chaperone/usher (CU) pathway to assemble virulence-associated surface fibers termed pili or fimbriae. Y. pestis has two well-characterized CU pathways: the caf genes coding for the F1 capsule and the psa genes coding for the pH 6 antigen. The Y. pestis genome contains additional CU pathways that are capable of assembling pilus fibers, but the roles of these pathways in the pathogenesis of plague are not understood. We constructed deletion mutations in the usher genes for six of the additional Y. pestis CU pathways. The wild-type (WT) and usher deletion strains were compared in the murine bubonic (subcutaneous) and pneumonic (intranasal) plague infection models. Y. pestis strains containing deletions in CU pathways y0348-0352, y1858-1862, and y1869-1873 were attenuated for virulence compared to the WT strain by the intranasal, but not subcutaneous, routes of infection, suggesting specific roles for these pathways during pneumonic plague. We examined binding of the Y. pestis WT and usher deletion strains to A549 human lung epithelial cells, HEp-2 human cervical epithelial cells, and primary human and murine macrophages. Y. pestis CU pathways y0348-0352 and y1858-1862 were found to contribute to adhesion to all host cells tested, whereas pathway y1869-1873 was specific for binding to macrophages. The correlation between the virulence attenuation and host cell binding phenotypes of the usher deletion mutants identifies three of the additional CU pathways of Y. pestis as mediating interactions with host cells that are important for the pathogenesis of plague.

Bacterial protein toxins and lipids: role in toxin targeting and activity

All bacterial toxins, which globally are hydrophilic proteins, interact first with their target cells by recognizing a surface receptor, which is either a lipid or a lipid derivative, or another compound but in a lipid environment. Intracellular active toxins follow various trafficking pathways, the sorting of which is greatly dependent on the nature of the receptor, notably lipidic receptor or receptor embedded into a distinct environment such as lipid microdomains. Numerous other toxins act locally on cell membrane. Indeed, phospholipase activity is a common mechanism shared by several membrane-damaging toxins. In addition, many toxins active intracellularly or on cell membrane modulate host cell phospholipid pathways. Unusually, a few bacterial toxins require a lipid post-translational modification to be active. Thereby, lipids are obligate partners of bacterial toxins.

Recent findings regarding maintenance of enzootic variants of Yersinia pestis in sylvatic reservoirs and their significance in the evolution of epidemic plague

Despite the widespread presence of bubonic plague in sylvatic reservoirs throughout the world, the causative agent (Yersinia pestis) evolved in its present form within the last 20,000 years from enteropathogenic Yersinia pseudotuberculosis. Comparison of the genomes from the two species revealed that Y. pestis possesses only a few unique plasmid-encoded genes that contribute to acute disease, whereas this organism has lost about 13% of the chromosomal genes that remain active in Y. pseudotuberculosis. These losses reflect readily detectable additions, deletions, transpositions, inversions, and acquisition of about 70 insertion sequence (IS) inserts, none of which are likely to promote increased virulence. In contrast, major enzymes of intermediary metabolism, including glucose 6-phosphate dehydrogenase (Zwf ) and aspartase, are present but not catalytically functional due to the presence of missense mutations. The latter are generally not detectable by the technology of bioinformatics and, in the case of Y. pestis, result in radical changes in the metabolic flow of carbon. As an important consequence, plague bacilli exhibit a stringent low-calcium response characterized by conversion of L-glutamate (and metabolically related amino acids) to L-aspartate with secretion of the latter into supernatant fluid at 37 degrees C in culture media containing Na(+) but lacking added Ca(2+). This phenomenon also occurs in vivo and likely adversely affects the bioenergetics of host amino acid pools. Curiously, aspartase is functional in all tested enzootic (pestoides) strains of Y. pestis. These isolates are typically restricted to the ancient plague reservoirs of Central Asia and Africa and are fully virulent in members of the rodent Superfamily Muroidea but avirulent in guinea pigs and man. The implications of these findings for the distribution and ecology of Y. pestis could be significant.

CD8(+) T cells restrict Yersinia pseudotuberculosis infection: bypass of anti-phagocytosis by targeting antigen-presenting cells

All Yersinia species target and bind to phagocytic cells, but uptake and destruction of bacteria are prevented by injection of anti-phagocytic Yop proteins into the host cell. Here we provide evidence that CD8⁺ T cells, which canonically eliminate intracellular pathogens, are important for restricting Yersinia, even though bacteria are primarily found in an extracellular locale during the course of disease. In a model of infection with attenuated Y. pseudotuberculosis, mice deficient for CD8⁺ T cells were more susceptible to infection than immunocompetent mice. Although exposure to attenuated Y. pseudotuberculosis generated T_H1-type antibody responses and conferred protection against challenge with fully virulent bacteria, depletion of CD8⁺ T cells during challenge severely compromised protective immunity. Strikingly, mice lacking the T cell effector molecule perforin also succumbed to Y. pseudotuberculosis infection. Given that the function of perforin is to kill antigen-presenting cells, we reasoned that cell death marks bacteria-associated host cells for internalization by neighboring phagocytes, thus allowing ingestion and clearance of the attached bacteria. Supportive of this model, cytolytic T cell killing of Y. pseudotuberculosis–associated host cells results in engulfment by neighboring phagocytes of both bacteria and target cells, bypassing anti-phagocytosis. Our findings are consistent with a novel function for cell-mediated immune responses protecting against extracellular pathogens like Yersinia: perforin and CD8⁺ T cells are critical for hosts to overcome the anti-phagocytic action of Yops.

Pathogenic Yersinia are bacteria that cause diverse diseases such as gastroenteritis and plague. Yersinia binds to specialized immune cells called macrophages, which attempt to engulf and destroy the bacteria. The bacteria resist destruction by injecting proteins called Yops into macrophages, which stops the engulfment process. Yersinia thus survives as attached but extracellular bacteria to cause disease. Yersinia disease can be prevented by immunization. In this study, we identified one mechanism of protective immunity—that host cells called CD8⁺ T lymphocytes are important to restrict Yersinia infection. This observation is unusual because CD8⁺ T cells generally protect against intracellular pathogens: T cells destroy the host cell harboring the pathogen, thus preventing the pathogen's replication. We present data consistent with the model that CD8⁺ T cells can also restrict extracellular bacteria by showing that T cells target host cells with extracellularly attached Yersinia, thus allowing the host cells and associated bacteria to be engulfed and removed by neighboring macrophages.

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.

Detection of viable Yersinia pestis by fluorescence in situ hybridization using peptide nucleic acid probes

A successful method has been developed for the detection of live Yersinia pestis, the plague bacillus, which incorporates nascent RNA synthesis. A fluorescent in situ hybridization (FISH) assay using peptide nucleic acid (PNA) probes was developed specifically to differentiate Y. pestis strains from closely related bacteria. PNA probes were chosen to target high copy mRNA of the Y. pestis caf1 gene, encoding the Fraction 1 (F1) antigen, and 16S ribosomal RNA. Among Yersinia strains tested, PNA probes Yp-16S-426 and Yp-F1-55 exhibited binding specificities of 100% and 98%, respectively. Y. pestis grown in the presence of competing bacteria, as might be encountered when recovering Y. pestis from environmental surfaces in a post-release bioterrorism event, was recognized by PNA probes and neither hybridization nor fluorescence was inhibited by competing bacterial strains which exhibited faster growth rates. Using fluorescence microscopy, individual Y. pestis bacteria were clearly differentiated from competing bacteria with an average detection sensitivity of 7.9x10(3) cells by fluorescence microscopy. In the current system, this would require an average of 2.56x10(5) viable Y. pestis organisms be recovered from a post-release environmental sample in order to achieve the minimum threshold for detection. The PNA-FISH assays described in this study allow for the sensitive and specific detection of viable Y. pestis bacteria in a timely manner.

Highly efficient method for introducing successive multiple scarless gene deletions and markerless gene insertions into the Yersinia pestis chromosome

An efficient two-step recombination method for markerless gene deletion and insertion that can be used for repetitive genetic modification in Yersinia pestis was developed. The method combines lambda Red recombination and counterselective screening (sacB gene) and can be used for genetic modification of Y. pestis to construct live attenuated vaccines.

Biogenesis of the fraction 1 capsule and analysis of the ultrastructure of Yersinia pestis

Analysis of a Yersinia pestis Delta caf1A mutant demonstrated that the Caf1A usher is required for the assembly and secretion of the fraction 1 capsule. The capsule assembled into thin fibrils and denser aggregates on the bacterial surface. Pilus-like fibers were also detected on the surface of Y. pestis. The capsule occasionally coated these fibers, suggesting how the capsule may cloak surface features to prevent host recognition.

Intermediary metabolism, Na+, the low calcium-response, and acute disease

The variables carriage of pCD, CO2 tension, exogenous ATP, L-glutamate, Mg2+, Na+, pH, source of energy, and temperature are known to modulate the low calcium response of Yersinia pestis in vitro. The role of these effectors and the basis of their interactions are defined here with emphasis on known Y. pestis-specific missense mutations in glucose 6-phosphate dehydrogenase and aspartase, which preclude use of the hexose monophosphate pathway and prevent efficient catabolism of L-glutamic acid, respectively. A physiological Ca2+-deficient rescue scenario is provided that permits essentially full-scale growth of virulent Y. pestis (<0.1 mM Na+ and 25 mM L-glutamate at pH 6.5) with expression of pCD-encoded virulence effectors and their attendant type III secretion system. Multiplication in this environment indicates that Ca2+ prevents innate toxicity of Na+. However, Na+ actually promotes growth in Ca2+-deficient medium at pH 9.0 due to the evident action of Na+-translocating NADH-ubiquinone oxidoreductase. Another Ca2+-deficient rescue scenario (100 mM Na+ and 25 mM L-glutamate at pH 5.5) permitted growth while downregulating pCD-encoded functions. A consequence of the abrupt Na+-mediated bacteriostasis typical of aspartase-deficient Y. pestis is conversion of L-glutamate to L-aspartate with release of the latter into culture supernatant fluids. Occurrence of this event in vivo would radically alter the equilibrium of host amino acid pools thereby contributing to enhanced lethality.

A surface-focused biotinylation procedure identifies the Yersinia pestis catalase KatY as a membrane-associated but non-surface-located protein

This study identified major surface proteins of the plague bacterium Yersinia pestis. We applied a novel surface biotinylation method, followed by NeutrAvidin (NA) bead capture, on-bead digestion, and identification by liquid chromatography-tandem mass spectrometry (LC-MS-MS). The use of stachyose during biotinylation focused the reaction to the surface. Coupled with NA pulldown and immunoblot analysis, this method determined whether a protein was accessible to the surface. We applied the method to test the hypothesis that the catalase KatY is a surface protein of the plague bacterium Y. pestis. A rabbit serum recognized the catalase KatY as a major putative outer membrane-associated antigen expressed by Y. pestis cells grown at 37 degrees C. Similar findings by other groups had led to speculations that this protein might be exposed to the surface and might be a candidate for evaluation as a protective antigen for an improved plague vaccine. KatY was obtained only in the total membrane fraction, and stachyose greatly reduced its biotinylation as well as that of the periplasmic maltose binding protein, indicating that KatY is not on the bacterial surface. LC-MS-MS analysis of on-bead digests representing ca. 10(9) cells identified highly abundant species, including KatY, Pal, and OmpA, as well as the lipoprotein Pcp, all of which bound in a biotin-specific manner. Pla, Lpp, and OmpX (Ail) bound to the NA beads in a non-biotin-specific manner. There was no contamination from abundant cytoplasmic proteins. We hypothesize that OmpX and Pcp are highly abundant and likely to be important for the Y. pestis pathogenic process. We speculate that a portion of KatY associates with the outer membrane in intact cells but that it is located on the periplasmic side. Consistent with this idea, it did not protect C57BL/6 mice against bubonic plague.

Current trends in plague research: from genomics to virulence

Yersinia pestis is the causative agent of plague, which diverged from Yersinia pseudotuberculosis within the past 20,000 years. Although these two species share a high degree of homology at the DNA level (>90%), they differ radically in their pathogenicity and transmission. In this review, we briefly outline the known virulence factors that differentiate these two species and emphasize genetic studies that have been conducted comparing Y. pestis and Y. pseudotuberculosis. These comparisons have led to a better understanding of the genetic contributions to the differences in the virulence and pathogenicity between these two organisms and have generated information that can be applied in future diagnostic and vaccine development. Comparison of the genetic differences between Y. pestis and Y. pseudotuberculosis has also lent insight into the emergence of acute pathogens from organisms causing milder diseases.

Homology Models of theYersinia PseudotuberculosisandYersinia PestisGeneral Porins and Comparative Analysis of Their Functional and Antigenic Regions

The amino acid sequences of the Yersinia pseudotuberculosis porin (YPS) and Y. pestis porin (YPT) have recently deduced but their three-dimensional structures were not known. These sequences were analyzed using the servers 3D-PSSM and PredPort. The YPS and YPT porins were shown to have a high degree of identity (above 50%) in primary and secondary structures. The three-dimensional models of the Yersinia pseudotuberculosis porin (YPS) and Y. pestis porin (YPT) were obtained using the homology modeling approach, SWISS-MODEL Protein Modeling Server and 3-D structure of PhoE porin from E. coli as template. The superposition of the Calpha-atoms of the monomers of the Yersinia porins and PhoE porin gave a root mean square deviations of 0.47 A and 0.43 A for YPS and YPT respectively. Yersinia porins were found to be very similar in their three-dimensional structure to other non-specific enterobacterial porins, having the same features of overall fold and disposition of loop L3. The intrinsic structures of the monomer pores of YPS and YPT were investigated and their conductances were predicted with the program HOLE. The good correspondence between the theoretical and experimental magnitudes of YPS conductance was found. The Yersinia porins were determined to be unusual in containing the substitution, Glu replaced by Val, in a highly conserved pentapeptide (Pro-Glu-Phe-Gly-Gly-Asp), located in the loop L3 tip that disturbs the functionally important cluster of the acidic amino acids in the constriction site. Comparative analysis of structural organization of YPS and E. coli OmpF porin in the regions involved in subunit association and pore lumen was performed. The YPS porin functional properties were predicted. The differences between these porins in polar interactions playing a significant role in stabilization of the porin trimers were found and discussed in term of the variations in trimer stability. The Yersinia porins were shown to have the highest degree of the structural similarity. The differences between the porins were observed in their external loops. Their loops L6 and loops L8 showed 71.4 and 52.9% of sequence identity, respectively. The arrangement of charged residues clustered in the channel external vestibule of these porins was found to be also different suggesting the possible differences in their functional properties. The surface exposed regions of Yersinia porins involved in their potential sequential antigenic determinants were compared. The structural basis of their cross reactivity and antigenic differences is discussed.

Horizontal transfer of the high-pathogenicity island of Yersinia pseudotuberculosis

The horizontal transfer of genetic elements plays a major role in bacterial evolution. The high-pathogenicity island (HPI), which codes for an iron uptake system, is present and highly conserved in various Enterobacteriaceae, suggesting its recent acquisition by lateral gene transfer. The aim of this work was to determine whether the HPI has kept its ability to be transmitted horizontally. We demonstrate here that the HPI is indeed transferable from a donor to a recipient Yersinia pseudotuberculosis strain. This transfer was observable only when the donor and recipient bacteria were cocultured at low temperatures in a liquid medium. When optimized conditions were used (bacteria actively growing in an iron-deprived medium at 4 degrees C), the frequency of HPI transfer reached approximately 10(-8). The island was transferable to various serotype I strains of Y. pseudotuberculosis and to Yersinia pestis, but not to Y. pseudotuberculosis strains of serotypes II and IV or to Yersinia enterocolitica. Upon transfer, the HPI was inserted almost systematically into the asn3 tRNA locus. Acquisition of the HPI resulted in the loss of the resident island, suggesting an incompatibility between two copies of the HPI within the same strain. Transfer of the island did not require a functional HPI-borne insertion-excision machinery and was RecA dependent in the recipient but not the donor strain, suggesting that integration of the island into the recipient chromosome occurs via a mechanism of homologous recombination. This lateral transfer also involved the HPI-adjacent sequences, leading to the mobilization of a chromosomal region at least 46 kb in size.

Microarray analysis of temperature-induced transcriptome of Yersinia pestis

Yersinia pestis, the etiologic agent of plague, must acclimatize itself to temperature shifts between the temperature (26 C) for flea blockage and the body temperature (37 C) of warm-blooded hosts during its life cycle. Here a whole-genome DNA microarray was used to investigate transcriptional regulation upon the upshift of growth temperature from 26 to 37 C in a chemically defined medium. Four hundred and one genes were regulated differentially under the two temperatures. About 39% of these genes were up-regulated at 37 C, whereas 61% were down-regulated. Temperature-induced changes occurred at the level of transcription of genes encoding proven or predicted virulence factors, regulators, metabolism-associated proteins, prophages, and hypothetical proteins. Strikingly, many gene clusters displayed a co-transcription pattern in response to temperature upshift. Our data provided a genome-wide profile of gene transcription induced by temperature shift and should shed light on the pathogenicity and host-microbe interaction of this deadly pathogen.

Intraspecific diversity of Yersinia pestis

Increased interest in the pathogenic potential of Yersinia pestis has emerged because of the potential threats from bioterrorism. Pathogenic potential is based on genetic factors present in a population of microbes, yet most studies evaluating the role of specific genes in virulence have used a limited number of strains. For Y. pestis this issue is complicated by the fact that most strains available for study in the Americas are clonally derived and thus genetically restricted, emanating from a strain of Y. pestis introduced into the United States in 1902 via marine shipping and subsequent spread of this strain throughout North and South America. In countries from the former Soviet Union (FSU), Mongolia, and China there are large areas of enzootic foci of Y. pestis infection containing genetically diverse strains that have been intensely studied by scientists in these countries. However, the results of these investigations are not generally known outside of these countries. Here we describe the variety of methods used in the FSU to classify Y. pestis strains based on genetic and phenotypic variation and show that there is a high level of diversity in these strains not reflected by ones obtained from sylvatic areas and patients in the Americas.

Rapid detection of Yersinia pestis with multiplex real-time PCR assays using fluorescent hybridisation probes

The objective of the present study was to establish a system of real-time polymerase chain reactions (PCRs) for the specific detection of Yersinia pestis using the LightCycler (LC) instrument. Twenty-five strains of Y. pestis, 94 strains of other Yersinia species and 33 clinically relevant bacteria were investigated. Assays for the 16S rRNA gene target and the plasminogen activator gene (resides on the 9.5-kb plasmid) and for the Y. pestis murine toxin gene and the fraction 1 antigen gene (both on the 100-kb plasmid) were combined for the use in two multiplex assays including an internal amplification control detecting bacteriophage lambda-DNA. Applying these multiplex assays, Y. pestis was selectively identified; other bacteria yielded no amplification products. The lower limit of detection was approximately 0.1 genome equivalent. Rat or flea DNA had no inhibitory effects on the detection of Y. pestis. The results obtained using the multiplex real-time assays showed 100% accuracy when compared with combinations of conventional PCR assays. We developed and evaluated a highly specific real-time PCR strategy for the detection of Y. pestis, obtaining results within 3 h including DNA preparation.

Structural characterization of lipo-oligosaccharide (LOS) from Yersinia pestis: regulation of LOS structure by the PhoPQ system

The two-component regulatory system PhoPQ has been shown to regulate the expression of virulence factors in a number of bacterial species. For one such virulence factor, lipopolysaccharide (LPS), the PhoPQ system has been shown to regulate structural modifications in Salmonella enterica var Typhimurium. In Yersinia pestis, which expresses lipo-oligosaccharide (LOS), a PhoPQ regulatory system has been identified and an isogenic mutant constructed. To investigate potential modifications to LOS from Y. pestis, which to date has not been fully characterized, purified LOS from wild-type plague and the phoP defective mutant were analysed by mass spectrometry. Here we report the structural characterization of LOS from Y. pestis and the direct comparison of LOS from a phoP mutant. Structural modifications to lipid A, the host signalling portion of LOS, were not detected but analysis of the core revealed the expression of two distinct molecular species in wild-type LOS, differing in terminal galactose or heptose. The phoP mutant was restricted to the expression of a single molecular species, containing terminal heptose. The minimum inhibitory concentration of cationic antimicrobial peptides for the two strains was determined and compared with the wild-type: the phoP mutant was highly sensitive to polymyxin. Thus, LOS modification is under the control of the PhoPQ regulatory system and the ability to alter LOS structure may be required for survival of Y. pestis within the mammalian and/or flea host.

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.

Protein regions important for plasminogen activation and inactivation of alpha2-antiplasmin in the surface protease Pla of Yersinia pestis

The plasminogen activator, surface protease Pla, of the plague bacterium Yersinia pestis is an important virulence factor that enables the spread of Y. pestis from subcutaneous sites into circulation. Pla-expressing Y. pestis and recombinant Escherichia coli formed active plasmin in the presence of the major human plasmin inhibitor, alpha2-antiplasmin, and the bacteria were found to inactivate alpha2-antiplasmin. In contrast, only poor plasminogen activation and no cleavage of alpha2-antiplasmin was observed with recombinant bacteria expressing the homologous gene ompT from E. coli. A beta-barrel topology model for Pla and OmpT predicted 10 transmembrane beta-strands and five surface-exposed loops L1-L5. Hybrid Pla-OmpT proteins were created by substituting each of the loops between Pla and OmpT. Analysis of the hybrid molecules suggested a critical role of L3 and L4 in the substrate specificity of Pla towards plasminogen and alpha2-antiplasmin. Substitution analysis at 25 surface-located residues showed the importance of the conserved residues H101, H208, D84, D86, D206 and S99 for the proteolytic activity of Pla-expressing recombinant E. coli. The mature alpha-Pla of 292 amino acids was processed into beta-Pla by an autoprocessing cleavage at residue K262, and residues important for the self-recognition of Pla were identified. Prevention of autoprocessing of Pla, however, had no detectable effect on plasminogen activation or cleavage of alpha2-antiplasmin. Cleavage of alpha2-antiplasmin and plasminogen activation were influenced by residue R211 in L4 as well as by unidentified residues in L3. OmpT, which is not associated with invasive bacterial disease, was converted into a Pla-like protease by deleting residues D214 and P215, by substituting residue K217 for R217 in L4 of OmpT and also by substituting the entire L3 with that from Pla. This simple modification of the surface loops and the substrate specificity of OmpT exemplifies the evolution of a housekeeping protein into a virulence factor by subtle mutations at critical protein regions. We propose that inactivation of alpha2-antiplasmin by Pla of Y. pestis promotes uncontrolled proteolysis and contributes to the invasive character of plague.

Identification of immunodominant epitope of F1 antigen of Yersinia pestis

The F1 antigen of Yersinia pestis has been identified as one of the major protective antigens of this bacterium. The present study aims to delineate major and minor antigenic sites of F1 antigen. Using algorithmic predictions, five peptide sequences (P1, P2, P3, P4 and P5) spanning the C-terminal region were identified and synthesized. Antibodies were generated in mice against the peptides, native F1 protein and polymerized F1 antigen using liposomes as mode of immunization. Cross-reactivity between F1 antigen and peptides was tested using both solid and solution phase assays. Similar assays were done with rabbit anti-F1 sera. Competitive inhibition assays using a different combination of antisera and competing antigen identified P2 peptide FFVRSIGSKGGKLAAGKYTDAVTV (142-165) as the immunodominant sequence. The results indicate that this sequence appears to be exposed on the surface of F1 molecule. In a solid phase binding assay, P2 peptide was recognized even at high F1 antisera dilution. However, when antisera raised to different peptides were tested for binding to F1 antigen, antisera to P4 peptide showed maximal immunoreactivity. This implies more accessibility of this region during immobilization on solid surface. There was consistency in the results obtained for different strains of mice as well as for the rabbit antisera. Such a sequence of F1 antigen, which is recognized widely in animals of different genetic background, would be useful for diagnosis and subunit vaccine.

Signal transduction-mediated adherence and entry of Helicobacter pylori into cultured cells

BACKGROUND & AIMS

An ability to invade host cells could be a means for Helicobacter pylori to achieve resistance to antibiotic therapy. The aim of this study was to investigate the mechanisms involved in adherence and entry of H. pylori into cultured cells.

METHODS

Coinfection with Yersinia expressing mutant or wild-type YopH tyrosine phosphatase was used. Genistein and cytochalasin D were used as inhibitors of adherence and entry; entry was monitored by a gentamicin-protection assay. Target cells were AGS cells and a beta1-integrin-deficient cell line with its corresponding beta1-integrin-expressing transfectant.

RESULTS

H. pylori induced phosphorylation of 125-130-kilodalton proteins, similar in size to the target proteins of Yersinia YopH. Adherence of H. pylori was inhibited by Yersinia organisms expressing enzymatically active YopH but not by inactive YopH. Adherence and entry of H. pylori was considerably higher with beta1-integrin-transfected cells than with beta1-integrin-deficient cells. Antibodies directed against alpha5- and beta1-integrin chains reduced adherence to the alpha5beta1-integrin-expressing gastric cell line AGS. Entry was inhibited by both cytochalasin D and genistein. Entry, but not adherence, was higher for 2 type I strains than for a type II isolate.

CONCLUSIONS

Invasion of gastric epithelium via an integrin-mediated pathway could contribute to the ability of H. pylori to establish persistent infection.

YopD of Yersinia pseudotuberculosis is translocated into the cytosol of HeLa epithelial cells: evidence of a structural domain necessary for translocation

Yersinia pseudotuberculosis YopB and YopD proteins are essential for translocation of Yop effector proteins into the target cell cytosol. YopB is suggested to mediate pore formation in the target cell plasma membrane, allowing translocation of Yop effector proteins, although the function of YopD is unclear. To investigate the role in translocation for YopD, a mutant strain in Y. pseudotuberculosis was constructed containing an in frame deletion of essentially the entire yopD gene. As shown recently for the Y. pestis YopD protein, we found that the in vitro low calcium response controlling virulence gene expression was negatively regulated by YopD. This yopD null mutant (YPIII/pIB621) was also non-cytotoxic towards HeLa cell monolayers, supporting the role for YopD in the translocation process. Although other constituents of the Yersinia translocase apparatus (YopB, YopK and YopN) are not translocated into the host cell cytosol, fractionation of infected HeLa cells allowed us to identify the cytosolic localization of YopD by the wild-type strain (YPIII/pIB102), but not by strains defective in either YopD or YopB. YopD was also identified by immunofluorescence in the cytoplasm of HeLa cell monolayers infected with a multiple yop mutant strain (YPIII/pIB29MEKA). These results demonstrate a dual function for YopD in negative regulation of Yop production and Yop effector translocation, including the YopD protein itself. To investigate whether an amphipathic domain near the C-terminus of YopD is involved in the translocation process, a mutant strain (YPIII/pIB155deltaD278-292) was constructed that is devoid of this region. Phenotypically, this small in frame deltayopD278-292 deletion mutant was indistinguishable from the yopD null mutant. The truncated YopD protein and Yop effectors were not translocated into the cytosol of HeLa cell monolayers infected with this mutant. The comparable regulatory and translocation phenotypes displayed by the small in frame deltayopD278-292 deletion and deltayopD null mutants suggest that regulation of Yop synthesis and Yop translocation are intimately coupled. We present an intriguing scenario to the Yersinia infection process that highlights the need for polarized translocation of YopD to specifically establish translocation of Yop effectors. These observations are contrary to previous suggestions that members of the translocase apparatus were not translocated into the host cell cytosol.

Homology with a repeated Yersinia pestis DNA sequence IS100 correlates with pesticin sensitivity in Yersinia pseudotuberculosis

We have identified IS100 sequences in a specific subset of Yersinia pseudotuberculosis isolates that were also sensitive to the Y. pestis-produced bacteriocin, pesticin. In contrast, Y. pseudotuberculosis strains which did not contain IS100 sequences were not sensitive to pesticin. We propose that IS100 serves as a molecular marker that identifies a subset of Y. pseudotuberculosis isolates that have a particularly close evolutionary and/or ecological relationship with Y. pestis.

Detection and identification of Yersinia pestis by polymerase chain reaction (PCR) using multiplex primers

A PCR method for detection of Yersinia pestis-virulence determinants by the use of multiplex primers was developed. Four pairs of oligonucleotide primers were designed from each gene of three kinds of virulent plasmids and a chromosomal DNA; 60-Md plasmid-located gene (caf1)encoding Y.pestis-specific capsular antigen fraction 1, a Y.pestis-specific region of a yopM gene encoded on 42-Md virulent plasmid, a plasminogen activator gene (pla) encoded on Y.pestis-specific 7-Md plasmid and an invasin protein gene (inv) encoded on chromosomal DNA. This multiplex-primer system was specific for the detection of Y.pestis among pathogenic Yersinia species and other enterobacteriaceae having antigens common to Y.pestis. Since this method is simple and safe, it will be useful to identify and confirm Y.pestis in cases of emergency and for the surveillance of epidemics.

YadA mediates specific binding of enteropathogenic Yersinia enterocolitica to human intestinal submucosa

The binding of live Yersinia enterocolitica to frozen sections of human intestine was investigated qualitatively by monitoring the binding of bacteria by using Gram or immunoperoxidase staining as well as quantitatively by a new enzyme immunoassay-on-slide method. We have demonstrated that the binding of various Y. enterocolitica serotypes and Escherichia coli clones to frozen sections of human intestine is mediated by the Yersinia adhesin, YadA. The YadA-mediated binding occurs mainly at the submucosal layer of the intestinal wall and only to a limited extent at the mucosal layer; there binding is mostly to the mucin threads. In addition, partially purified YadA binds to frozen sections with a pattern similar to that of intact bacteria. Collagen, laminin, or partially purified YadA only partially inhibited the YadA-mediated binding of bacteria, presumably because YadA is multifunctional. A combination of collagen and laminin inhibited the binding more efficiently. Therefore, YadA may be involved in the interactions with the extracellular matrix molecules after the invasion of the intestinal tissue.

PCR-based random amplified polymorphic DNA fingerprinting of Yersinia pseudotuberculosis and its practical applications

The random amplified polymorphic DNA (RAPD) fingerprinting method was used to distinguish between various strains of Yersinia pseudotuberculosis, a causative agent of gastroenteritis in humans. The RAPD test uses arbitrarily chosen DNA oligomers of 10 nucleotides to prime DNA synthesis from genomic sites to which they are accidentally matched or almost matched. Most 10-nucleotide primers yielded strain-specific arrays. Ten Y. pseudotuberculosis type strains were distinguishable from each other by analyzing the RAPD arrays produced by using primers with a 50% G+C content. The RAPD patterns of Y. pseudotuberculosis strains were found to be constant regardless of the presence or absence of the large plasmid. RAPD tests were subsequently used to identify 30 clinical isolates of Y. pseudotuberculosis that were collected as the causative agent of an outbreak of Izumi fever, a disease showing clinical symptoms characteristic of atypical scarlet fever, in Japan. The RAPD arrays from all of the isolates yielded common patterns that were unique to each primer used. Since those 30 isolates belonged to serotype 5a and the restriction digest patterns of their large plasmids were all the same, the results of the RAPD tests confirmed the view that those isolates were from a single source and indicated that the RAPD test can be practically applied to survey transmission of the bacterium in humans.

Yersinia pestis pH 6 antigen forms fimbriae and is induced by intracellular association with macrophages

Ability to express pH 6 antigen (Ag) is necessary for full virulence of Yersinia pestis; however, the function of the Ag in pathogenesis remains unclear. We determined the nucleotide sequence of a 4232 bp region of Y. pestis DNA which encoded the pH 6 Ag structural gene (psaA) and accessory loci necessary for Ag synthesis. Protein sequences encoded by the Y. pestis DNA were similar to accessory proteins which function in the biosynthesis of Escherichia coli fimbriae Pap, K88, K99 and CS3 as well as the molecular chaperone for the Y. pestis capsule protein. Electron microscopy and immunogold labelling studies revealed that pH 6 Ag expressing E. coli or Yersinia produced flexible 'fibrillar' organelles composed of individual linear strands, multiple strand bundles or wiry aggregates of PsaA. Y. pestis associated with the murine macrophage-like cell line, RAW264.7, expressed pH 6 Ag in an intracellular acidification-dependent manner. Together with an earlier study showing that a Y. pestis psaA mutant was reduced in virulence, these results demonstrate that the expression of fimbriae which are induced in host macrophages is involved in plague pathogenesis.

Determination of genome size, macrorestriction pattern polymorphism, and nonpigmentation-specific deletion in Yersinia pestis by pulsed-field gel electrophoresis

Of 16 restriction endonucleases known to hydrolyze rare 6- or 8-base recognition sequences that were tested, only SpeI, NotI, AscI, and SfiI generated fragments of chromosomal DNA from Yersinia pestis, the causative agent of bubonic plague, of sufficient length to permit physical analysis by pulsed-field gel electrophoresis (PFGE). Of the individual bands detected after single-dimensional PFGE of these digests, the largest sum was obtained with SpeI (3,575.6 +/- 114.6 kb). Of these 41 bands, 3 were found to contain comigrating fragments, as judged by the results of two-dimensional SpeI-ApaI PFGE; addition of these fragments and the three plasmids of the species yielded a refined estimate of 4,397.9 +/- 134.6 kb for the genome. This size was similar for eight strains of diverse geographical origin that exhibited distinct DNA macrorestriction patterns closely related to their biotypes. The high-frequency chromosomal deletion known to exist in nonpigmented mutants (unable to assimilate Fe3+ at 37 degrees C or store hemin at 26 degrees C) was shown by two-dimensional PFGE analysis with SpeI and ApaI or with SfiI and SpeI to be 92.5 and 106 kb in size, respectively. The endpoints of this deletion were precise, and its size was more than sufficient to encode the eight known peptides reported to be absent in nonpigmented mutants. This deletion had not occurred (but was able to do so) in a rare mutant capable of hemin storage but not iron transport.

Analysis of Yersinia pestis chromosomal determinants Pgm+ and Psts associated with virulence

The ability to absorb exogenous pigments (Pgm+) has, until now, been considered an established virulence factor of Yersinia pestis, the causative agent of plague. This property correlates with the sensitivity to the bacteriocin pesticin (Psts). Both functions are chromosomally encoded. In the present study, using Hfr donors and isogenic Pgm-Psts and Pgm-Pstr mutants, these functions were shown to be determined by discrete but closely linked genes. These markers designated pgm and psn, respectively, were preliminarily located within a linkage group including 11 loci. It was also found that pigmentation is not essential for mouse virulence but is necessary for survival of Y. pestis in the flea, the plague vector. At the same time, conversion of an avirulent Pstr mutant to pesticin sensitivity restored some degree of virulence.

Plasmid composition and virulence-associated factors of Yersinia pestis isolates from a plague outbreak at the Paraiba State, Brazil

Pathogenic Yersinia pestis isolates were collected during a plague outbreak at the Paraiba State in 1986. The Y. pestis isolates were investigated for the presence of virulence-associated factors and plasmid content. All strains analysed were proficient in the expression of the VW and fraction 1 antigens, pigment adsorption and pesticin-fibrinolysin-coagulase production. A similar plasmid profile composed by four plasmid with molecular weight of 60, 44, 14.9, and 6.4 Megadaltons (MD) was found in all strains. DNA cleavage with EcoRI restriction enzyme further demonstrated the uniform plasmid content of the Y. pestis isolates. Seven additional Y. pestis strains, previously isolated in the same region but in an endemic state, showed the same plasmid fingerprint. The lack of any detectable difference between epidemic and endemic isolates as well as the value of plasmid fingerprints in epidemiology of Y. pestis is discussed.

Mutations in the hemolytic-phospholipase C operon result in decreased virulence of Pseudomonas aeruginosa PAO1 grown under phosphate-limiting conditions

The phospholipase C (PLC) operon of Pseudomonas aeruginosa consists of plcS, which encodes a heat-labile secreted hemolysin, and two in-phase, overlapping genes, plcR1 and plcR2, which may encode Pi-regulatory genes. A 2.8-kilobase-pair deletion mutation in this operon was constructed, and a tetracycline resistance (Tcr) cartridge replaced the deleted sequences. A deletion mutant of strain PAO1 was obtained through recombination between the flanking regions of the mutated cloned PLC operon and the homologous chromosomal regions. The deletion of the chromosomal PLC operon and its replacement by the Tcr cartridge was confirmed by Southern hybridization. The deletion strain, PLC SR, is nonhemolytic. However, it retains PLC activity when measured on a synthetic substrate. A second mutant strain, PLC R, contains a deletion in the plcR genes. This mutant is more hemolytic and produces more enzymatic activity than PAO1. The virulence of both of these mutants was compared with that of PAO1 in the mouse burn model of infection. When mice were infected with cultures grown in a high-Pi medium, there was a 10-fold increase in the 50% lethal dose of the mutants compared with PAO1. In contrast, when the inoculum originated from low-Pi cultures, there was a 200- to 10,000-fold increase in the 50% lethal dose of the mutants over PAO1.

Differential clearance and host-pathogen interactions of YopE- and YopK- YopL- Yersinia pestis in BALB/c mice

This study characterized infections in BALB/c mice by the nonpigmented Yersinia pestis KIM and its derivatives lacking the low-Ca2+-response virulence plasmid pCD1 or failing to express selected yersinial outer membrane proteins (YOPs). The parent Y. pestis showed net growth in the spleen by 2 h and in the liver after 7 h; exponential growth in both the liver and spleen culminated in death of the mice starting on day 4, with total bacterial numbers of less than 10(8) in the blood, liver, and spleen together. The histopathology progressed from microabscesses to extensive coagulative necrosis unaccompanied by further immigration of inflammatory cells. This, together with the relatively low bacterial numbers, suggests a toxigenic mechanism. YopE- or YopK- YopL- yersiniae were cleared from the spleen but grew in the liver after an initial lag. Their growth was curbed after 1 to 2 days and entered a plateau that lasted 5 to 6 days; viable numbers then decline rapidly. This suggests that these Yop- mutations distinguish, at least kinetically, between host responses in liver and spleen. Both strains caused acute inflammation in liver that evolved into structured lesions surrounded by progressively mononuclear inflammation suggestive of a granulomatous response. Accordingly, YOP E and YOPs K and L are necessary in the early days of the infection for net growth in spleen and prolonged growth in the liver; their absence is reflected morphologically by the emergence of cell-mediated immunity in the liver. The YopE- and YopK- YopL- mutants bound only slightly increased amounts of C3, suggesting that YOPs E, K, and L are protective through mechanisms other than interfering with the binding of complement.

Outer membrane peptides of Yersinia pestis mediating siderophore-independent assimilation of iron

It is established that wild-type cells of Yersinia pestis absorb exogenous hemin or Congo red and thus grow as pigmented colonies at 26 degrees C on media containing these chromatophores (Pgm+). Pgm+ isolated are known to possess a siderophore-independent mechanism of iron-transport (required for growth in iron-deficient medium) which is absent in avirulent Pgm- mutants. Production of the bacteriocin pesticin and linked invasins (Pst+) is an additional defined virulence factor of yersiniae; mutation of Pgm+,Pst- organisms to pesticin-resistance (Pstr) results in concomitant conversion to Pgm-. In this study, autoradiograms of two-dimensional gels of [35S]methionine-labeled outer membranes from Pgm- mutants were compared to those of the Pgm+,Pst+ or Pgm+,Pst- parent. An apparently single predominant peptide present in these preparations (greater than 10% of total membrane protein) existed as a family of iron-modifiable 17.9-kDa molecules focusing down to isoelectric points of about 4.6 and up to 5.89. Expression of eight detectable Pst(+)-specific peptides was not significantly influenced by exogenous iron. Pgm+ yersiniae constitutively produced pigmentation-specific peptide F and five iron-repressible peptides termed IrpA to IrpE. Typical spontaneous mutation to Pgm- resulted in loss of peptide F and IrpB-E. A rare Pgm+,Pstr mutant, selected on Congo red agar containing pesticin, also lost IrpB-E but retained peptide F. This isolate, like Pgm- mutants, failed to grow in iron-deficient medium. Regardless of phenotype, all yersiniae utilized hemin, hemopexin, myoglobin, hemoglobin, and ferritin, but not transferrin or lactoferrin, as sole sources of iron.

Identification of invasin: a protein that allows enteric bacteria to penetrate cultured mammalian cells

Bacterial strains harboring the Yersinia pseudotuberculosis inv locus were analyzed in order to investigate the mechanism of host cell penetration by an invasive pathogen. The inv locus was found to be necessary for Y. pseudotuberculosis to enter HEp-2 cells and sufficient to convert E. coli into a microorganism able to penetrate cultured cells. Both E. coli and Y. pseudotuberculosis strains harboring inv mutations were defective for entry into HEp-2 cells. Furthermore, molecular clones containing inv, and little additional DNA, converted E. coli into a microorganism that was indistinguishable from the parental Yersinia strain with regard to the entry of cultured cells. Data from in vitro protein synthesis indicated that a 103 kd protein was synthesized from inv, saturating the coding capacity of the locus. The nucleotide sequence shows an open reading frame corresponding to a protein of similar size. This protein, called invasin, is necessary for the microorganisms to penetrate HEp-2 cells, and is compartmentalized on the outer surface of the bacterium.

Plasmid-mediated resistance to phagocytosis in Yersinia enterocolitica

Results of our previous studies have shown that the chemiluminescence response of human neutrophils (polymorphonuclear leukocytes [PMNs]) is inhibited by plasmid-mediated cell surface components from Yersinia enterocolitica. In this study we examined the susceptibility to phagocytosis of Y. enterocolitica cells with or without plasmid-mediated surface structure and the effect of isolated outer membrane fragments on phagocytosis of Escherichia coli by PMNs in vitro. Y. enterocolitica cells with expressed plasmid-mediated surface structure were much less sensitive to ingestion by PMNs than those without it, and the resistance to phagocytosis was readily eliminated in a dose-dependent fashion by pronase treatment of whole cells, which was shown to remove plasmid-encoded outer membrane proteins. Ingestion and intracellular killing of E. coli were inhibited significantly in the presence of isolated outer membrane fragments derived from plasmid-bearing Y. enterocolitica cells. To assess the interaction of Y. enterocolitica with phagocytic cells in vivo, two isogenic strains of Y. enterocolitica, differing only in the presence or absence of the virulence plasmid, were inoculated intradermally into the backs of rabbits; and tissue sections obtained at 12 h postinoculation were examined by light and electron microscopy. The plasmidless strain was found almost entirely in PMNs or mononuclear cells. In contrast, the plasmid-bearing strain was found to be surrounded by, or interspersed with, PMNs and mononuclear cells; but most bacteria were extracellular, with little evidence of phagocytosis. These results suggest that plasmid-mediated cell surface components of Y. enterocolitica act as antiphagocytic factors, thus facilitating the survival and proliferation of the organism in the host tissue.

Plasmid-like properties of the four virulence-associated factors of Yersinia pestis

The virulence-associated factors of Yersinia pestis, which determine the abilities to produce pesticin I (Pst+), capsular fraction I antigen (Fra+), V and W antigen complex (Vwa+) and a cell-surface component for adsorption of exogenous pigments (Pgm+), were independently eliminated by cultivation of the cells in the presence of acridine orange, ethidium bromide or sodium dodecyl sulfate at a subinhibitory concentration. A virulent Y. pestis strain, Yreka, harbored at least five extrachromosomal DNA molecules of different sizes. In these molecules, a novel 13-megadalton DNA which was cured concomitantly with the elimination of the Fra factor was found, in addition to the known species of 7 and 44 megadaltons which were lost with the conversions to Pst- and Vwa-, respectively. Although the conversion to Pgm- could not be correlated with the lack of any proper extra-chromosomal DNA, the factor was transmitted to Pgm- cells with the aid of self-conjugative RP4 plasmid. The cells acquiring the Pgm factor regained virulence for mice.

Virulence plasmid-associated HeLa cell induced cytotoxicity of Yersinia pseudotuberculosis

A virulent plasmid-containing strain and an isogenic plasmid-free avirulent derivative strain of Y. pseudotuberculosis were compared with respect to the interaction with cultured HeLa cells. No difference in the ability to adhere to HeLa cells was observed. Both strains invaded the HeLa cells to the same degree. The internalized bacteria did not multiply over a 24-h period. When incubated at 37 degrees C HeLa cells infected with the virulent plasmid-containing strain YPIII(pIB1) showed morphological changes in contrast to the avirulent plasmid-free strain YPIII which showed no such effect. This cytotoxic effect was recorded after about 2 hours of incubation at 37 degrees C. This event could be prevented by addition of gentamicin 1 hour before the predicted time of the morphological changes of the HeLa cells. A number of transposon Tn5 derived plasmid mutants were analyzed and a correlation was found between virulence and ability to induce cytotoxicity. The cytotoxic effect was not absolutely linked to Ca2+-dependence because one Ca2+-independent mutant was found to be cytotoxic. This mutant was able to express the temperature-inducible plasmid-coded outer membrane proteins (YOPs) as well as the V-antigen. On the molecular level a strong correlation was found between expression of some of the YOPs, the V-antigen and the cytotoxic effect. However, one of these YOPs, YOP1, could be eliminated as being an important determinant of cytotoxicity because an insertion mutant of the corresponding structural gene was still able to induce cytotoxicity.

Virulence genes regulated at the transcriptional level by Ca2+ in Yersinia pestis include structural genes for outer membrane proteins

Yersinia pestis, the causative agent of plague, has a virulence determinant called the low-Ca2+ response (Lcr+ phenotype) that confers on the bacterium Ca2+ dependence for growth at 37 degrees C and expression of V antigen. This virulence determinant is common to the three species of Yersinia and is mediated by Lcr plasmids (called pCD in Y. pestis). In this study, we generated insertions of Mu dI1(Ap lac) in pCD1 of Y. pestis KIM, screened for cells showing transcriptional regulation by Ca2+, and obtained inserts that define at least four pCD1 genes. Their patterns of transcription under different growth conditions closely paralleled the pattern of expression of the V antigen. We tested for expression of Lcr-specific yersinial outer membrane proteins (Yops) by the pCD1::Mu dI1(Ap lac) plasmids. Four of the inserts each eliminated expression of a different Yop; one of these Yops was unique to Y. pestis. Two of the insertions affecting Yops caused avirulence, and one caused strongly decreased virulence of Y. pestis in mice. These data indicate that Yops, like the V antigen, are virulence attributes regulated in the low-Ca2+ response.