Expression of the low calcium response in Yersinia pestis

Pathogenicity and virulence of Yersinia

The genus Yersinia includes human, animal, insect, and plant pathogens as well as many symbionts and harmless bacteria. Within this genus are Yersinia enterocolitica and the Yersinia pseudotuberculosis complex, with four human pathogenic species that are highly related at the genomic level including the causative agent of plague, Yersinia pestis. Extensive laboratory, field work, and clinical research have been conducted to understand the underlying pathogenesis and zoonotic transmission of these pathogens. There are presently more than 500 whole genome sequences from which an evolutionary footprint can be developed that details shared and unique virulence properties. Whereas the virulence of Y. pestis now seems in apparent homoeostasis within its flea transmission cycle, substantial evolutionary changes that affect transmission and disease severity continue to ndergo apparent selective pressure within the other Yersiniae that cause intestinal diseases. In this review, we will summarize the present understanding of the virulence and pathogenesis of Yersinia, highlighting shared mechanisms of virulence and the differences that determine the infection niche and disease severity.

Development of a dual antigen lateral flow immunoassay for detecting Yersinia pestis

Background

Yersinia pestis is the causative agent of plague, a zoonosis associated with small mammals. Plague is a severe disease, especially in the pneumonic and septicemic forms, where fatality rates approach 100% if left untreated. The bacterium is primarily transmitted via flea bite or through direct contact with an infected host. The 2017 plague outbreak in Madagascar resulted in more than 2,400 cases and was highlighted by an increased number of pneumonic infections. Standard diagnostics for plague include laboratory-based assays such as bacterial culture and serology, which are inadequate for administering immediate patient care for pneumonic and septicemic plague.

Principal findings

The goal of this study was to develop a sensitive rapid plague prototype that can detect all virulent strains of Y. pestis. Monoclonal antibodies (mAbs) were produced against two Y. pestis antigens, low-calcium response V (LcrV) and capsular fraction-1 (F1), and prototype lateral flow immunoassays (LFI) and enzyme-linked immunosorbent assays (ELISA) were constructed. The LFIs developed for the detection of LcrV and F1 had limits of detection (LOD) of roughly 1–2 ng/mL in surrogate clinical samples (antigens spiked into normal human sera). The optimized antigen-capture ELISAs produced LODs of 74 pg/mL for LcrV and 61 pg/mL for F1 when these antigens were spiked into buffer. A dual antigen LFI prototype comprised of two test lines was evaluated for the detection of both antigens in Y. pestis lysates. The dual format was also evaluated for specificity using a small panel of clinical near-neighbors and other Tier 1 bacterial Select Agents.

Conclusions

LcrV is expressed by all virulent Y. pestis strains, but homologs produced by other Yersinia species can confound assay specificity. F1 is specific to Y. pestis but is not expressed by all virulent strains. Utilizing highly reactive mAbs, a dual-antigen detection (multiplexed) LFI was developed to capitalize on the diagnostic strengths of each target.

An Improvement in Diagnostic Blood Culture Conditions Allows for the Rapid Detection and Isolation of the Slow Growing Pathogen Yersinia pestis

Plague, caused by the human pathogen Yersinia pestis, is a severe and rapidly progressing lethal disease that has caused millions of deaths globally throughout human history and still presents a significant public health concern, mainly in developing countries. Owing to the possibility of its malicious use as a bio-threat agent, Y. pestis is classified as a tier-1 select agent. The prompt administration of an effective antimicrobial therapy, essential for a favorable patient prognosis, requires early pathogen detection, identification and isolation. Although the disease rapidly progresses and the pathogen replicates at high rates within the host, Y. pestis exhibits a slow growth in vitro under routinely employed clinical culturing conditions, complicating the diagnosis and isolation. In the current study, the in vitro bacterial growth in blood cultures was accelerated by the addition of nutritional supplements. We report the ability of calcium (Ca+2)- and iron (Fe+2)-enriched aerobic blood culture media to expedite the growth of various virulent Y. pestis strains. Using a supplemented blood culture, a shortening of the doubling time from ~110 min to ~45 min could be achieved, resulting in increase of 5 order of magnitude in the bacterial loads within 24 h of incubation, consequently allowing the rapid detection and isolation of the slow growing Y. pestis bacteria. In addition, the aerobic and anaerobic blood culture bottles used in clinical set-up were compared for a Y. pestis culture in the presence of Ca+2 and Fe+2. The comparison established the superiority of the supplemented aerobic cultures for an early detection and achieved a significant increase in the yields of the pathogen. In line with the accelerated bacterial growth rates, the specific diagnostic markers F1 and LcrV (V) antigens could be directly detected significantly earlier. Downstream identification employing MALDI-TOF and immunofluorescence assays were performed directly from the inoculated supplemented blood culture, resulting in an increased sensitivity and without any detectable compromise of the accuracy of the antibiotic susceptibility testing (E-test), critical for subsequent successful therapeutic interventions.

Adaptivity and dynamics in type III secretion systems

The type III secretion system is the common core of two bacterial molecular machines: the flagellum and the injectisome. The flagellum is the most widely distributed prokaryotic locomotion device, whereas the injectisome is a syringe-like apparatus for inter-kingdom protein translocation, which is essential for virulence in important human pathogens. The successful concept of the type III secretion system has been modified for different bacterial needs. It can be adapted to changing conditions, and was found to be a dynamic complex constantly exchanging components. In this review, we highlight the flexibility, adaptivity, and dynamic nature of the type III secretion system.

Yersinia pestis Plasminogen Activator

The Gram-negative bacterium Yersinia pestis causes plague, a fatal flea-borne anthropozoonosis, which can progress to aerosol-transmitted pneumonia. Y. pestis overcomes the innate immunity of its host thanks to many pathogenicity factors, including plasminogen activator, Pla. This factor is a broad-spectrum outer membrane protease also acting as adhesin and invasin. Y. pestis uses Pla adhesion and proteolytic capacity to manipulate the fibrinolytic cascade and immune system to produce bacteremia necessary for pathogen transmission via fleabite or aerosols. Because of microevolution, Y. pestis invasiveness has increased significantly after a single amino-acid substitution (I259T) in Pla of one of the oldest Y. pestis phylogenetic groups. This mutation caused a better ability to activate plasminogen. In paradox with its fibrinolytic activity, Pla cleaves and inactivates the tissue factor pathway inhibitor (TFPI), a key inhibitor of the coagulation cascade. This function in the plague remains enigmatic. Pla (or pla) had been used as a specific marker of Y. pestis, but its solitary detection is no longer valid as this gene is present in other species of Enterobacteriaceae. Though recovering hosts generate anti-Pla antibodies, Pla is not a good subunit vaccine. However, its deletion increases the safety of attenuated Y. pestis strains, providing a means to generate a safe live plague vaccine.

Life After Secretion-Yersinia enterocolitica Rapidly Toggles Effector Secretion and Can Resume Cell Division in Response to Changing External Conditions

Many pathogenic bacteria use the type III secretion system (T3SS) injectisome to manipulate host cells by injecting virulence-promoting effector proteins into the host cytosol. The T3SS is activated upon host cell contact, and its activation is accompanied by an arrest of cell division; hence, many species maintain a T3SS-inactive sibling population to propagate efficiently within the host. The enteric pathogen Yersinia enterocolitica utilizes the T3SS to prevent phagocytosis and inhibit inflammatory responses. Unlike other species, almost all Y. enterocolitica are T3SS-positive at 37°C, which raises the question, how these bacteria are able to propagate within the host, that is, when and how they stop secretion and restart cell division after a burst of secretion. Using a fast and quantitative in vitro secretion assay, we have examined the initiation and termination of type III secretion. We found that effector secretion begins immediately once the activating signal is present, and instantly stops when this signal is removed. Following effector secretion, the bacteria resume division within minutes after being introduced to a non-secreting environment, and the same bacteria are able to re-initiate effector secretion at later time points. Our results indicate that Y. enterocolitica use their type III secretion system to promote their individual survival when necessary, and are able to quickly switch their behavior toward replication afterwards, possibly gaining an advantage during infection.

Going around in circles: virulence plasmids in enteric pathogens

Plasmids have a major role in the development of disease caused by enteric bacterial pathogens. Virulence plasmids are usually large (>40 kb) low copy elements and encode genes that promote host-pathogen interactions. Although virulence plasmids provide advantages to bacteria in specific conditions, they often impose fitness costs on their host. In this Review, we discuss virulence plasmids in Enterobacteriaceae that are important causes of diarrhoea in humans, Shigella spp., Salmonella spp., Yersinia spp and pathovars of Escherichia coli. We contrast these plasmids with those that are routinely used in the laboratory and outline the mechanisms by which virulence plasmids are maintained in bacterial populations. We highlight examples of virulence plasmids that encode multiple mechanisms for their maintenance (for example, toxin-antitoxin and partitioning systems) and speculate on how these might contribute to their propagation and success.

An Experimental Pipeline for Initial Characterization of Bacterial Type III Secretion System Inhibitor Mode of Action Using Enteropathogenic Yersinia

Dozens of Gram negative pathogens use one or more type III secretion systems (T3SS) to disarm host defenses or occupy a beneficial niche during infection of a host organism. While the T3SS represents an attractive drug target and dozens of compounds with T3SS inhibitory activity have been identified, few T3SS inhibitors have been validated and mode of action determined. One issue is the lack of standardized orthogonal assays following high throughput screening. Using a training set of commercially available compounds previously shown to possess T3SS inhibitory activity, we demonstrate the utility of an experiment pipeline comprised of six distinct assays to assess the stages of type III secretion impacted: T3SS gene copy number, T3SS gene expression, T3SS basal body and needle assembly, secretion of cargo through the T3SS, and translocation of T3SS effector proteins into host cells. We used enteropathogenic Yersinia as the workhorse T3SS-expressing model organisms for this experimental pipeline, as Yersinia is sensitive to all T3SS inhibitors we tested, including those active against other T3SS-expressing pathogens. We find that this experimental pipeline is capable of rapidly distinguishing between T3SS inhibitors that interrupt the process of type III secretion at different points in T3SS assembly and function. For example, our data suggests that Compound 3, a malic diamide, blocks either activity of the assembled T3SS or alters the structure of the T3SS in a way that blocks T3SS cargo secretion but not antibody recognition of the T3SS needle. In contrast, our data predicts that Compound 4, a haloid-containing sulfonamidobenzamide, disrupts T3SS needle subunit secretion or assembly. Furthermore, we suggest that misregulation of copy number control of the pYV virulence plasmid, which encodes the Yersinia T3SS, should be considered as a possible mode of action for compounds with T3SS inhibitory activity against Yersinia.

Lipopolysaccharide-associated resistance to killing of yersiniae by complement

Yersinia pestis, Y. pseudotuberculosis, and Y. enterocolitica share ~70 kb low calcium response (Lcr) plasmids encoding virulence factors expressed at 37°C that, except for the adhesin YadA, are repressed by Ca2+(Lcr+). Virulence factors encoded on both the Lcr plasmid and chromosome have been reported to protect yersiniae against complement-dependent killing. In this study, LPS was isolated from yersiniae of serum-sensitive phenotypes (Lcr+and Lcr-Y. enterocolitica and Y. pseudotuberculosis grown at 26°C and Lcr-Y. enterocolitica grown at 37°C) and incorporated into liposomes containing radioactive chromium. These vesicles lysed with release of free51Cr in normal but not decomplemented serum. Liposomes prepared from serum-resistant phenotypes (Lcr+and Lcr-Y. pestis grown at 26°C or 37°C, Lcr+and Lcr-Y. pseudotubercu losis grown at 37°C, and Lcr+Y. enterocolitica grown at 37°C) did not undergo complement-dependent lysis. LPS from serum-resistant Y. pestis and Y. pseudotuberculosis was rough as judged by deficiency of O-groups.

Role of Yersinia pestis toxin complex family proteins in resistance to phagocytosis by polymorphonuclear leukocytes

Yersinia pestis carries homologues of the toxin complex (Tc) family proteins, which were first identified in other Gram-negative bacteria as having potent insecticidal activity. The Y. pestis Tc proteins are neither toxic to fleas nor essential for survival of the bacterium in the flea, even though tc gene expression is highly upregulated and much more of the Tc proteins YitA and YipA are produced in the flea than when Y. pestis is grown in vitro. We show that Tc(+) and Tc(-) Y. pestis strains are transmitted equivalently from coinfected fleas, further demonstrating that the Tc proteins have no discernible role, either positive or negative, in transmission by the flea vector. Tc proteins did, however, confer Y. pestis with increased resistance to killing by polymorphonuclear leukocytes (PMNs). Resistance to killing was not the result of decreased PMN viability or increased intracellular survival but instead correlated with a Tc protein-dependent resistance to phagocytosis that was independent of the type III secretion system (T3SS). Correspondingly, we did not detect T3SS-dependent secretion of the native Tc proteins YitA and YipA or the translocation of YitA- or YipA-β-lactamase fusion proteins into CHO-K1 (CHO) cells or human PMNs. Thus, although highly produced by Y. pestis within the flea and related to insecticidal toxins, the Tc proteins do not affect interaction with the flea or transmission. Rather, the Y. pestis Tc proteins inhibit phagocytosis by mouse PMNs, independent of the T3SS, and may be important for subverting the mammalian innate immune response immediately following transmission from the flea.

Regulation of the Yersinia type III secretion system: traffic control

Yersinia species, as well as many other Gram-negative pathogens, use a type III secretion system (T3SS) to translocate effector proteins from the bacterial cytoplasm to the host cytosol. This T3SS resembles a molecular syringe, with a needle-like shaft connected to a basal body structure, which spans the inner and outer bacterial membranes. The basal body of the injectisome shares a high degree of homology with the bacterial flagellum. Extending from the T3SS basal body is the needle, which is a polymer of a single protein, YscF. The distal end of the needle serves as a platform for the assembly of a tip complex composed of LcrV. Though never directly observed, prevailing models assume that LcrV assists in the insertion of the pore-forming proteins YopB and YopD into the host cell membrane. This completes a bridge between the bacterium and host cell to provide a continuous channel through which effectors are delivered. Significant effort has gone into understanding how the T3SS is assembled, how its substrates are recognized and how substrate delivery is controlled. Arguably the latter topic is the least understood; however, recent advances have provided new insight, and therefore, this review will focus primarily on summarizing the current state of knowledge regarding the control of substrate delivery by the T3SS. Specifically, we will discuss the roles of YopK, as well as YopN and YopE, which have long been linked to regulation of translocation. We also propose models whereby the YopK regulator communicates with the basal body of the T3SS to control translocation.

Three Yersinia pestis adhesins facilitate Yop delivery to eukaryotic cells and contribute to plague virulence

To establish a successful infection, Yersinia pestis requires the delivery of cytotoxic Yops to host cells. Yops inhibit phagocytosis, block cytokine responses, and induce apoptosis of macrophages. The Y. pestis adhesin Ail facilitates Yop translocation and is required for full virulence in mice. To determine the contributions of other adhesins to Yop delivery, we deleted five known adhesins of Y. pestis. In addition to Ail, plasminogen activator (Pla) and pH 6 antigen (Psa) could mediate Yop translocation to host cells. The contribution of each adhesin to binding and Yop delivery was dependent upon the growth conditions. When cells were pregrown at 28°C and pH 7, the order of importance for adhesins in cell binding and cytotoxicity was Ail > Pla > Psa. Y. pestis grown at 37°C and pH 7 had equal contributions from Ail and Pla but an undetectable role for Psa. At 37°C and pH 6, both Ail and Psa contributed to binding and Yop delivery, while Pla contributed minimally. Pla-mediated Yop translocation was independent of protease activity. Of the three single mutants, the Δail mutant was the most defective in mouse virulence. The expression level of ail was also the highest of the three adhesins in infected mouse tissues. Compared to an ail mutant, additional deletion of psaA (encoding Psa) led to a 130,000-fold increase in the 50% lethal dose for mice relative to that of the KIM5 parental strain. Our results indicate that in addition to Ail, Pla and Psa can serve as environmentally specific adhesins to facilitate Yop secretion, a critical virulence function of Y. pestis.

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.

Proteomic characterization of Yersinia pestis virulence

The Yersinia pestis proteome was studied as a function of temperature and calcium by two-dimensional differential gel electrophoresis. Over 4,100 individual protein spots were detected, of which hundreds were differentially expressed. A total of 43 differentially expressed protein spots, representing 24 unique proteins, were identified by mass spectrometry. Differences in expression were observed for several virulence-associated factors, including catalase-peroxidase (KatY), murine toxin (Ymt), plasminogen activator (Pla), and F1 capsule antigen (Caf1), as well as several putative virulence factors and membrane-bound and metabolic proteins. Differentially expressed proteins not previously reported to contribute to virulence are candidates for more detailed mechanistic studies, representing potential new virulence determinants.

Real-time characterization of virulence factor expression in Yersinia pestis using a GFP reporter system

A real-time reporter system was developed to monitor the thermal induction of virulence factors in Yersinia pestis, the etiological agent of plague. The reporter system consists of a plasmid in Y. pestis in which the expression of green fluorescent protein (GFP) is under the control of the promoters for six virulence factors, yopE, sycE, yopK, yopT, yscN, and lcrE yopN, which are all components of the Type III secretion virulence mechanism of Y. pestis. Induction of the expression of these genes in vivo was determined by the increase in fluorescence intensity of GFP in real time, in 96-well format. Different basal levels of expression at 26 degrees C were observed for the Y. pestis promoters. Expressed as percentages of the level measured for the lac promoter (positive control), the basal expression levels before temperature shift were: yopE (15%), sycE (15%), yopK (13%), yopT (4%), lcrE (3.3%), and yscN (0.8%). Following the shift in temperature from 26 to 37 degrees C, the rates of expression of these genes increased with the yopE reporter showing the strongest degree of induction. The rates of induction of the other virulence factors after the temperature, expressed as percentages of yopE induction, were: yopK (57%), sycE (9%), yscN (3%), lcrE (3%), and yopT (2%). The thermal induction of each of these promoter fusions was repressed by calcium, and the ratios of the initial rates of thermal induction without calcium supplementation compared to the rate with calcium supplementation were: yopE (11-fold), yscN (7-fold), yopK (6-fold), lcrE (3-fold), yopT (2-fold), and sycE (1-fold). This work demonstrates a novel approach to quantify gene induction and provides a method to rapidly determine the effects of external stimuli on expression of Y. pestis virulence factors in real time, in living cells, as a means to characterize virulence determinants.

Molecular characterization of KatY (antigen 5), a thermoregulated chromosomally encoded catalase-peroxidase of Yersinia pestis

The first temperature-dependent proteins (expressed at 37 degrees C, but not 26 degrees C) to be identified in Yersinia pestis were antigens 3 (fraction 1), 4 (pH 6 antigen), and 5 (hereafter termed KatY). Antigens 3 and 4 are now established virulence factors, whereas little is known about KatY, except that it is encoded chromosomally, produced in abundance, possesses modest catalase activity, and is shared by Yersinia pseudotuberculosis, but not Yersinia enterocolitica. We report here an improved chromatographic method (DEAE-cellulose, calcium hydroxylapatite, and Sephadex G-150) that yields enzymatically active KatY (2,423 U/mg of protein). Corresponding mouse monoclonal antibody 1B70.1 detected plasminogen activator-mediated hydrolysis of KatY, and a polyclonal rabbit antiserum raised against outer membranes of Y. pestis was enriched for anti-KatY. A sequenced approximately 16-kb Y. pestis DNA insert of a positive pLG338 clone indicated that katY encodes an 81.4-kDa protein (pI 6.98) containing a leader sequence of 2.6 kDa; the deduced molecular mass and pI of processed KatY were 78.8 kDa and 6. 43, respectively. A minor truncated variant (predicted molecular mass of 53.6 kDa) was also expressed. KatY is similar (39 to 59% identity) to vegetative bacterial catalase-peroxidases (KatG in Escherichia coli) and is closely related to plasmid-encoded KatP of enterohemorrhagic E. coli O157:H7 (75% identity). katY encoded a putative Ca2+-binding site, and its promoter contained three homologues to the consensus recognition sequence of the pCD-encoded transcriptional activator LcrF. rbsA was located upstream of katY, and cybB, cybC, dmsABC, and araD were mapped downstream. These genes are not linked to katG or katP in E. coli.

Expression of the plague plasminogen activator in Yersinia pseudotuberculosis and Escherichia coli

Enteropathogenic yersiniae (Yersinia pseudotuberculosis and Yersinia enterocolitica) typically cause chronic disease as opposed to the closely related Yersinia pestis, the causative agent of bubonic plague. It is established that this difference reflects, in part, carriage by Y. pestis of a unique 9.6-kb pesticin or Pst plasmid (pPCP) encoding plasminogen activator (Pla) rather than distinctions between shared approximately 70-kb low-calcium-response, or Lcr, plasmids (pCD in Y. pestis and pYV in enteropathogenic yersiniae) encoding cytotoxic Yops and anti-inflammatory V antigen. Pla is known to exist as a combination of 32.6-kDa (alpha-Pla) and slightly smaller (beta-Pla) outer membrane proteins, of which at least one promotes bacterial dissemination in vivo and degradation of Yops in vitro. We show here that only alpha-Pla accumulates in Escherichia coli LE392/pPCP1 cultivated in enriched medium and that either autolysis or extraction of this isolate with 1.0 M NaCl results in release of soluble alpha and beta forms possessing biological activity. This process also converted cell-bound alpha-Pla to beta-Pla and smaller forms in Y. pestis KIM/pPCP1 and Y. pseudotuberculosis PB1/+/pPCP1 but did not promote solubilization. Pla-mediated posttranslational hydrolysis of pulse-labeled Yops in Y. pseudotuberculosis PB1/+/pPCP1 occurred more slowly than that in Y. pestis but was otherwise similar except for accumulation of stable degradation products of YadA, a pYV-mediated fibrillar adhesin not encoded in frame by pCD. Carriage of pPCP by Y. pseudotuberculosis did not significantly influence virulence in mice.

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.

Membrane protein variations associated with in vitro passage of Borrelia burgdorferi

Borrelia burgdorferi, the causative agent of Lyme disease, undergoes a loss in virulence with repeated passage in vitro. Defining the changes which occur after conversion to avirulence may assist in identifying virulence factors and mechanisms of pathogenesis. We have used a cross-adsorption technique and two-dimensional nonequilibrium pH gradient electrophoresis to compare virulent (low-passage) and avirulent (high-passage) variants of B. Burgdorferi B31. Using cross-adsorbed rabbit sera to probe immunoblots, we identified 10 low-passage-associated proteins (relative molecular masses of 78, 58, 49, 34, 33, 28, 24, 20, and 16 kDa) unique to the virulent strain B31. Cross-adsorbed human serum detected five proteins of similar sizes (78, 58, 34, 28, and 20 kDa), suggesting that several of of these proteins were expressed during human infection. By probing inner and outer membranes, two proteins (58 and 33 kDa) that localized specifically to the outer membrane were observed. An additional low-passage-associated protein (28 kDa) was identified when outer membranes from low- and high-pressure variants of strain B31 were compared by two-dimensional nonequilibrium pH gradient electrophoresis.

Suppression of cytokines in mice by protein A-V antigen fusion peptide and restoration of synthesis by active immunization

It is established that an approximately 70-kb Lcr plasmid enables Yersinia pestis, the causative agent of bubonic plague, to multiply in focal necrotic lesions within visceral organs of mice by preventing net synthesis of the cytokines tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma), thereby minimizing inflammation (Lcr+). Rabbit antiserum raised against cloned staphylococcal protein A-V antigen fusion peptide (PAV) is known to passively immunize mice against 10 minimum lethal doses of intravenously injected Lcr+ cells of Y. pestis. In this study, injected PAV suppressed TNF-alpha and IFN-gamma in mice challenged with avirulent V antigen-deficient Y. pestis (lcrV or Lcr-) and promoted survival in vivo of these isolates as well as salmonellae and Listeria monocytogenes (with which the outcome was lethal). Active immunization of mice with PAV protected against 1,000 minimum lethal doses of intravenously injected Lcr+ cells of Y. pestis and Yersinia pseudotuberculosis but not Yersinia enterocolitica. The progressive necrosis provoked by Lcr+ cells of Y. pestis in visceral organs of nonimmunized mice was replaced after active immunization with PAV by massive infiltration of neutrophils and mononuclear cells (which generated protective granulomas indistinguishable from those formed against avirulent Lcr- mutants in nonimmunized mice). Distinct multiple abscesses typical of Lcr+ cells of Y. pseudotuberculosis were prevented by similar immunization. Significant synthesis of TNF-alpha and IFN-gamma occurred in spleens of mice actively immunized with PAV after challenge with Lcr+ cells of Y. pestis. These findings suggest that V antigen contributes to disease by suppressing the normal inflammatory response.

Physiological basis of the low calcium response in Yersinia pestis

It is established that duplication in vitro of that amount of Ca2+ (2.5 mM) and Mg2+ (1.5 mM) present in blood permits vegetative growth of Yersinia pestis with repression of virulence factors encoded by the Lcr plasmid (Lcr+); similar simulation of intracellular fluid (no Ca2+ and 20 mM Mg2+) promotes bacteriostasis with induction of these virulence determinants. However, proliferation of yersiniae in mice occurs primarily within necrotic focal lesions (supplied by Ca(2+)-deficient host cell cytoplasm) within visceral organs rather than in Ca(2+)-sufficient blood. The present study addressed this enigma by defining conditions necessary for achieving vegetative growth of Lcr+ yersiniae at 37 degrees C in simulated intracellular fluid. Maximum optical densities were increased by substitution of K+ for Na+ and elimination of Cl-; the combination of Na+ plus L-glutamate was selectively toxic to Lcr+ cells. This phenomenon was attributed in part to the absence of aspartase in Y. pestis (a lesion known to facilitate massive accumulation of L-aspartate via transamination of the oxalacetate pool by L-glutamate). Replacement of L-glutamate by exogenous L-aspartate or alpha-ketoglutarate reversed this toxicity by favoring retention of oxalacetate. Proliferation of Lcr+ cells in a medium containing K+ and L-aspartate but lacking added Ca2+ and Na+ was markedly enhanced by increasing the concentration of fermentable carbohydrate. Accordingly, in the worst-case scenario (i.e., added Na+, Cl-, and L-glutamate), Lcr+ yersiniae underwent restriction of growth after one doubling, and in the best-case scenario (i.e., added K+ and L-aspartate), the organisms completed more than five doublings, thereby achieving full-scale growth. Both of these Ca(2+)-deficient media promoted maximum induction of Mg(2+)-induced V antigen, a virulence factor encoded by the Lcr plasmid.

Passive immunity to yersiniae mediated by anti-recombinant V antigen and protein A-V antigen fusion peptide

LcrV (V antigen), a known unstable 37.3-kDa monomeric peptide encoded on the ca. 70-kb Lcr plasmid of Yersinia pestis, Yersinia pseudotuberculosis, and Yersinia enterocolitica, has been implicated as a regulator of the low-calcium response, virulence factor, and protective antigen. In this study, lcrV of Y. pestis was cloned into protease-deficient Escherichia coli BL21. The resulting recombinant V antigen underwent marked degradation from the C-terminal end during purification, yielding major peptides of 36, 35, 34, and 32 to 29 kDa. Rabbit gamma globulin raised against this mixture of cleavage products provided significant protection against 10 minimum lethal doses of Y. pestis (P < 0.01) and Y. pseudotuberculosis (P < 0.02). To both stabilize V antigen and facilitate its purification, plasmid pPAV13 was constructed so as to encode a fusion of lcrV and the structural gene for protein A (i.e., all but the first 67 N-terminal amino acids of V antigen plus the signal sequence and immunoglobulin G-binding domains but not the cell wall-associated region of protein A). The resulting fusion peptide, termed PAV, could be purified to homogeneity in one step by immunoglobulin G affinity chromatography and was stable thereafter. Rabbit polyclonal gamma globulin directed against PAV provided excellent passive immunity against 10 minimum lethal doses of Y. pestis (P < 0.005) and Y. pseudotuberculosis (P < 0.005) but was ineffective against Y. enterocolitica. Protection failed after absorption with excess PAV, cloned whole V antigen, or a large (31.5-kDa) truncated derivative of the latter but was retained (P < 0.005) upon similar absorption with a smaller (19.3-kDa) truncated variant, indicating that at least one protective epitope resides internally between amino acids 168 and 275.

Storage reservoirs of hemin and inorganic iron in Yersinia pestis

It is established that a high-frequency chromosomal deletion of ca. 100 kb accounts for the loss of properties making up the pigmented phenotype (Pgm+) of wild-type Yersinia pestis. These determinants are known to include virulence by peripheral routes of injection, sensitivity to the bacteriocin pesticin, adsorption of exogenous hemin or Congo red at 26 degrees C, and growth in iron-sequestered medium at 37 degrees C. We have now identified the outer membrane as the primary site of exogenous hemin storage in Pgm+ cells grown at 26 degrees C. Significant outer membrane storage of hemin did not occur in Pgm- mutants or in Pgm+ cells cultivated at 37 degrees C. However, both Pgm+ and Pgm- organisms grown at 37 degrees C contained a periplasmic reservoir of hemin, which may be associated with a temperature-dependent ca. 70-kDa peptide recently equated with antigen 5. At 37 degrees C, Pgm+ and Pgm- yersiniae also utilized a cytoplasmic ca. 19-kDa bacterioferritin-like peptide for deposition of inorganic iron. Incorporation of [55Fe]hemin into pools at 37 degrees C was not significantly inhibited by competition with excess unlabeled Fe3+. However, excess unlabeled hemin modestly competed with incorporation of label from 55FeCl3. This relative independence of storage pools observed at 37 degrees C is consistent with physiological linkage to in vivo acquisition and transport of Fe3+ from ferritin and of hemin from hemoglobin, myoglobin, or hemopexin.

Association between virulence of Yersinia pestis and suppression of gamma interferon and tumor necrosis factor alpha

It is established that Yersinia pestis, the causative agent of bubonic plague, and enteropathogenic Yersinia pseudotuberculosis and Yersinia enterocolitica share a ca. 70-kb low-calcium response or Lcr plasmid (Lcr+). The latter is known to encode regulatory functions that restrict growth at 37 degrees C in Ca(2+)-deficient medium and virulence factors that are expressed only in vitro within this environment (e.g., certain Yops and V antigen). In this study, gamma interferon (IFN-gamma) was never detected in mice infected with 10 minimum lethal doses (MLD) of Lcr+ cells of Y. pestis, and significant levels of tumor necrosis factor alpha (TNF-alpha) arose only prior to death. Prompt and marked synthesis of these cytokines was observed upon infection with avirulent Lcr- mutants. Treatment of mice with exogenous IFN-gamma plus TNF-alpha inhibited multiplication of Lcr+ yersiniae in vivo, thereby providing protection against challenge with 10 MLD. Administration of both cytokines was required for absolute survival, suggesting a synergistic rather than cumulative interaction. This protective effect entailed cytokine priming as judged by subsequent detection of substantial levels of endogenous IFN-gamma and TNF-alpha. Monospecific anti-V-antigen, known to provide passive immunity against 10 MLD of Lcr+ Y. pestis, permitted significant synthesis of endogenous IFN-gamma and TNF-alpha. These findings demonstrate that Lcr+ yersiniae suppress synthesis of cytokines and suggest that this effect is mediated by one or more Lcr plasmid-encoded virulence factors.

Major stable peptides of Yersinia pestis synthesized during the low-calcium response

It is established that the medically significant yersiniae require the presence of physiological levels of Ca2+ (ca. 2.5 mM) for sustained growth at 37 degrees C and that this nutritional requirement is mediated by a shared ca. 70-kb Lcr plasmid. The latter also encodes virulence factors (Yersinia outer membrane proteins [Yops] and V antigen) known to be selectively synthesized in vitro at 37 degrees C in Ca(2+)-deficient medium. In this study, cells of Yersinia pestis KIM were first starved for Ca2+ at 37 degrees C to prevent synthesis of bulk vegetative protein and then, after cell division had ceased, pulsed with [35S]methionine. After sufficient chase to ensure plasminogen activator-mediated degradation of Yops, the remaining major radioactive peptides were separated by conventional chromatographic methods and identified as Lcr plasmid-encoded V antigen and LcrH (and possibly LcrG), ca. 10-kb Pst plasmid-encoded pesticin and plasminogen activator, ca. 100-kb Tox plasmid-encoded fraction 1 (capsular) antigen and murine exotoxin, and chromosomally encoded antigen 4 (pH 6 antigen) and antigen 5 (a novel hemin-rich peptide possessing modest catalase activity but not superoxide dismutase activity). Also produced at high concentration was a chromosome-encoded GroEL-like chaperone protein. Accordingly, the transcriptional block preventing synthesis of bulk vegetative protein at 37 degrees C in Ca(2+)-deficient medium may not apply to genes encoding virulence factors or to highly conserved GroEL (known in other species to utilize a secondary stress-induced sigma factor).

Surface-exposed antigenic determinants in outer membranes of wild Yersinia pestis isolates

Immunogenic surface exposed envelope proteins of Yersinia pestis strains were investigated with SDS-PAGE and immunoblots with antisera of immunized guinea pigs and convalescent patients. The sarkosyl-insoluble outer membrane proteins (OMPs) of three human isolates and one laboratory strain of Y. pestis grown in different rich media to exponential or stationary phase, and cultivated at 28 degrees C or 37 degrees C, were assayed for the presence of immunogenic peptides. Seven guinea-pig sera immunized with one of the four Y. pestis strains and four human sera from plague-infected patients indicated that at least four outer membrane proteins with molecular weights of 45 KDal, 42 KDal, 21 KDal and 16.5 KDal were strongly recognized by at least one of them. All but one of the guinea-pig serum and one human serum recognized the 45 KDal protein. The 42 KDal was identified only in outer membrane isolated from Y. pestis cells grown at 28 degrees C. All immunogenic OMPs were found in the four strains investigated. Labelling of intact Y. pestis cells with Iodogen and 131I further demonstrated the surface-exposed location of the immunogenic OMPs.

Factors promoting acute and chronic diseases caused by yersiniae

The experimental system constructed with the medically significant yersiniae provides a powerful basic model for comparative study of factors required for expression of acute versus chronic disease. The system exploits the close genetic similarity between Yersinia pestis, the etiological agent of bubonic plague, and enteropathogenic Yersinia pseudotuberculosis and Yersinia enterocolitica. Y. pestis possesses three plasmids, of which one, shared by the enteropathogenic species, mediates a number of virulence factors that directly or indirectly promote survival within macrophages and immunosuppression. The two remaining plasmids are unique and encode functions that promote acute disease by enhancing bacterial dissemination in tissues and resistance to phagocytosis by neutrophils and monocytes. These properties are replaced in the enteropathogenic yersiniae by host cell invasins and an adhesin which promote chronic disease; the latter are cryptic in Y. pestis. Additional distinctions include specific mutational losses in Y. pestis which result in loss of fitness in natural environments plus gain of properties that facilitate transmission and infection via fleabite.

Pathogenicity of Yersinia kristensenii for mice

Forty-seven strains of Yersinia kristensenii from widely differing sources, representing all known O serogroups of this species, were investigated for virulence with a variety of animal and in vitro assays. Twenty-four (51%) of the isolates were lethal for mice pretreated with iron dextran. Mouse-lethal strains occurred predominantly within O serogroups O:11, O:12,25, and O:16. Virulent Y. kristensenii strains generally did not express the virulence-associated phenotype (Ca2+ dependence and binding of Congo red and crystal violet) which characterizes virulent strains of Y. enterocolitica, nor did they carry the Yersinia virulence plasmid. Although all strains hybridized with a DNA probe derived from the inv (invasin) gene of Y. enterocolitica, none was able to invade HEp-2 epithelial cell culture. Y. kristensenii strains were virulent only when inoculated parenterally into iron-loaded mice. Animals infected in this way succumbed rapidly to infection, generally within 24 h. This finding suggested that the pathogenicity of these bacteria may be attributable to a secreted toxin, but a search for such a substance and for other in vitro correlates of pathogenicity was unsuccessful. These observations indicate that some strains of Y. kristensenii kill mice by a mechanism not previously recognized in yersiniae.

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.