A surface protease and the invasive character of plague

Substrate specificity and screening of the integral membrane protease Pla

This paper reports a study to find small peptide substrates for the important virulence factor of Yersinia pestis, plasminogen activator, Pla. The method used to find small substrates for this protease is reported along with studies examining the ability of these peptides to inhibit activity of the enzyme. Through the use of parallel synthesis and positional scanning, small tripeptides were identified that are viable substrates for the protease.

Microseconds dynamics simulations of the outer-membrane protease T

Conformational fluctuations of enzymes may play an important role for substrate recognition and/or catalysis, as it has been suggested in the case of the protease enzymatic superfamily. Unfortunately, theoretically addressing this issue is a problem of formidable complexity, as the number of the involved degrees of freedom is enormous: indeed, the biological function of a protein depends, in principle, on all its atoms and on the surrounding water molecules. Here we investigated a membrane protease enzyme, the OmpT from Escherichia coli, by a hybrid molecular mechanics/coarse-grained approach, in which the active site is treated with the GROMOS force field, whereas the protein scaffold is described with a Go-model. The method has been previously tested against results obtained with all-atom simulations. Our results show that the large-scale motions and fluctuations of the electric field in the microsecond timescale may impact on the biological function and suggest that OmpT employs the same catalytic strategy as aspartic proteases. Such a conclusion cannot be drawn within the 10- to 100-ns timescale typical of current molecular dynamics simulations. In addition, our studies provide a structural explanation for the drop in the catalytic activity of two known mutants (S99A and H212A), suggesting that the coarse-grained approach is a fast and reliable tool for providing structure/function relationships for both wild-type OmpT and mutants.

Yersinia pestis biofilm in the flea vector and its role in the transmission of plague

Transmission by fleabite is a relatively recent evolutionary adaptation of Yersinia pestis, the bacterial agent of bubonic plague. To produce a transmissible infection, Y. pestis grows as an attached biofilm in the foregut of the flea vector. Biofilm formation both in the flea foregut and in vitro is dependent on an extracellular matrix (ECM) synthesized by the Yersinia hms gene products. The hms genes are similar to the pga and ica genes of Escherichia coli and Staphylococcus epidermidis, respectively, that act to synthesize a poly-beta-1,6-N-acetyl-d-glucosamine ECM required for biofilm formation. As with extracellular polysaccharide production in many other bacteria, synthesis of the Hms-dependent ECM is controlled by intracellular levels of cyclic-di-GMP. Yersinia pseudotuberculosis, the food- and water-borne enteric pathogen from which Y. pestis evolved recently, possesses identical hms genes and can form biofilm in vitro but not in the flea. The genetic changes in Y. pestis that resulted in adapting biofilm-forming capability to the flea gut environment, a critical step in the evolution of vector-borne transmission, have yet to be identified. During a flea bite, Y. pestis is regurgitated into the dermis in a unique biofilm phenotype, and this has implications for the initial interaction with the mammalian innate immune response.

How the structural gene products of Yersinia pestis relate to virulence

Bubonic plague is the most devastating acute infectious disease known to man. The causative agent, Yersinia pestis, is now more firmly entrenched in sylvatic reservoirs throughout the world than at any time in the past. Consequently, the organism increasingly causes casual human disease and is readily available for use as a bioweapon. Recent attempts to understand the severe nature of plague have focused upon its very recent divergence from Yersinia pseudotuberculosis, an etiological instrument of chronic enteropathogenic infection. This review emphasizes that the invasive nature of plague and its dissemination by fleabite is mediated by plasmids not shared by enteropathogenic yersiniae. The basis for high lethality is considered within the context of chromosomal degeneration causing loss of normal metabolic functions and modification of virulence factors, permitting a terminal anti-inflammatory phase associated with pronounced septicemia.

Relationship of the lipopolysaccharide structure of Yersinia pestis to resistance to antimicrobial factors

Disruption of lipopolysaccharide (LPS) biosynthesis genes in an epidemiologically significant Yersinia pestis strain showed that the ability to synthesize the full inner core of the LPS is crucial for resistances to the bactericidal action of antimicrobial peptides and to complement-mediated serum killing. Resistance to polymyxin B also requires a high content of the cationic sugar, 4-amino-4-deoxy-L-arabinose, in lipid A.

The lack of binding of VEK-30, an internal peptide from the group A streptococcal M-like protein, PAM, to murine plasminogen is due to two amino acid replacements in the plasminogen kringle-2 domain

VEK-30, a 30-amino acid internal peptide present within a streptococcal M-like plasminogen (Pg)-binding protein (PAM) from Gram-positive group-A streptococci (GAS), represents an epitope within PAM that shows high affinity for the lysine binding site (LBS) of the kringle-2 (K2) domain of human (h)Pg. VEK-30 does not interact with this same region of mouse (m)Pg, despite the high conservation of the mK2- and hK2-LBS. To identify the molecular basis for the species specificity of this interaction, hPg and mPg variants were generated, including an hPg chimera with the mK2 sequence and an mPg chimera containing the hK2 sequence. The binding of synthetic VEK-30 to these variants was studied by surface plasmon resonance. The data revealed that, in otherwise intact Pg, the species specificity of VEK-30 binding in these two cases is entirely dictated by two K2 residues that are different between hPg and mPg, namely, Arg-220 of hPg, which is a Gly in mPg, and Leu-222 of hPg, which is a Pro in mPg, neither of which are members of the canonical K2-LBS. Neither the activation of hPg, nor the enzymatic activity of its activated product, plasmin (hPm), are compromised by replacing these two amino acids by their murine counterparts. It is also demonstrated that hPg is more susceptible to activation to hPm after complexation with VEK-30 and that this property is greatly reduced as a result of the R220G and L222P replacements in hPg. These mechanisms for accumulation of protease activity on GAS likely contribute to the virulence of PAM(+)-GAS strains and identify targets for new therapeutic interventions.

Phenotypic characterization of OmpX, an Ail homologue of Yersinia pestis KIM

The goal of this study was to characterize the Yersinia pestis KIM OmpX protein. Yersinia spp. provide a model for studying several virulence processes including attachment to, and internalization by, host cells. For Yersinia enterocolitica and Yersinia pseudotuberculosis, Ail, YadA and Inv, have been implicated in these processes. In Y. pestis, YadA and Inv are inactivated. Genomic analysis of two Y. pestis strains revealed four loci with sequence homology to Ail. One of these genes, designated y1324 in the Y. pestis KIM database, encodes a protein designated OmpX. The mature protein has a predicted molecular mass of 17.47 kDa, shares approximately 70 % sequence identity with Y. enterocolitica Ail, and has an identical homologue, designated Ail, in the Y. pestis CO92 database. The present study compared the Y. pestis KIM6(+) parental strain with a mutant derivative having an engineered disruption of the OmpX structural gene. The parental strain (and a merodiploid control strain) expressed OmpX at 28 and 37 degrees C, and the protein was detectable throughout all phases of growth. OmpX was required for efficient adherence to, and internalization by, cultured HEp-2 cell monolayers and conferred resistance to the bactericidal effect of human serum. Deletion of ompX resulted in a significantly reduced autoaggregation phenotype and loss of pellicle formation in vitro. These results suggest that Y. pestis OmpX shares functional homology with Y. enterocolitica Ail in adherence, internalization into epithelial cells and serum resistance.

Transcriptome analysis of Yersinia pestis in human plasma: an approach for discovering bacterial genes involved in septicaemic plague

Yersinia pestis is the aetiologic agent of plague. Without appropriate treatment, the pathogen rapidly causes septicaemia, the terminal and fatal phase of the disease. In order to identify bacterial genes which are essential during septicaemic plague in humans, we performed a transcriptome analysis on the fully virulent Y. pestis CO92 strain grown in either decomplemented human plasma or Luria-Bertani medium, incubated at either 28 or 37 degrees C and harvested at either the mid-exponential or the stationary growth phase. Y. pestis genes involved in 12 iron-acquisition systems and one iron-storage system (bfr, bfd) were specifically induced in human plasma. Of these, the ybt and tonB genes (encoding the yersiniabactin siderophore virulence factor and the siderophore transporter, respectively) were induced at 37 degrees C, i.e. under conditions mimicking the mammalian environment. Growth in human plasma also upregulated genes involved in the synthesis of five fimbrial-like structures (including the Psa virulence factor), and in purine/pyrimidine metabolism (the nrd genes). Genes known to play a role in the virulence of several bacterial pathogens (such as those encoding the Lpp lipoprotein and non-iron metal-uptake proteins) were induced in human plasma, during either the exponential or the stationary phase. Finally, 120 genes encoding proteins of unknown function were upregulated in human plasma. Eleven of these genes were specifically transcribed at 37 degrees C and may thus represent new virulence factors that are important during the septicaemic phase of human plague.

Direct transcriptional control of the plasminogen activator gene of Yersinia pestis by the cyclic AMP receptor protein

Horizontal gene transfer events followed by proper regulatory integration of a gene drive rapid evolution of bacterial pathogens. A key event in the evolution of the highly virulent plague bacterium Yersinia pestis was the acquisition of plasmid pPCP1, which carries the plasminogen activator gene, pla. This promoted the bubonic form of the disease by increasing bacterial dissemination from flea bite sites and incidentally enhanced replication in respiratory airways during pneumonic infection. We determined that expression of pla is controlled by the global regulator cyclic AMP (cAMP) receptor protein (Crp). This transcription factor is well conserved among distantly related bacteria, where it acts as a soluble receptor for the ubiquitous signaling molecule cAMP and controls a global network of metabolic and stress-protective genes. Crp has a similar physiological role in Y. pestis since loss of its function resulted in an inability to metabolize a variety of nonglucose substrates. Activation of pla expression requires a transcription activation element of the pla promoter that serves as a Crp binding site. Crp interaction with this site was demonstrated to occur only in the presence of cAMP. Alteration of the Crp binding site nucleotide sequence prevented in vitro formation of Crp-DNA complexes and inhibited in vivo expression of pla. The placement of pla under direct regulatory control of Crp highlights how highly adapted pathogens integrate laterally acquired genes to coordinate virulence factor expression with global gene networks to maintain homeostasis through the infectious life cycle.

Activation of pro-matrix metalloproteinase-9 and degradation of gelatin by the surface protease PgtE of Salmonella enterica serovar Typhimurium

Mammalian matrix metalloproteinases (MMPs) degrade collagen networks in extracellular matrices by cleaving collagen and its denatured form gelatin, and thus enhance migration of mammalian cells. The gastrointestinal pathogen Salmonella enterica survives and grows within host macrophages and dendritic cells, and can disseminate in the host by travelling within infected host cells. Here, we report that S. enterica serovar Typhimurium activates proMMP-9 (gelatinase B) secreted by human primary macrophages, and degrades gelatin after growth within J774A.1 murine macrophage-like cells. Both proMMP-9 activation and gelatin degradation were due to expression of the Salmonella surface protease PgtE. Following intraperitoneal infection in BALB/c mice, the amount of a pgtE deletion derivative was nearly ten-fold lower in the livers and spleens of mice than the amount of wild-type S. enterica, suggesting that PgtE contributes to dissemination of Salmonella in the host. PgtE belongs to the omptin family of bacterial beta-barrel transmembrane proteases. The ortholog of PgtE in Yersinia pestis, Pla, which is central for bacterial virulence in plague, was poor in proMMP-9 activation and in gelatin degradation. To model the evolution of these activities in the omptin barrel, we performed a substitution analysis in Pla and genetically modified it into a PgtE-like gelatinase. Our results indicate that PgtE and Pla have diverged in substrate specificity, and suggest that Salmonella PgtE has evolved to functionally mimic mammalian MMPs.

Modulation of dendritic cell differentiation and function by YopJ ofYersinia pestis

Yersinia pestis evades immune responses in part by injecting into host immune cells several effector proteins called Yersinia outer proteins (Yops) that impair cellular function. This has been best characterized in the innate effector cells, but much less so for cells involved in adaptive immune responses. Dendritic cells (DC) sit at the crossroads between innate and adaptive immunity, and can function to initiate or inhibit adaptive immune responses. Although Y. pestis can target and inactivate DC, the mechanism responsible for this remains unclear. We have found that injection of Y. pestis YopJ into DC progenitors disrupts key signal transduction pathways and interferes with DC differentiation and subsequent function. YopJ injection prevents up-regulation of the NF-kappaB transcription factor Rel B and inhibits MAPK/ERK activation--both having key roles in DC differentiation. Furthermore, YopJ injection prevents costimulatory ligand up-regulation, LPS-induced cytokine expression, and yields differentiated DC with diminished capability to induce T cell proliferation and IFN-gamma induction. By modulating DC function through YopJ-mediated disruption of signaling pathways during progenitor to DC differentiation, Yersinia may interfere with the adaptive responses necessary to clear the infection as well as establish a tolerant immune environment that leads to chronic infection/carrier state in the surviving host.

The skin as interface in the transmission of arthropod-borne pathogens

Animal skin separates the inner world of the body from the largely hostile outside world and is actively involved in the defence against microbes. However, the skin is no perfect defence barrier and many microorganisms have managed to live on or within the skin as harmless passengers or as disease-causing pathogens. Microbes have evolved numerous strategies that allow them to gain access to the layers underneath the epidermis where they either multiply within the dermis or move to distant destinations within the body for replication. A number of viruses, bacteria and parasites use arthropod vectors, like ticks or mosquitoes, to deliver them into the dermis while taking their blood meal. Within the dermis, successful pathogens subvert the function of a variety of skin resident cells or cells of the innate immune system that rush to the site of infection. In this review several interactions with cells of the skin by medically relevant vector-borne pathogens are discussed to highlight the different ways in which these pathogens have come to survive within the skin and to usurp the defence mechanisms of the host for their own ends.

Fibrin and fibrinolysis in infection and host defense

Bacterial pathogens have frequently evolved and maintained the capacity to engage and/or activate hemostatic system components of their vertebrate hosts. Recent studies of mice with selected alterations in host plasminogen and other hemostatic factors have begun to reveal a seminal role of bacterial plasminogen activators and fibrin clearance in microbial pathogenesis. Bacterial pathogens appear to exploit host plasmin-mediated proteolysis to both support microbial dissemination and evade innate immune surveillance systems. The contribution of bacterial plasminogen activation to the evasion of the inflammatory response is particularly conspicuous with the plague agent, Yersinia pestis. Infection of control mice with wild-type Y. pestis leads to the formation of widespread foci containing massive numbers of free bacteria with little inflammatory cell infiltrate, whereas the loss of either the bacterial plasminogen activator, Pla, or the elimination of host plasminogen results in the accumulation of robust inflammatory cell infiltrates at sites of infection and greatly improved survival. Interestingly, fibrin(ogen) deficiency undermines the local inflammatory response observed with Pla-deficient Y. pestis and effectively eliminates the survival benefits posed by the elimination of either host plasminogen or bacterial Pla. These studies, and complementary studies with other human pathogens, illustrate that plasminogen and fibrinogen are extremely effective modifiers of the inflammatory response in vivo and critical determinants of bacterial virulence and host defense. Detailed studies of the inflammatory response in mice with genetically-imposed modifications in coagulation and fibrinolytic factors underscore the regulatory crosstalk between the hemostatic and immune systems.

Identification and type III-dependent secretion of the Yersinia pestis insecticidal-like proteins

Plague, or the Black Death, is a zoonotic disease that is spread from mammal to mammal by fleas. This mode of transmission demands that the causative agent of this disease, Yersinia pestis, is able to survive and multiply in both mammals and insects. In recent years the complete genome sequence of a number of Y. pestis strains have been determined. This sequence information indicates that Y. pestis contains a cluster of genes with homology to insecticidal toxin encoding genes of the insect pathogen Photorhabdus luminescens. Here we demonstrate that Y. pestis KIM strains produced the encoded proteins. Production of the locus-encoded proteins was dependent on a gene (yitR) encoding a member of the LysR family of transcriptional activators. Evidence suggests the proteins are type III secretion substrates. N terminal amino acids (100 to 367) of each protein fused to an epitope tag were secreted by the virulence plasmid type III secretion type. A fusion protein comprised of the N-terminus of YipB and the enzymatic active component of Bordetella pertussis adenylate cyclase (Cya) was translocated into both mammalian and insect cells. In conclusion, a new class of Y. pestis type III secreted and translocated proteins has been identified. We hypothesize that these proteins function to promote transmission of and infection by Y. pestis.

Evaluation of the role of LcrV-Toll-like receptor 2-mediated immunomodulation in the virulence of Yersinia pestis

Pathogenic members of the Yersinia genus require the translocator protein LcrV for proper function of the type III secretion apparatus, which is crucial for virulence. LcrV has also been reported to play an independent immunosuppressive role via the induction of interleukin-10 (IL-10) through stimulation of Toll-like receptor 2 (TLR2). To investigate the LcrV-TLR2 interaction in vitro, His-tagged recombinant LcrV (rLcrV) from Yersinia pestis was cloned and expressed in Escherichia coli and purified through Ni-nitrilotriacetic acid column chromatography. High concentrations (5 microg/ml) of rLcrV stimulated TLR2 in vitro. Fractionation of rLcrV preparations via gel filtration revealed that only a minor component consisting of high-molecular-weight multimers or aggregates has TLR2 stimulating activity. Dimer and tetramer forms of rLcrV, which constitute the bulk of the material, do not have this activity. To investigate the potential role of LcrV/TLR2 in plague pathogenesis, we infected wild-type and TLR2(-/-) mice with virulent Y. pestis. No discernible difference between the two mouse strains in severity of disease or kinetics of survival after subcutaneous challenge was observed. IL-6, tumor necrosis factor, and IL-10 levels from spleen homogenates; bacterial load; and the extent of inflammation observed in organs from mice infected intravenously were also indistinguishable in both mouse strains. Taken together, our data indicate that the most abundant molecular species of Y. pestis LcrV do not efficiently activate TLR2-signaling and that TLR2-mediated immunomodulation is unlikely to play a significant role in plague.

Plague

Bubonic plague is an often fulminant systemic zoonosis, caused by Yersinia pestis. Conventional microbiology, bacterial population genetics, and genome sequence data, all suggest that Y pestis is a recently evolved clone of the enteric pathogen Yersinia pseudotuberculosis. The genetic basis of this organism's rapid adaptation to its insect vector (the flea) with transmission between mammalian hosts by novel subcutaneous and pneumonic routes of infection is becoming clearer. This transition provides a paradigm for the way in which new pathogens could emerge. Plague in humans is controlled by suppression of rodent reservoir hosts and their fleas and by early detection and treatment of cases of disease. Detection systems for plague in non-endemic regions might now be needed because of a bioterrorism threat. Rapid diagnostic tests are available and a subunit vaccine is in clinical trials.

The surface protease PgtE of Salmonella enterica affects complement activity by proteolytically cleaving C3b, C4b and C5

Complement activity in mammalian serum is fundamentally based on three homologous components C3b, C4b and C5. During systemic infection, the gastrointestinal pathogen Salmonella enterica disseminates within host phagocytic cells but also extracellularly. Consequently, systemic Salmonella transiently confronts the complement system. We show here that the surface protease PgtE of S. enterica proteolytically cleaves C3b, C4b and C5 and that the expression of PgtE enhances bacterial resistance to human serum. Degradation of C3b was further enhanced by PgtE-mediated plasminogen activation.

Plasmids and rickettsial evolution: insight from Rickettsia felis

BACKGROUND

The genome sequence of Rickettsia felis revealed a number of rickettsial genetic anomalies that likely contribute not only to a large genome size relative to other rickettsiae, but also to phenotypic oddities that have confounded the categorization of R. felis as either typhus group (TG) or spotted fever group (SFG) rickettsiae. Most intriguing was the first report from rickettsiae of a conjugative plasmid (pRF) that contains 68 putative open reading frames, several of which are predicted to encode proteins with high similarity to conjugative machinery in other plasmid-containing bacteria.

METHODOLOGY/PRINCIPAL FINDINGS

Using phylogeny estimation, we determined the mode of inheritance of pRF genes relative to conserved rickettsial chromosomal genes. Phylogenies of chromosomal genes were in agreement with other published rickettsial trees. However, phylogenies including pRF genes yielded different topologies and suggest a close relationship between pRF and ancestral group (AG) rickettsiae, including the recently completed genome of R. bellii str. RML369-C. This relatedness is further supported by the distribution of pRF genes across other rickettsiae, as 10 pRF genes (or inactive derivatives) also occur in AG (but not SFG) rickettsiae, with five of these genes characteristic of typical plasmids. Detailed characterization of pRF genes resulted in two novel findings: the identification of oriV and replication termination regions, and the likelihood that a second proposed plasmid, pRFdelta, is an artifact of the original genome assembly.

CONCLUSION/SIGNIFICANCE

Altogether, we propose a new rickettsial classification scheme with the addition of a fourth lineage, transitional group (TRG) rickettsiae, that is unique from TG and SFG rickettsiae and harbors genes from possible exchanges with AG rickettsiae via conjugation. We offer insight into the evolution of a plastic plasmid system in rickettsiae, including the role plasmids may have played in the acquirement of virulence traits in pathogenic strains, and the likely origin of plasmids within the rickettsial tree.

Cytosolic proteins contribute to surface plasminogen recruitment of Neisseria meningitidis

Plasminogen recruitment is a common strategy of pathogenic bacteria and results in a broad-spectrum surface-associated protease activity. Neisseria meningitidis has previously been shown to bind plasminogen. In this study, we show by several complementary approaches that endolase, DnaK, and peroxiredoxin, which are usually intracellular proteins, can also be located in the outer membrane and act as plasminogen receptors. Internal binding motifs, rather than C-terminal lysine residues, are responsible for plasminogen binding of the N. meningitidis receptors. Recombinant receptor proteins inhibit plasminogen association with N. meningitidis in a concentration-dependent manner. Besides binding purified plasminogen, N. meningitidis can also acquire plasminogen from human serum. Activation of N. meningitidis-associated plasminogen by urokinase results in functional activity and allows the bacteria to degrade fibrinogen. Furthermore, plasmin bound to N. meningitidis is protected against inactivation by alpha(2)-antiplasmin.

A Plasminogen-Activating Protease Specifically Controls the Development of Primary Pneumonic Plague

Primary pneumonic plague is transmitted easily, progresses rapidly, and causes high mortality, but the mechanisms by which Yersinia pestis overwhelms the lungs are largely unknown. We show that the plasminogen activator Pla is essential for Y. pestis to cause primary pneumonic plague but is less important for dissemination during pneumonic plague than during bubonic plague. Experiments manipulating its temporal expression showed that Pla allows Y. pestis to replicate rapidly in the airways, causing a lethal fulminant pneumonia; if unexpressed, inflammation is aborted, and lung repair is activated. Inhibition of Pla expression prolonged the survival of animals with the disease, offering a therapeutic option to extend the period during which antibiotics are effective.

Distribution of virulence-associated genes inYersinia enterocoliticabiovar 1A correlates with clonal groups and not the source of isolation

Yersinia enterocolitica, an important food- and water-borne enteric pathogen, is represented by six biovars viz. 1A, 1B and 2-5. Some biovar 1A strains, despite lacking virulence plasmid (pYV) and chromosomal virulence genes, have been reported to cause symptoms similar to that produced by isolates belonging to known pathogenic biovars. Virulence-associated genes viz. ail, virF, inv, myfA, ystA, ystB, ystC, tccC, hreP, fepA, fepD, fes, ymoA and sat were studied in 81 clinical and nonclinical strains of Y. enterocolitica biovar 1A by PCR amplification. All strains lacked ail, virF, ystA and ystC genes. The distribution of other genes with respect to clonal groups revealed that four genes viz. ystB, hreP, myfA and sat were associated exclusively with strains belonging to clonal group A. The clonal groups A and B were differentiated previously based on rep (REP-/ERIC) - PCR genomic fingerprinting. The distribution of virulence-associated genes, however, did not differ significantly between clinical and nonclinical strains. In strains of Y. enterocolitica biovar 1A, clonal groups seem to reflect virulence potential better than the source (clinical vs. nonclinical) of isolation.

Advances in antimicrobial peptide immunobiology

Antimicrobial peptides are ancient components of the innate immune system and have been isolated from organisms spanning the phylogenetic spectrum. Over an evolutionary time span, these peptides have retained potency, in the face of highly mutable target microorganisms. This fact suggests important coevolutionary influences in the host-pathogen relationship. Despite their diverse origins, the majority of antimicrobial peptides have common biophysical parameters that are likely essential for activity, including small size, cationicity, and amphipathicity. Although more than 900 different antimicrobial peptides have been characterized, most can be grouped as belonging to one of three structural classes: (1) linear, often of alpha-helical propensity; (2) cysteine stabilized, most commonly conforming to beta-sheet structure; and (3) those with one or more predominant amino acid residues, but variable in structure. Interestingly, these biophysical and structural features are retained in ribosomally as well as nonribosomally synthesized peptides. Therefore, it appears that a relatively limited set of physicochemical features is required for antimicrobial peptide efficacy against a broad spectrum of microbial pathogens. During the past several years, a number of themes have emerged within the field of antimicrobial peptide immunobiology. One developing area expands upon known microbicidal mechanisms of antimicrobial peptides to include targets beyond the plasma membrane. Examples include antimicrobial peptide activity involving structures such as extracellular polysaccharide and cell wall components, as well as the identification of an increasing number of intracellular targets. Additional areas of interest include an expanding recognition of antimicrobial peptide multifunctionality, and the identification of large antimicrobial proteins, and antimicrobial peptide or protein fragments derived thereof. The following discussion highlights such recent developments in antimicrobial peptide immunobiology, with an emphasis on the biophysical aspects of host-defense polypeptide action and mechanisms of microbial resistance.

The Psa fimbriae of Yersinia pestis interact with phosphatidylcholine on alveolar epithelial cells and pulmonary surfactant

The pH 6 antigen (Psa) of Yersinia pestis consists of fimbriae with adhesive properties of potential importance for the pathogenesis of plague, including pneumonic plague. The Psa fimbriae mediate bacterial binding to human alveolar epithelial cells. The Psa fimbriae bound mostly to one component present in the total lipid extract from type II alveolar epithelial cells of the cell line A549 separated by thin-layer chromatography (TLC). The Psa receptor was identified as phosphatidylcholine (PC) by TLC using alkali treatment, molybdenum blue staining, and Psa overlays. The Psa fimbriae bound to PC in a dose-dependent manner, and binding was inhibited by phosphorylcholine (ChoP) and choline. Binding inhibition was dose dependent, although only high concentrations of ChoP completely blocked Psa binding to PC. In contrast, less than 1 muM of a ChoP-polylysine polymer inhibited specifically the adhesion of Psa-fimbriated Escherichia coli to PC, and type I (WI-26 VA4) and type II alveolar epithelial cells. These results indicated that the homopolymeric Psa fimbriae are multimeric adhesins. Psa also bound to pulmonary surfactant, which covers the alveolar surface as a product of type II alveolar epithelial cells and includes PC as the major component. The observed dose-dependent interaction of Psa with pulmonary surfactant was blocked by ChoP. Interestingly, surfactant did not inhibit Psa-mediated bacterial binding to alveolar cells, suggesting that both surfactant and cell membrane PC retain Psa-fimbriated bacteria on the alveolar surface. Altogether, the results indicate that Psa uses the ChoP moiety of PC as a receptor to mediate bacterial binding to pulmonary surfactant and alveolar epithelial cells.

Effects of Psa and F1 on the adhesive and invasive interactions of Yersinia pestis with human respiratory tract epithelial cells

Yersinia pestis, the causative agent of plague, expresses the Psa fimbriae (pH 6 antigen) in vitro and in vivo. To evaluate the potential virulence properties of Psa for pneumonic plague, an Escherichia coli strain expressing Psa was engineered and shown to adhere to three types of human respiratory tract epithelial cells. Psa binding specificity was confirmed with Psa-coated polystyrene beads and by inhibition assays. Individual Y. pestis cells were found to be able to express the capsular antigen fraction 1 (F1) concomitantly with Psa on their surface when analyzed by flow cytometry. To better evaluate the separate effects of F1 and Psa on the adhesive and invasive properties of Y. pestis, isogenic Deltacaf (F1 genes), Deltapsa, and Deltacaf Deltapsa mutants were constructed and studied with the three respiratory tract epithelial cells. The Deltapsa mutant bound significantly less to all three epithelial cells compared to the parental wild-type strain and the Deltacaf and Deltacaf Deltapsa mutants, indicating that Psa acts as an adhesin for respiratory tract epithelial cells. An antiadhesive effect of F1 was clearly detectable only in the absence of Psa, underlining the dominance of the Psa+ phenotype. Both F1 and Psa inhibited the intracellular uptake of Y. pestis. Thus, F1 inhibits bacterial uptake by inhibiting bacterial adhesion to epithelial cells, whereas Psa seems to block bacterial uptake by interacting with a host receptor that doesn't direct internalization. The Deltacaf Deltapsa double mutant bound and invaded all three epithelial cell types well, revealing the presence of an undefined adhesin(s) and invasin(s).

Developing insect models for the study of current and emerging human pathogens

The study of human diseases requires the testing of microorganisms in model systems. Although mammals are typically used, we argue the validity of using insects as models in order to examine human diseases, particularly the growing number of opportunistic microorganisms. Insects can be used in large numbers, are easily manipulated, and are not subject to the same ethical concerns as mammalian systems. Insects and mammals have many parallels with respect to microbial pathogenesis, from proteinaceous integuments that require breaching before infection to similarities in their innate immune responses. Reactions of insects to Candida and Pseudomonas spp. infections show good correlation with mouse models, providing precedent-setting examples of the study of human pathogens using insects. Insects as pathogen hosts also warrant study because they may act as reservoirs for emerging human pathogens. Finally, insect models may be used to examine the evolutionary processes involved in the acquisition of virulence factors and host-jumping mechanisms indispensable to emerging pathogens. Insect models may be used in 'niche' investigations where large sample sizes can facilitate rapid, informative screening of opportunistic diseases and provide insights into pathogen evolution, while reducing the cost and ethical concerns associated with mammalian models.

The interaction between pathogens and the host coagulation system

There is mounting evidence that the hemostatic system is critical in host responses to bacterial infection. Invasive bacteria have evolved virulence strategies to interact with host hemostatic factors such as plasminogen and fibrinogen for infection. Furthermore, genetic variations in host hemostatic factors also influence host response to bacterial infection.

Virulence factors of Yersinia pestis are overcome by a strong lipopolysaccharide response

At mammalian body temperature, the plague bacillus Yersinia pestis synthesizes lipopolysaccharide (LPS)-lipid A with poor Toll-like receptor 4 (TLR4)-stimulating activity. To address the effect of weak TLR4 stimulation on virulence, we modified Y. pestis to produce a potent TLR4-stimulating LPS. Modified Y. pestis was completely avirulent after subcutaneous infection even at high challenge doses. Resistance to disease required TLR4, the adaptor protein MyD88 and coreceptor MD-2 and was considerably enhanced by CD14 and the adaptor Mal. Both innate and adaptive responses were required for sterilizing immunity against the modified strain, and convalescent mice were protected from both subcutaneous and respiratory challenge with wild-type Y. pestis. Despite the presence of other established immune evasion mechanisms, the modified Y. pestis was unable to cause systemic disease, demonstrating that the ability to evade the LPS-induced inflammatory response is critical for Y. pestis virulence. Evading TLR4 activation by lipid A alteration may contribute to the virulence of various Gram-negative bacteria.

Reciprocal upregulation of urokinase plasminogen activator and its inhibitor, PAI-2, by Borrelia burgdorferi affects bacterial penetration and host-inflammatory response

The mammalian plasminogen activation system (PAS) is a complex system involved in multiple physiological and pathological processes. Borrelia burgdorferi interacts with certain components of the PAS. Here we further investigate this interaction to determine its effect on bacterial dissemination and host cell migration in vitro. We show that stimulation of monocytic cells with B. burgdorferi induces the transient production and secretion of urokinase plasminogen activator (uPA), shortly followed by its physiological inhibitor, plasminogen activator inhibitor-2 (PAI-2). Mono Mac 6 (MM6) cells as well as peripheral blood monocytes enhanced transmigration of B. burgdorferi across a barrier coated with fibronectin mediated by uPA. Moreover, the induction of PAI-2 or the addition of recombinant PAI-2 did not have a significant effect on the uPA-potentiated transmigration of B. burgdorferi. In contrast, the induction of PAI-2 by B. burgdorferi resulted in significantly diminished invasion by monocytic cells across a reconstituted basement membrane (matrigel), which could be partially restored by treatment with purified uPA. These results show that the PAS plays a twofold role in the pathogenesis of B. burgdorferi infection, both by enhancing bacterial dissemination and by diminishing host-cell inflammatory migration.

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.

Adhesive properties of the purified plasminogen activator Pla ofYersinia pestis

The beta-barrel outer membrane protease Pla from Yersinia pestis is an important virulence factor in plague and enables initiation of the bubonic plague. Pla is a multifunctional protease whose expression also enhances bacterial adherence to extracellular matrix. It has remained uncertain whether the increase in cellular adhesiveness results from modification of the bacterial surface by Pla, or whether the Pla molecule is an adhesin. Pla was purified as a His6-fusion protein from Escherichia coli and reconstituted with lipopolysaccharide to an enzymatically active form. Purified His6-Pla was coated onto fluorescent micro-particles (FMPs) that expressed plasminogen activity. Pla-coated FMPs also bound to laminin and to reconstituted basement membrane (Matrigel) immobilized on permanox slides, whereas only poor activity was seen with lipopolysaccharide-coated FMPs or bovine serum albumin-coated FMPs. The results show that the Pla molecule has intrinsic adhesive properties and that purified transmembrane proteins coated onto FMPs can be used for functional assays.

RovA, a global regulator of Yersinia pestis, specifically required for bubonic plague

The pathogenic species of Yersinia contain the transcriptional regulator RovA. In Yersinia pseudotuberculosis and Yersinia enterocolitica, RovA regulates expression of the invasion factor invasin (inv), which mediates translocation across the intestinal epithelium. A Y. enterocolitica rovA mutant has a significant decrease in virulence by LD(50) analysis and an altered rate of dissemination compared with either wild type or an inv mutant, suggesting that RovA regulates multiple virulence factors. Here, we show the involvement of RovA in the virulence of Yersinia pestis, which naturally lacks a functional inv gene. A Y. pestis DeltarovA mutant is attenuated approximately 80-fold by LD(50) and is defective in dissemination/colonization of spleens and lungs after s.c. inoculation. However, the DeltarovA mutant is only slightly attenuated when given via an intranasal or i.p. route, indicating a more important role for RovA in bubonic plague than pneumonic plague or systemic infection. Microarray analysis was used to define the RovA regulon. The psa locus was among the most highly down-regulated loci in the DeltarovA mutant. A DeltapsaA mutant had a significant dissemination defect after s.c. infection but only slight attenuation by the pneumonic-disease model, closely mimicking the virulence defect seen with the DeltarovA mutant. DNA-binding studies revealed that RovA specifically interacts with the psaE and psaA promoter regions, indicating a direct role for RovA in regulating this locus. Thus, RovA appears to be a global transcription factor in Y. pestis and, through its regulatory influence on genes such as psaEFABC, contributes to the virulence of Y. pestis.

Proteases and protease inhibitors: a balance of activities in host-pathogen interaction

The immune system is the collection of effector molecules and cells of the host that act against invading parasites and their products. Secreted proteases serve important roles in parasitic metabolism and virulence and the several families of protein protease inhibitors of the plasma and blood cells play an important role in immunity by inactivating and clearing the protease virulence factors of parasites. The protease inhibitors are of two classes, the active-site inhibitors and the alpha2-macroglobulins. Inhibitors for the first class bind and inactivate the active site of the target protease. Proteins of the second class bind proteases by a unique molecular trap mechanism and deliver the bound protease to a receptor-mediated endocytic system for degradation in secondary lysosomes. Proteins of the alpha2-macroglobulin family are present in a variety of animal phyla, including the nematodes, arthropods, mollusks, echinoderms, urochordates, and vertebrates. A shared suite of unique functional characteristics have been documented for the alpha2-macroglobulins of vertebrates, arthropods, and mollusks. The alpha2-macroglobulins of nematodes, arthropods, mollusks, and vertebrates show significant sequence identity in key functional domains. Thus, the alpha2-macroglobulins comprise an evolutionarily conserved arm of the innate immune system with similar structure and function in animal phyla separated by 0.6 billion years of evolution.

Remodelling of the Escherichia coli outer membrane by two small regulatory RNAs

Small non-coding RNAs that play important regulatory roles exist in numerous organisms. In Escherichia coli, about 60 small RNAs have been found and those that have been studied are involved in the response and adaptation to different stresses. RygA and RygB, two of these small RNAs, were identified on the basis of their conservation between different species and their ability to bind Hfq. They are adjacent on the chromosome and have sequence similarity at their 5' and 3' ends but distinct central regions, suggesting that they could regulate the expression of both common and distinct genes. A screen using a multicopy E. coli library led to identification of the response regulator OmpR and its associated sensor kinase EnvZ as positive regulators of rygA and rygB transcription. Therefore, RygA and RygB were renamed OmrA and OmrB respectively (for OmpR-regulated sRNAs A and B). When expressed at high levels, OmrA and OmrB RNAs negatively regulate the expression of several genes encoding multiple outer membrane proteins, including cirA, fecA, fepA and ompT. Taken together, these data suggest that OmrA and OmrB participate in the regulation of outer membrane composition in response to environmental conditions.

Role of the Yersinia pestis plasminogen activator in the incidence of distinct septicemic and bubonic forms of flea-borne plague

Yersinia pestis is transmitted by fleas and causes bubonic plague, characterized by severe local lymphadenitis that progresses rapidly to systemic infection and life-threatening septicemia. Here, we show that although flea-borne transmission usually leads to bubonic plague in mice, it can also lead to primary septicemic plague. However, intradermal injection of Y. pestis, commonly used to mimic transmission by fleabite, leads only to bubonic plague. A Y. pestis strain lacking the plasmid-encoded cell-surface plasminogen activator, which is avirulent by intradermal or s.c. injection, was able to cause fatal primary septicemic plague at low incidence, but not bubonic plague, when transmitted by fleas. The results clarify a long-standing uncertainty about the etiology of primary septicemic plague and support an evolutionary scenario in which plague first emerged as a flea-borne septicemic disease of limited transmissibility. Subsequent acquisition of the plasminogen activator gene by horizontal transfer enabled the bubonic form of disease and increased the potential for epidemic spread.

Borrelia burgdorferi sensu stricto invasiveness is correlated with OspC-plasminogen affinity

Borrelia burgdorferi sensu lato, the causative agent of Lyme borreliosis, is transmitted through tick bite. Lyme borreliosis evolves in two stages: a primary red skin lesion called erythema migrans; later on, invasive bacteria disseminate to distant sites inducing secondary manifestations (neuropathies, arthritis, carditis, late skin disorders). It has been previously suggested that the ospC gene could be associated with invasiveness in humans depending on its sequence. Here, we confirm the pattern of invasiveness, according to B. burgdorferi sensu stricto (B. b. ss) ospC group, using the mouse as an experimental host of B. b. ss. As it has been shown that the host plasminogen activation system is used by B. burgdorferi to disseminate throughout the host, we studied the interaction of plasminogen with OspC proteins from invasive and non-invasive groups of B. b. ss. Using two methods, ELISA and surface plasmon resonance, we demonstrate that indeed OspC is a plasminogen-binding protein. Moreover, significant differences in binding affinity for plasminogen are correlated with different invasiveness patterns in mice. These results suggest that the correlation between ospC polymorphism and Borrelia invasiveness in humans is linked, at least in part, to differences in OspC affinity for plasminogen.

Structural basis for reduced staphylocoagulase-mediated bovine prothrombin activation

Staphylocoagulase (SC) is a protein secreted by the human pathogen, Staphylococcus aureus, that activates human prothrombin (ProT) by inducing a conformational change. SC-bound ProT efficiently clots fibrinogen, thus bypassing the physiological blood coagulation pathway. The crystal structure of a fully active SC fragment, SC-(1-325), bound to human prethrombin 2 showed that the SC-(1-325) N terminus inserts into the Ile(16) pocket of prethrombin 2, thereby inducing expression of a functional catalytic site in the cognate zymogen without peptide bond cleavage. As shown here, SC-(1-325) binds to bovine and human ProT with similar affinity but activates the bovine zymogen only very poorly. By contrast to the approximately 2-fold difference in chromogenic substrate kinetic constants between human thrombin and the SC-(1-325).human (pro)thrombin complexes, SC-(1-325).bovine ProT shows a 3,500-fold lower k(cat)/K(m) compared with free bovine thrombin, because of a 47-fold increase in K(m) and a 67-fold decrease in k(cat). The SC-(1-325).bovine ProT complex is approximately 5,800-fold less active compared with its human counterpart. Comparison of human and bovine fibrinogen as substrates of human and bovine thrombin and the SC-(1-325).(pro)thrombin complexes indicates that the species specificity of SC-(1-325) cofactor activity is determined primarily by differences in conformational activation of bound ProT. These results suggest that the catalytic site in the SC-(1-325).bovine ProT complex is incompletely formed. The current crystal structure of SC-(1-325).bovine thrombin reveals that SC would dock similarly to the bovine proenzyme, whereas the bovine (pro)thrombin-characteristic residues Arg(144) and Arg(145) would likely interfere with insertion of the SC N terminus, thus explaining the greatly reduced activation of bovine ProT.

Molecular and physiological insights into plague transmission, virulence and etiology

Plague is caused by Yersinia pestis, which evolved from the enteric pathogen Y. pseudotuberculosis, which normally causes a chronic and relatively mild disease. Y. pestis is not only able to parasitize the flea but also highly virulent to rodents and humans, causing epidemics of a systemic and often fatal disease. Y. pestis could be used as a bio-weapon and for bio-terrorism. It uses a number of strategies that allow the pathogen to change its lifestyle rapidly to survive in fleas and to grow in the mammalian hosts. Extensive studies reviewed here give an overall picture of the determinants responsible for plague pathogenesis in mammalians and the transmission by fleas. The availability of multiple genomic sequences and more extensive use of genomics and proteomics technologies should allow a comprehensive dissection of the complex of host-adaptation and virulence in Y. pestis.

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.

Evaluation of O-antigen inactivation on Pla activity and virulence of Yersinia pseudotuberculosis harbouring the pPla plasmid

Yersinia pestis is a species that emerged recently from Yersinia pseudotuberculosis and gained an exceptional pathogenicity potential. Among the major genetic differences between the plague bacillus and its ancestor is the acquisition of the pPla plasmid, which has been associated with the increased virulence of Y. pestis. In a previous study, introduction of pPla into Y. pseudotuberculosis did not lead to any modification of the virulence of the host bacterium. However, it was subsequently demonstrated that the presence of smooth lipopolysaccharide (LPS) inhibits the activity of Pla. In this study, pPla was introduced into a Y. pseudotuberculosis strain expressing smooth LPS, and into a variant in which a mutation that abrogates the formation of O-antigen (O-Ag) repeats (as in natural isolates of Y. pestis) was generated. It was found that in both strains, Pla was synthesized, exported to the bacterial membrane and processed as in Y. pestis. However, the ability of Pla to activate plasminogen was weak and observed only at 37 degrees C in the smooth strain, while this activity was similar to that of Y. pestis and expressed at both 28 and 37 degrees C in the O-Ag mutant strain. Similarly, Pla-mediated inactivation of the antiprotease alpha2-antiplasmin was not detected in the smooth Y. pseudotuberculosis strain grown at 28 degrees C, but was expressed at both temperatures in the O-Ag mutant strain. Despite the more efficient activity of Pla, the Y. pseudotuberculosis O-Ag mutant strain exhibited a lower pathogenicity upon subcutaneous infection of mice. The results thus indicate that, although abrogation of O side chain synthesis in a Y. pseudotuberculosis strain harbouring pPla potentiates the two proteolytic activities of Pla, this is not sufficient to confer to Y. pseudotuberculosis a higher pathogenicity potential. These results also suggest that acquisition of pPla may not have been sufficient to confer an immediate higher pathogenic potential to the ancestor Y. pestis strain.

Progression of primary pneumonic plague: a mouse model of infection, pathology, and bacterial transcriptional activity

Although pneumonic plague is the deadliest manifestation of disease caused by the bacterium Yersinia pestis, there is surprisingly little information on the cellular and molecular mechanisms responsible for Y. pestis-triggered pathology in the lung. Therefore, to understand the progression of this unique disease, we characterized an intranasal mouse model of primary pneumonic plague. Mice succumbed to a purulent multifocal severe exudative bronchopneumonia that closely resembles the disease observed in humans. Analyses revealed a strikingly biphasic syndrome, in which the infection begins with an antiinflammatory state in the first 24-36 h that rapidly progresses to a highly proinflammatory state by 48 h and death by 3 days. To assess the adaptation of Y. pestis to a mammalian environment, we used DNA microarray technology to analyze the transcriptional responses of the bacteria during interaction with the mouse lung. Included among the genes up-regulated in vivo are those comprising the yop-ysc type III secretion system and genes contained within the chromosomal pigmentation locus, validating the use of this technology to identify loci essential to the virulence of Y. pestis.

The many and varied niches occupied by Yersinia pestis as an arthropod-vectored zoonotic pathogen

Yersinia pestis, the causative agent of bubonic and pneumonic plague, has a complex lifestyle, cycling between both arthropod and mammalian hosts. This pathogen has previously been shown to survive intracellularly within macrophages and to be capable of biofilm formation within the flea, suggesting the development of a range of strategies to ensure survival throughout its life cycle, including expression of virulence factors and tight regulation of its genes.

Influence of Na(+), dicarboxylic amino acids, and pH in modulating the low-calcium response of Yersinia pestis

The virulence of yersiniae is promoted in part by shared approximately 70-kb plasmids (pCD in Yersinia pestis and pYV in enteropathogenic Yersinia pseudotuberculosis and Yersinia enterocolitica) that mediate a low-calcium response. This phenotype is characterized at 37 degrees C by either bacteriostasis in Ca(2+)-deficient medium with expression of pCD/pYV-encoded virulence effectors (Yops and LcrV) or vegetative growth and repression of Yops and LcrV with > or =2.5 mM Ca(2+) (Lcr(+)). Regulation of Yops and LcrV is well defined but little is known about bacteriostasis other than that Na(+) plus l-glutamate promotes prompt restriction of Y. pestis. As shown here, l-aspartate substituted for l-glutamate in this context but only Na(+) exacerbated the nutritional requirement for Ca(2+). Bacteriostasis of Y. pestis (but not enteropathogenic yersiniae) was abrupt in Ca(2+)-deficient medium at neutral to slightly alkaline pH (7.0 to 8.0), although increasing the pH to 8.5 or 9.0, especially with added Na(+) (but not l-glutamate), facilitated full-scale growth. Added l-glutamate (but not Na(+)) favored Ca(2+)-independent growth at acidic pH (5.0 to 6.5). Yops and LcrV were produced in Ca(2+)-deficient media at pH 6.5 to 9.0 regardless of the presence of added Na(+) or l-glutamate, although their expression at alkaline pH was minimal. Resting Ca(2+)-starved Lcr(+) cells of Y. pestis supplied with l-glutamate first excreted and then destroyed l-aspartate. These findings indicate that expression of Yops and LcrV is necessary but not sufficient for bacteriostasis of Ca(2+)-starved yersiniae and suggest that abrupt restriction of Y. pestis requires Na(+) and the known absence of aspartate ammonia-lyase in this species.

Replication of Yersinia pestis in interferon gamma-activated macrophages requires ripA, a gene encoded in the pigmentation locus

Yersinia pestis is a facultative intracellular bacterial pathogen that can replicate in macrophages. Little is known about the mechanism by which Y. pestis replicates in macrophages, and macrophage defense mechanisms important for limiting intracellular survival of Y. pestis have not been characterized. In this work, we investigated the ability of Y. pestis to replicate in primary murine macrophages that were activated with IFN-gamma. Y. pestis was able to replicate in macrophages that were activated with IFN-gamma after infection (postactivated). A region of chromosomal DNA known as the pigmentation (pgm) locus was required for replication in postactivated macrophages, and this replication was associated with reduced nitric oxide (NO) levels but not with reduced inducible NO synthase (iNOS) expression. Y. pestis delta pgm replicated in iNOS-/- macrophages that were postactivated with IFN-gamma, suggesting that killing of delta pgm Y. pestis is NO-dependent. A specific genetic locus within pgm, which shares similarity to a pathogenicity island in Salmonella, was shown to be required for replication of Y. pestis and restriction of NO levels in postactivated macrophages. These data demonstrate that intracellular Y. pestis can evade killing by macrophages that are exposed to IFN-gamma and identify a potential virulence gene encoded in the pgm locus that is required for this activity.

Activation, cytokine production, and intracellular survival of bacteria inSalmonella-infected human monocyte-derived macrophages and dendritic cells

Salmonella enterica serovar typhimurium (S. typhimurium) is an intracellular pathogen causing localized gastroenteritis in humans. Macrophages (Mphis) and dendritic cells (DCs) play an important role in innate immunity against Salmonella. In this report, we have compared the consequences of infection of human Mphis and DCs with wild-type S. typhimurium and an isogenic PgtE-defective strain. PgtE is an outer membrane protein hypothesized to have a role in intracellular survival of Salmonella. We observed that DCs undergo full maturation in response to Salmonella infection, as indicated by up-regulation of cell-surface marker proteins CD80, CD83, CD86, and human leukocyte antigen class II. CC chemokine ligand 5 (CCL5), CXC chemokine ligand 10, tumor necrosis factor alpha, interleukin (IL)-12, and IL-18 gene expression and protein production were readily induced by Salmonella-infected Mphis and DCs. CCL20 was preferentially produced by Mphis, whereas DCs secreted higher levels of CCL19 as compared with Mphis. DCs and Mphis infected with S. typhimurium also produced high levels of interferon-gamma (IFN-gamma). Cytokine neutralization and stimulation experiments suggest that the production was partly regulated by Salmonella-induced type I IFNs, IL-12, and IL-18. DC cytokine production induced by Salmonella was much higher as compared with the responses induced by Salmonella lipopolysaccharide or flagellin. Mphis and DCs were capable of internalizing and harboring Salmonella for several days. S. enterica PgtE provided no survival advantage for the bacteria in human Mphis or DCs. Our results demonstrate that although Mphis and DCs share similar functions, they may have different roles during Salmonella infection as a result of differential production of certain chemokines and cytokines.

Does Disseminated Intravascular Coagulation Lead to Multiple Organ Failure?

Microvascular dysfunction with its associated impaired regional oxygen transport and use is believed to be the final common pathway in the development of multiple organ failure. The precise mechanisms underlying this dysfunction, however, are uncertain. Activation of the coagulation system is a key feature in the pathogenesis of sepsis, but whether it is also the cause of multiple organ failure is unclear. This article discusses the evidence for and against a key role for disseminated intravascular coagulation in the pathogenesis of multiple organ failure.

Effective antiprotease-antibiotic treatment of experimental anthrax

Background

Inhalation anthrax is characterized by a systemic spread of the challenge agent, Bacillus anthracis. It causes severe damage, including multiple hemorrhagic lesions, to host tissues and organs. It is widely believed that anthrax lethal toxin secreted by proliferating bacteria is a major cause of death, however, the pathology of intoxication in experimental animals is drastically different from that found during the infectious process. In order to close a gap between our understanding of anthrax molecular pathology and the most prominent clinical features of the infectious process we undertook bioinformatic and experimental analyses of potential proteolytic virulence factors of B. anthracis distinct from lethal toxin.

Methods

Secreted proteins (other than lethal and edema toxins) produced by B. anthracis were tested for tissue-damaging activity and toxicity in mice. Chemical protease inhibitors and rabbit immune sera raised against B. anthracis proteases were used to treat mice challenged with B. anthracis (Sterne) spores.

Results

B. anthracis strain delta Ames (pXO1^-, pXO2^-) producing no lethal and edema toxins secrets a number of metalloprotease virulence factors upon cultivation under aerobic conditions, including those with hemorrhagic, caseinolytic and collagenolytic activities, belonging to M4 and M9 thermolysin and bacterial collagenase families, respectively.

These factors are directly toxic to DBA/2 mice upon intratracheal administration at 0.5 mg/kg and higher doses. Chemical protease inhibitors (phosphoramidon and 1, 10-phenanthroline), as well as immune sera against M4 and M9 proteases of B. anthracis, were used to treat mice challenged with B. anthracis (Sterne) spores. These substances demonstrate a substantial protective efficacy in combination with ciprofloxacin therapy initiated as late as 48 h post spore challenge, compared to the antibiotic alone.

Conclusion

Secreted proteolytic enzymes are important pathogenic factors of B. anthrasis, which can be considered as effective therapeutic targets in the development of anthrax treatment and prophylactic approaches complementing anti-lethal toxin therapy.

Bacterial metastasis: the host plasminogen system in bacterial invasion

Several pathogenic bacterial species intervene with the mammalian proteolytic plasminogen-plasmin system. Recent developments have been made in understanding the structure and the virulence-associated functions of bacterial plasminogen receptors and activators, in particular by using plasminogen-deficient or transgenic gain-of-function mice. Bacteria can affect the regulation of the plasminogen system by degrading circulating plasmin inhibitors and by influencing the expression levels of mammalian plasminogen activators and activation inhibitors. Interaction with the plasminogen system promotes damage of extracellular matrices as well as bacterial spread and organ invasion during infection, suggesting common mechanisms in migration of eukaryotic and prokaryotic cells.