Mutation of the maturase lipoprotein attenuates the virulence of Streptococcus equi to a greater extent than does loss of general lipoprotein lipidation

Investigation of cross-opsonic effect leads to the discovery of PPIase-domain containing protein vaccine candidate to prevent infections by Gram-positive ESKAPE pathogens

BACKGROUND

Enterococcus faecium and Staphylococcus aureus are the Gram-positive pathogens of the ESKAPE group, known to represent a great threat to human health due to their high virulence and multiple resistances to antibiotics. Combined, enterococci and S. aureus account for 26% of healthcare-associated infections and are the most common organisms responsible for blood stream infections. We previously showed that the peptidyl-prolyl cis/trans isomerase (PPIase) PpiC of E. faecium elicits the production of specific, opsonic, and protective antibodies that are effective against several strains of E. faecium and E. faecalis. Due to the ubiquitous characteristics of PPIases and their essential function within Gram-positive cells, we hypothesized a potential cross-reactive effect of anti-PpiC antibodies.

RESULTS

Opsonophagocytic assays combined with bioinformatics led to the identification of the foldase protein PrsA as a new potential vaccine antigen in S. aureus. We show that PrsA is a stable dimeric protein able to elicit opsonic antibodies against the S. aureus strain MW2, as well as cross-binding and cross-opsonic in several S. aureus, E. faecium and E. faecalis strains.

CONCLUSIONS

Given the multiple antibiotic resistances S. aureus and enterococci present, finding preventive strategies is essential to fight those two nosocomial pathogens. The study shows the potential of PrsA as an antigen to use in vaccine formulation against the two dangerous Gram-positive ESKAPE bacteria. Our findings support the idea that PPIases should be further investigated as vaccine targets in the frame of pan-vaccinomics strategy.

Study of the Role of Lipoprotein Maturation Enzymes in the Pathogenesis of the Infection Caused by the Streptococcus suis Serotype 2 Sequence Type 25 North American Prototype Strain

Streptococcus suis serotype 2 is an important swine bacterial pathogen causing sudden death, septic shock, and meningitis. However, serotype 2 strains are phenotypically and genotypically heterogeneous and composed of a multitude of sequence types (STs) whose distributions greatly vary worldwide. It has been previously shown that the lipoprotein (LPP) maturation enzymes diacylglyceryl transferase (Lgt) and signal peptidase (Lsp) significantly modulate the inflammatory host response and play a differential role in virulence depending on the genetic background of the strain. Differently from Eurasian ST1/ST7 strains, the capsular polysaccharide of a North American S. suis serotype 2 ST25 representative strain only partially masks sub-capsular domains and bacterial wall components. Thus, our hypothesis is that since LPPs would be more surface exposed in ST25 strains than in their ST1 or ST7 counterparts, the maturation enzymes would play a more important role in the pathogenesis of the infection caused by the North American strain. Using isogenic Δlgt and Δlsp mutants derived from the wild-type ST25 strain, our studies suggest that these enzymes do not seem to play a role in the interaction between S. suis and epithelial and endothelial cells, regardless of the genetics background of the strain used. However, a role in the formation of biofilms (also independently of the STs) has been demonstrated. Moreover, the involvement of LPP dendritic cell activation in vitro seems to be somehow more pronounced with the ST25 strain. Finally, the Lgt enzyme seems to play a more important role in the virulence of the ST25 strain. Although some differences between STs could be observed, our original hypothesis that LPPs would be significantly more important in ST25 strains due to a better bacterial surface exposition could not be confirmed.

Unique virulence role of post-translocational chaperone PrsA in shaping Streptococcus pyogenes secretome

Streptococcus pyogenes (group A Streptococcus, GAS) is a strict human pathogen causing a broad spectrum of diseases and a variety of autoimmune sequelae. The pathogenesis of GAS infection mostly relies on the production of an extensive network of cell wall-associated and secreted virulence proteins, such as adhesins, toxins, and exoenzymes. PrsA, the only extracellular parvulin-type peptidyl-prolyl isomerase expressed ubiquitously in Gram-positive bacteria, has been suggested to assist the folding and maturation of newly exported proteins to acquire their native conformation and activity. Two PrsA proteins, PrsA1 and PrsA2, have been identified in GAS, but the respective contribution of each PrsA in GAS pathogenesis remains largely unknown. By combining comparative proteomic and phenotypic analysis approaches, we demonstrate that both PrsA isoforms are required to maintain GAS proteome homeostasis and virulence-associated traits in a unique and overlapping manner. The inactivation of both PrsA in GAS caused remarkable impairment in biofilm formation, host adherence, infection-induced cytotoxicity, and in vivo virulence in a murine soft tissue infection model. The concordance of proteomic and phenotypic data clearly features the essential role of PrsA in GAS full virulence.

Role of Maturation of Lipoproteins in the Pathogenesis of the Infection Caused by Streptococcus suis Serotype 2

Streptococcus suis serotype 2 is an important porcine bacterial pathogen associated with multiple pathologies in piglets. Bacterial lipoproteins (LPPs) have been described as playing important roles in the pathogenesis of the infection of other Gram-positive bacteria as adhesins, pro-inflammatory cell activators and/or virulence factors. In the current study, we aimed to evaluate the role of the prolipoprotein diacylglyceryl transferase (Lgt) and lipoprotein signal peptidase (Lsp) enzymes, which are responsible for LPP maturation, on the pathogenesis of the infection caused by two different sequence types (STs) of S. suis serotype 2 strains (virulent ST1 and highly virulent ST7). Through the use of isogenic Δlgt, Δlsp and double Δlgt/Δlsp mutants, it was shown that lack of these enzymes did not influence S. suis adhesion/invasion to porcine respiratory epithelial cells. However, in the absence of the Lsp and/or Lgt, a significant reduction in the capacity of S. suis to activate phagocytic cells and induce pro-inflammatory mediators (in vitro and in vivo) was observed. In general, results obtained with the double mutant did not differ in comparison to single mutants, indicating lack of an additive effect. Finally, our data suggest that these enzymes play a differential role in virulence, depending on the genetic background of the strain and being more important for the highly virulent ST7 strain.

NisI Maturation and Its Influence on Nisin Resistance in Lactococcus lactis

NisI confers immunity against nisin, with high substrate specificity to prevent a suicidal effect in nisin-producing Lactococcus lactis strains. However, the NisI maturation process as well as its influence on nisin resistance has not been characterized. Here, we report the roles of lipoprotein signal peptidase II (Lsp) and prolipoprotein diacylglyceryl transferase (Lgt) in NisI maturation and nisin resistance of L. lactis F44. We found that the resistance of nisin of an Lsp-deficient mutant remarkably decreased, while no significant differences in growth were observed. We demonstrated that Lsp could cleave signal peptide of NisI precursor in vitro Moreover, diacylglyceryl modification of NisI catalyzed by Lgt played a decisive role in attachment of NisI on the cell envelope, while it exhibited no effects on cleavage of the signal peptides of NisI precursor. The dissociation constant (K_D ) for the interaction between nisin and NisI exhibited a 2.8-fold increase compared with that between nisin and pre-NisI with signal peptide by surface plasmon resonance (SPR) analysis, providing evidence that Lsp-catalyzed signal peptide cleavage was critical for the immune activity of NisI. Our study revealed the process of NisI maturation in L. lactis and presented a potential strategy to enhance industrial nisin production.IMPORTANCE Nisin, a safe and natural antimicrobial peptide, has a long and impressive history as a food preservative and is also considered a novel candidate to alleviate the increasingly serious threat of antibiotic resistance. Nisin is produced by certain L. lactis strains. The nisin immunity protein NisI, a membrane-bound lipoprotein, is expressed by nisin producers to avoid suicidal action. Here, we report the roles of Lsp and Lgt in NisI maturation and nisin resistance of L. lactis F44. The results verified the importance of Lsp to NisI-conferred immunity and Lgt to localization. Our study revealed the process of NisI maturation in L. lactis and presented a potential strategy to enhance industrial nisin production.

Lipoproteins Contribute to the Anti-inflammatory Capacity of Lactobacillus plantarum WCFS1

Bacterial lipoproteins are well-recognized microorganism-associated molecular patterns, which interact with Toll-like receptor (TLR) 2, an important pattern recognition receptor of the host innate immune system. Lipoproteins are conjugated with two- or three-acyl chains (di- or tri-acyl), which is essential for appropriate anchoring in the cell membrane as well as for the interaction with TLR2. Lipoproteins have mostly been studied in pathogens and have established roles in various biological processes, such as nutrient import, cell wall cross-linking and remodeling, and host-cell interaction. By contrast, information on the role of lipoproteins in the physiology and host interaction of probiotic bacteria is scarce. By deletion of lgt, encoding prolipoprotein diacylglyceryl transferase, responsible for lipidation of lipoprotein precursors, we investigated the roles of the collective group of lipoproteins in the physiology of the probiotic model strain Lactobacillus plantarum WCFS1 using proteomic analysis of secreted proteins. To investigate the consequences of the lgt mutation in host-cell interaction, the capacity of mutant and wild-type bacteria to stimulate TLR2 signaling and inflammatory responses was compared using (reporter-) cell-based models. These experiments exemplified the critical contribution of the acyl chains of lipoproteins in immunomodulation. To the best of our knowledge, this is the first study that investigated collective lipoprotein functions in a model strain for probiotic lactobacilli, and we show that the lipoproteins in L. plantarum WCFS1 are critical drivers of anti-inflammatory host responses toward this strain.

Intramuscular vaccination with Strangvac is safe and induces protection against equine strangles caused by Streptococcus equi

The equine disease strangles, caused by Streptococcus equi, remains a major cause of welfare and economic cost to the global horse industry. Here we report the safety, immunogenicity and efficacy of a novel multi-component chimeric fusion protein vaccine, called Strangvac, when administered to ponies via the intramuscular route. Across the four studies, Strangvac was safe and induced robust antibody responses towards the vaccine components in blood serum and the nasopharynx, which were boosted by revaccination up to 12 months after a primary course of 2 vaccinations 4 weeks apart. The vaccine response did not cross-react with a commercial strangles iELISA, which identifies horses that have been exposed to S. equi, demonstrating that it was possible to differentiate infected from vaccinated animals (DIVA). Following challenge with S. equi strain 4047 (Se4047), all 36 control ponies that had received an adjuvant-only placebo vaccine developed clinical signs of strangles. In contrast, intramuscular vaccination with Strangvac protected ponies significantly from challenge with Se4047 at two weeks (5 of 16 ponies protected (31%), P = 0.04) and two months (7 of 12 ponies protected (58%), P = 0.0046 (including pooled control data) after second vaccination. Optimal protection (15 of 16 ponies protected (94%), P < 0.0001) was observed following challenge at two weeks post-third vaccination. Our data demonstrate that Strangvac is safe, has DIVA capability and provides a rapid onset of protective immunity against strangles. We conclude that Strangvac is a valuable tool with which to protect horses from strangles, particularly during high-risk periods, whilst maintaining the mobility of horse populations as required by the global equine industry.

Identification of genes required for the fitness of Streptococcus equi subsp. equi in whole equine blood and hydrogen peroxide

The availability of next-generation sequencing techniques provides an unprecedented opportunity for the assignment of gene function. Streptococcus equi subspecies equi is the causative agent of strangles in horses, one of the most prevalent and important diseases of equids worldwide. However, the live attenuated vaccines that are utilized to control this disease cause adverse reactions in some animals. Here, we employ transposon-directed insertion-site sequencing (TraDIS) to identify genes that are required for the fitness of S. equi in whole equine blood or in the presence of H2O2 to model selective pressures exerted by the equine immune response during infection. We report the fitness values of 1503 and 1471 genes, representing 94.5 and 92.5 % of non-essential genes in S. equi, following incubation in whole blood and in the presence of H2O2, respectively. Of these genes, 36 and 15 were identified as being important to the fitness of S. equi in whole blood or H2O2, respectively, with 14 genes being important in both conditions. Allelic replacement mutants were generated to validate the fitness results. Our data identify genes that are important for S. equi to resist aspects of the immune response in vitro, which can be exploited for the development of safer live attenuated vaccines to prevent strangles.

Prominent Binding of Human and Equine Fibrinogen to Streptococcus equi subsp. zooepidemicus Is Mediated by Specific SzM Types and Is a Distinct Phenotype of Zoonotic Isolates

Streptococcus equi subsp. zooepidemicus is an important pathogen in horses that causes severe diseases such as pneumonia and abortion. Furthermore, it is a zoonotic agent, and contact with horses is a known risk factor. In this study, we investigated the working hypothesis that the zoonotic potential varies among S. equi subsp. zooepidemicus strains in association with differences in M-like protein-mediated binding of host plasma proteins. We demonstrate via in-frame deletion mutagenesis of two different S. equi subsp. zooepidemicus strains that the M-like protein SzM is crucial for the binding of fibrinogen to the bacterial surface and for survival in equine and human blood. S. equi subsp. zooepidemicus isolates of equine and human origins were compared with regard to SzM sequences and binding of equine and human fibrinogens. The N-terminal 216 amino acids of the mature SzM were found to exhibit a high degree of diversity, but the majority of human isolates grouped in three distinct SzM clusters. Plasma protein absorption assays and flow cytometry analysis revealed that pronounced binding of human fibrinogen is a common phenotype of human S. equi subsp. zooepidemicus isolates but much less so in equine S. equi subsp. zooepidemicus isolates. Furthermore, binding of human fibrinogen is associated with specific SzM types. These results suggest that SzM-mediated binding of human fibrinogen is an important virulence mechanism of zoonotic S. equi subsp. zooepidemicus isolates.

PrsA contributes to Streptococcus suis serotype 2 pathogenicity by modulating secretion of selected virulence factors

Streptococcus suis serotype 2 (S. suis 2) is a major zoonotic pathogen. Parvulin-type peptidyl-prolyl isomerase (PrsA) in S. suis 2 is found surface-associated, pro-inflammatory and cytotoxic. To further explore the roles of PrsA in S. suis 2 infection, we constructed a prsA deletion mutant (ΔprsA) and a complemented strain (CΔprsA). The ΔprsA mutant showed increased length of bacterial chains and decreased growth. Deletion of prsA increased bacterial adhesion to host epithelial cells but with weakened invasion. The ΔprsA mutant had reduced survival in RAW264.7 macrophages and pig whole blood, and significantly attenuated in virulence to mice. All these phenotypes of the mutant could be reversed largely to the levels of its parental strain by gene complementation. Western blotting revealed that suilysin was markedly reduced both in surface-associated (SAP) and secreted fractions (SecP) of ΔprsA, which might be responsible for reduced hemolytic activity. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and enolase were significantly increased in both SAP and SecP fractions as a result of prsA deletion. Increased adhesion of the ΔprsA mutant to bEND.3 cells was prevented using polyclonal antibodies against GAPDH and enolase. Overall, we propose that S. suis 2 deploys PrsA to control translocation of important virulence factors, thereby favoring its survival in the host with enhanced pathogenicity by compromising its interactions with the host cells. Further investigation is required to find out how PrsA modulates protein translocation to benefit S. suis infection and if there are other S. suis 2 substrates of potential virulence regulated by PrsA.

Diversity of Streptococcus equi subsp. zooepidemicus strains isolated from the Spanish sheep and goat population and the identification, function and prevalence of a novel arbutin utilisation system

The zoonotic bacterium Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is a diverse, opportunistic pathogen that can cause mastitis in dairy sheep and goats. We used multilocus sequence typing (MLST) to define the genetic diversity of 60 isolates of S. zooepidemicus, which were recovered from sheep and goats in Spain between 2003 and 2010. We identify a novel clonal complex based on sequence type (ST), ST-236, which accounted for 39 of the 60 isolates. A representative ST-236 strain, S. zooepidemicus strain C7 (SzC7), was sequenced and interrogated for the presence of novel nutritional uptake or utilisation systems, the acquisition of which have previously been shown to be important for environmental adaptation in other streptococcal pathogens. A novel phosphoenolpyruvate sugar phosphotransferase system (PTS), which enabled the utilisation of arbutin, was identified. Functionality of the PTS was confirmed following deletion of the PTS from SzC7. Arbutin is found in multiple animal foodstuffs and we propose that the ability to utilise arbutin may have conferred a selective advantage to strains infecting animals, the diet of which contains this sugar.

Investigation of the Fim1 putative pilus locus of Streptococcus equi subspecies equi

The Gram-positive bacterium Streptococcus equi subspecies equi (S. equi) is the causative agent of strangles, among the most frequently diagnosed infectious diseases of horses worldwide. Genome analysis of S. equi strain 4047 (Se4047) identified a putative operon, Fim1, with similarity to the pilus loci of other Gram-positive bacteria. The Fim1 locus was present in all strains of S. equi and its close relative S. equi subspecies zooepidemicus (S. zooepidemicus) that have been studied to date. In this study we provide evidence that the putative structural pilus proteins, SEQ_0936 and CNE, are produced on the cell surface during in vitro growth and in vivo infection. Although the proteins encoded within the Fim1 locus are not essential for attachment or biofilm formation, over-transcription of SEQ_0936 and CNE enhanced attachment to equine tissue in vitro. Our data suggest that whilst the Fim1 locus does not produce a polymerized pilus structure, the products of the Fim1 locus may fulfil an adhesive function. The putative pilus-associated regulator, tetR, which contains a nonsense mutation in S. equi, was able to regulate transcription of the Fim1 locus following repair and over-transcription, confirming its predicted role in the operon.

Defining the ABC of gene essentiality in streptococci

BACKGROUND

Utilising next generation sequencing to interrogate saturated bacterial mutant libraries provides unprecedented information for the assignment of genome-wide gene essentiality. Exposure of saturated mutant libraries to specific conditions and subsequent sequencing can be exploited to uncover gene essentiality relevant to the condition. Here we present a barcoded transposon directed insertion-site sequencing (TraDIS) system to define an essential gene list for Streptococcus equi subsp. equi, the causative agent of strangles in horses, for the first time. The gene essentiality data for this group C Streptococcus was compared to that of group A and B streptococci.

RESULTS

Six barcoded variants of pGh9:ISS1 were designed and used to generate mutant libraries containing between 33,000-66,000 unique mutants. TraDIS was performed on DNA extracted from each library and data were analysed separately and as a combined master pool. Gene essentiality determined that 19.5% of the S. equi genome was essential. Gene essentialities were compared to those of group A and group B streptococci, identifying concordances of 90.2% and 89.4%, respectively and an overall concordance of 83.7% between the three species.

CONCLUSIONS

The use of barcoded pGh9:ISS1 to generate mutant libraries provides a highly useful tool for the assignment of gene function in S. equi and other streptococci. The shared essential gene set of group A, B and C streptococci provides further evidence of the close genetic relationships between these important pathogenic bacteria. Therefore, the ABC of gene essentiality reported here provides a solid foundation towards reporting the functional genome of streptococci.

Immunogenicity of phospholipase A2 toxins and their role in Streptococcus equi pathogenicity

Streptococcus equi subsp. equi (S. equi) is the causative agent of strangles, one of the most frequently diagnosed infectious diseases of horses worldwide. Phospholipase A₂ toxins (PLA₂) cleave phospholipid molecules at position sn-2 contributing to the production of leukotrienes that are important inflammatory mediators. Two homologous phospholipases, SlaA and SlaB are encoded by the S. equi genome suggesting that PLA₂ toxins may contribute to its pathogenicity. Here we report the immunogenicity and role of PLA₂ toxins during natural and experimental infection of horses with S. equi. The levels of anti-PLA₂ specific antibodies in serum from horses naturally exposed to S. equi or without exposure were measured by indirect ELISA. Furthermore, the importance of PLA₂ was determined during experimental infection of Welsh Mountain ponies with a mutant strain of S. equi lacking slaA and slaB. Our results show that PLA₂ toxins are immunogenic, which supports their production during natural S. equi infection, but that these toxins are not essential for the development of strangles in a susceptible natural host.

Evaluation of Staphylococcus aureus Lipoproteins: Role in Nutritional Acquisition and Pathogenicity

Bacterial lipoproteins (Lpp) represent a major class of membrane proteins. They are distinguished by a lipid moiety at the N-terminus by which they are anchored either in the outer leaflet of the cytoplasmic membrane or, in Gram-negative bacteria, also in the inner leaflet of the outer membrane. In Gram-positive bacteria Lpp significantly contribute to nutrient transport, Toll-like receptor 2 activation and pathogenicity. Here we examine the Lpp of Staphylococcus aureus USA300, as a prototype for a multiple antibiotic resistant and community-acquired pathogen that is rapidly spreading worldwide. The compiled Lpp were grouped according to the postulated function and dissemination of homologs in the genus Staphylococcus and beyond. Based on this evaluation we also point out Lpp as promising vaccine candidates.

Characterisation of SEQ0694 (PrsA/PrtM) of Streptococcus equi as a functional peptidyl-prolyl isomerase affecting multiple secreted protein substrates

Peptidyl-prolyl isomerase (PPIase) lipoproteins have been shown to influence the virulence of a number of Gram-positive bacterial human and animal pathogens, most likely through facilitating the folding of cell envelope and secreted virulence factors. Here, we used a proteomic approach to demonstrate that the Streptococcus equi PPIase SEQ0694 alters the production of multiple secreted proteins, including at least two putative virulence factors (FNE and IdeE2). We demonstrate also that, despite some unusual sequence features, recombinant SEQ0694 and its central parvulin domain are functional PPIases. These data add to our knowledge of the mechanisms by which lipoprotein PPIases contribute to the virulence of streptococcal pathogens.

Lipoproteins of Gram-Positive Bacteria: Key Players in the Immune Response and Virulence

Since the discovery in 1973 of the first of the bacterial lipoproteins (Lpp) in Escherichia coli, Braun's lipoprotein, the ever-increasing number of publications indicates the importance of these proteins. Bacterial Lpp belong to the class of lipid-anchored proteins that in Gram-negative bacteria are anchored in both the cytoplasmic and outer membranes and in Gram-positive bacteria are anchored only in the cytoplasmic membrane. In contrast to the case for Gram-negative bacteria, in Gram-positive bacteria lipoprotein maturation and processing are not vital. Physiologically, Lpp play an important role in nutrient and ion acquisition, allowing particularly pathogenic species to better survive in the host. Bacterial Lpp are recognized by Toll-like receptor 2 (TLR2) of the innate immune system. The important role of Lpp in Gram-positive bacteria, particularly in the phylum Firmicutes, as key players in the immune response and pathogenicity has emerged only in recent years. In this review, we address the role of Lpp in signaling and modulating the immune response, in inflammation, and in pathogenicity. We also address the potential of Lpp as promising vaccine candidates.

Determinants of Genetic Diversity of Spontaneous Drug Resistance in Bacteria

Any pathogen population sufficiently large is expected to harbor spontaneous drug-resistant mutants, often responsible for disease relapse after antibiotic therapy. It is seldom appreciated, however, that while larger populations harbor more mutants, the abundance distribution of these mutants is expected to be markedly uneven. This is because a larger population size allows early mutants to expand for longer, exacerbating their predominance in the final mutant subpopulation. Here, we investigate the extent to which this reduction in evenness can constrain the genetic diversity of spontaneous drug resistance in bacteria. Combining theory and experiments, we show that even small variations in growth rate between resistant mutants and the wild type result in orders-of-magnitude differences in genetic diversity. Indeed, only a slight fitness advantage for the mutant is enough to keep diversity low and independent of population size. These results have important clinical implications. Genetic diversity at antibiotic resistance loci can determine a population's capacity to cope with future challenges (i.e., second-line therapy). We thus revealed an unanticipated way in which the fitness effects of antibiotic resistance can affect the evolvability of pathogens surviving a drug-induced bottleneck. This insight will assist in the fight against multidrug-resistant microbes, as well as contribute to theories aimed at predicting cancer evolution.

Genome specialization and decay of the strangles pathogen, Streptococcus equi, is driven by persistent infection

Strangles, the most frequently diagnosed infectious disease of horses worldwide, is caused by Streptococcus equi. Despite its prevalence, the global diversity and mechanisms underlying the evolution of S. equi as a host-restricted pathogen remain poorly understood. Here, we define the global population structure of this important pathogen and reveal a population replacement in the late 19th or early 20th Century. Our data reveal a dynamic genome that continues to mutate and decay, but also to amplify and acquire genes despite the organism having lost its natural competence and become host-restricted. The lifestyle of S. equi within the horse is defined by short-term acute disease, strangles, followed by long-term infection. Population analysis reveals evidence of convergent evolution in isolates from post-acute disease samples as a result of niche adaptation to persistent infection within a host. Mutations that lead to metabolic streamlining and the loss of virulence determinants are more frequently found in persistent isolates, suggesting that the pathogenic potential of S. equi reduces as a consequence of long-term residency within the horse post-acute disease. An example of this is the deletion of the equibactin siderophore locus that is associated with iron acquisition, which occurs exclusively in persistent isolates, and renders S. equi significantly less able to cause acute disease in the natural host. We identify several loci that may similarly be required for the full virulence of S. equi, directing future research toward the development of new vaccines against this host-restricted pathogen.

The M949_1556 gene plays a role on the bacterial antigenicity and pathogenicity of Riemerella anatipestifer

Riemerella anatipestifer is one of the most economically important pathogens of farm ducks worldwide. However, the molecular mechanisms regarding its antigenicity and pathogenicity are poorly understood. We previously constructed a library of random Tn4351 transposon mutants using R. anatipestifer strain CH3. In this study, M949_1556 gene inactivated mutant strain CH3ΔM949_1556 was identified by screening of the library using monoclonal antibody against R. anatipestifer serotype 1 lipopolysaccharide (LPS) (anti-LPS MAb) followed by sequence analysis. The mutant strain presented no reactivity to the anti-LPS MAb in an indirect ELISA. Animal studies showed that the median lethal dose (LD50) of CH3ΔM949_1556 was >10(10) colony forming units (CFU), which was attenuated more than 50 times, compared with that of wild-type strain CH3 (LD50=2×10(8) CFU). The bacterial loads in the blood of CH3ΔM949_1556 infected ducks were significantly decreased, compared with those of CH3-infected ducks. In addition, CH3ΔM949_1556 presented significant, higher susceptibility to complement-dependent killing than CH3 did in vitro. Furthermore, CH3ΔM949_1556 showed increased bacterial adhesion and invasion capacities to Vero cells. Immunization with CH3ΔM949_1556-inactived vaccine was effective in protecting the ducks from challenge with R. anatipestifer serotype 1 strain WJ4, serotype 2 strain Yb2 and serotype 10 strain HXb2, suggesting that the mutant strain CH3ΔM949_1556 could provide a broad cross-protection among R. anatipestifer serotypes 1, 2 and 10 strains. Our results demonstrated that the M949_1556 gene plays a role on the bacterial antigenicity and pathogenicity of R. anatipestifer.

A group A Streptococcus ADP-ribosyltransferase toxin stimulates a protective interleukin 1β-dependent macrophage immune response

The M1T1 clone of group A Streptococcus (GAS) is associated with severe invasive infections, including necrotizing fasciitis and septicemia. During invasive M1T1 GAS disease, mutations in the covRS regulatory system led to upregulation of an ADP-ribosyltransferase, SpyA. Surprisingly, a GAS ΔspyA mutant was resistant to killing by macrophages and caused higher mortality with impaired bacterial clearance in a mouse intravenous challenge model. GAS expression of SpyA triggered macrophage cell death in association with caspase-1-dependent interleukin 1β (IL-1β) production, and differences between wild-type (WT) and ΔspyA GAS macrophage survival levels were lost in cells lacking caspase-1, NOD-like receptor protein 3 (NLRP3), apoptosis-associated speck-like protein (ASC), or pro-IL-1β. Similar in vitro findings were identified in macrophage studies performed with pseudomonal exotoxin A, another ADP-ribosylating toxin. Thus, SpyA triggers caspase-1-dependent inflammatory cell death in macrophages, revealing a toxin-triggered IL-1β-dependent innate immune response pathway critical in defense against invasive bacterial infection.

Group A Streptococcus (GAS) is a leading human pathogen capable of producing invasive infections even in healthy individuals. GAS bacteria produce a toxin called SpyA that modifies host proteins through a process called ADP ribosylation. We describe how macrophages, frontline defenders of the host innate immune system, respond to SpyA by undergoing a specialized form of cell death in which they are activated to release the proinflammatory cytokine molecule interleukin 1β (IL-1β). Release of IL-1β activates host immune cell clearance of GAS, as we demonstrated in tissue culture models of macrophage bacterial killing and in vivo mouse infectious-challenge experiments. Similar macrophage responses to a related toxin of Pseudomonas bacteria were also shown. Thus, macrophages recognize certain bacterial toxins to activate a protective immune response in the host.

PinR mediates the generation of reversible population diversity in Streptococcus zooepidemicus

Opportunistic pathogens must adapt to and survive in a wide range of complex ecosystems. Streptococcus zooepidemicus is an opportunistic pathogen of horses and many other animals, including humans. The assembly of different surface architecture phenotypes from one genotype is likely to be crucial to the successful exploitation of such an opportunistic lifestyle. Construction of a series of mutants revealed that a serine recombinase, PinR, inverts 114 bp of the promoter of SZO_08560, which is bordered by GTAGACTTTA and TAAAGTCTAC inverted repeats. Inversion acts as a switch, controlling the transcription of this sortase-processed protein, which may enhance the attachment of S. zooepidemicus to equine trachea. The genome of a recently sequenced strain of S. zooepidemicus, 2329 (Sz2329), was found to contain a disruptive internal inversion of 7 kb of the FimIV pilus locus, which is bordered by TAGAAA and TTTCTA inverted repeats. This strain lacks pinR and this inversion may have become irreversible following the loss of this recombinase. Active inversion of FimIV was detected in three strains of S. zooepidemicus, 1770 (Sz1770), B260863 (SzB260863) and H050840501 (SzH050840501), all of which encoded pinR. A deletion mutant of Sz1770 that lacked pinR was no longer capable of inverting its internal region of FimIV. The data highlight redundancy in the PinR sequence recognition motif around a short TAGA consensus and suggest that PinR can reversibly influence the wider surface architecture of S. zooepidemicus, providing this organism with a bet-hedging solution to survival in fluctuating environments.

Vaccination with a live multi-gene deletion strain protects horses against virulent challenge with Streptococcus equi

Strangles, caused by Streptococcus equi subspecies equi (S. equi) is one of the most frequently diagnosed infectious diseases of horses and there remains a significant need to develop new preventative vaccines. We generated a live vaccine strain of S. equi containing deletions in six genes: sagA, hasA, aroB, pyrC, seM and recA, which was administered to nine Welsh mountain ponies via the intramuscular route. Four vaccinated ponies developed adverse reactions following the first vaccination from which the live vaccine strain was isolated. Two of these ponies were withdrawn from the study and seven ponies received a second vaccination, one of which then developed an adverse reaction. Nine control ponies injected with culture media alone developed no adverse reactions. Following challenge with a virulent strain of S. equi, none of the seven vaccinated ponies had developed clinical signs of strangles eleven days post-challenge, compared to six of nine control ponies over the same period (P=0.0114). A lymph node abscess was identified in one of the seven vaccinated ponies at post-mortem examination, whilst all nine control ponies had at least one lymph node abscess (P=0.0009). Three of the six vaccinated ponies that were protected from strangles had not developed an adverse reaction following vaccination, suggesting that a better understanding of the pro-inflammatory responses to S. equi could lead to the development of a live attenuated vaccine against strangles that is safe for administration via intramuscular injection.

Development and use of a polarized equine upper respiratory tract mucosal explant system to study the early phase of pathogenesis of a European strain of equine arteritis virus

The upper respiratory tract mucosa represents the first line of defense, which has to be overcome by pathogens before invading the host. Considering the economic and ethical aspects involved in using experimental animals for pathogenesis studies, respiratory mucosal explants, in which the tissue’s three-dimensional architecture is preserved, may be ideal alternatives. Different respiratory mucosal explant cultures have been developed. However, none of them could be inoculated with pathogens solely at the epithelium side. In the present study, equine nasal and nasopharyngeal explants were embedded in agarose (3%), leaving the epithelium side exposed to allow apical inoculation. Morphometric analysis did not show degenerative changes during 72 h of cultivation. The number of apoptotic cells in the mucosa slightly increased over time. After validation, the system was used for apical infection with a European strain (08P178) of equine arteritis virus (EAV) (10^7.6TCID₅₀/mL per explant). Impermeability of agarose to virus particles was demonstrated by the absence of labeled microspheres (40nm) and a lack of EAV-antigens in RK13 cells seeded underneath the agarose layer in which inoculated explants were embedded. At 72 hpi, 27% of the EAV-positive cells were CD172a⁺ and 19% were CD3⁺ in nasal explants and 45% of the EAV-positive cells were CD172a⁺ and 15% were CD3⁺ in nasopharyngeal explants. Only a small percentage of EAV-positive cells were IgM⁺. This study validates the usefulness of a polarized mucosal explant system and shows that CD172a⁺ myeloid cells and CD3⁺ T lymphocytes represent important EAV-target cells in the respiratory mucosa.

Detection of Streptococcus equi subspecies equi using a triplex qPCR assay

Genome sequencing data for Streptococcus equi subspecies equi and zooepidemicus were used to develop a novel diagnostic triplex quantitative PCR (qPCR) assay targeting two genes specific to S. equi (eqbE and SEQ2190) and a unique 100 base pair control DNA sequence (SZIC) inserted into the SZO07770 pseudogene of S. zooepidemicus strain H70. This triplex strangles qPCR assay can provide results within 2 h of sample receipt, has an overall sensitivity of 93.9% and specificity of 96.6% relative to the eqbE singlex assay and detects S. equi at levels below the threshold of the culture assay, even in the presence of contaminating bacteria.

Effects of deletion of the Streptococcus pneumoniae lipoprotein diacylglyceryl transferase gene lgt on ABC transporter function and on growth in vivo

Lipoproteins are an important class of surface associated proteins that have diverse roles and frequently are involved in the virulence of bacterial pathogens. As prolipoproteins are attached to the cell membrane by a single enzyme, prolipoprotein diacylglyceryl transferase (Lgt), deletion of the corresponding gene potentially allows the characterisation of the overall importance of lipoproteins for specific bacterial functions. We have used a Δlgt mutant strain of Streptococcus pneumoniae to investigate the effects of loss of lipoprotein attachment on cation acquisition, growth in media containing specific carbon sources, and virulence in different infection models. Immunoblots of triton X-114 extracts, flow cytometry and immuno-fluorescence microscopy confirmed the Δlgt mutant had markedly reduced lipoprotein expression on the cell surface. The Δlgt mutant had reduced growth in cation depleted medium, increased sensitivity to oxidative stress, reduced zinc uptake, and reduced intracellular levels of several cations. Doubling time of the Δlgt mutant was also increased slightly when grown in medium with glucose, raffinose and maltotriose as sole carbon sources. These multiple defects in cation and sugar ABC transporter function for the Δlgt mutant were associated with only slightly delayed growth in complete medium. However the Δlgt mutant had significantly reduced growth in blood or bronchoalveolar lavage fluid and a marked impairment in virulence in mouse models of nasopharyngeal colonisation, sepsis and pneumonia. These data suggest that for S. pneumoniae loss of surface localisation of lipoproteins has widespread effects on ABC transporter functions that collectively prevent the Δlgt mutant from establishing invasive infection.

Involvement of peptidylprolyl cis/trans isomerases in Enterococcus faecalis virulence

Peptidylprolyl cis/trans isomerases (PPIases) are enzymes involved in protein folding. Analysis of the genome sequence of Enterococcus faecalis V583 allowed for identification of 3 PPIases carrying genes. ef2898 encodes an intracellular PPIase which was not shown to be important for the E. faecalis stress response or virulence. The other two PPIases, the parvulin family rotamase EF0685 and the cyclophilin family member EF1534, are expected to be surface-exposed proteins. They were shown to be important for virulence and resistance to NaCl. A Δef0685 Δef1534 mutant was also more resistant to oxidative stress, was able to grow under a high manganese concentration, and showed altered resistance to ampicillin and quinolone antibiotics.

Phosphatidylglycerol::prolipoprotein diacylglyceryl transferase (Lgt) of Escherichia coli has seven transmembrane segments, and its essential residues are embedded in the membrane

Lgt of Escherichia coli catalyzes the transfer of an sn-1,2-diacylglyceryl group from phosphatidylglycerol to prolipoproteins. The enzyme is essential for growth, as demonstrated here by the analysis of an lgt depletion strain. Cell fractionation demonstrated that Lgt is an inner membrane protein. Its membrane topology was determined by fusing Lgt to β-galactosidase and alkaline phosphatase and by substituted cysteine accessibility method (SCAM) studies. The data show that Lgt is embedded in the membrane by seven transmembrane segments, that its N terminus faces the periplasm, and that its C terminus faces the cytoplasm. Highly conserved amino acids in Lgt of both Gram-negative and Gram-positive bacteria were identified. Lgt enzymes are characterized by a so-called Lgt signature motif in which four residues are invariant. Ten conserved residues were replaced with alanine, and the activity of these Lgt variants was analyzed by their ability to complement the lgt depletion strain. Residues Y26, N146, and G154 are absolutely required for Lgt function, and R143, E151, R239, and E243 are important. The results demonstrate that the majority of the essential residues of Lgt are located in the membrane and that the Lgt signature motif faces the periplasm.

The prolipoprotein diacylglyceryl transferase (Lgt) of Enterococcus faecalis contributes to virulence

Enterococcus faecalis is an opportunistic pathogen responsible for nosocomial infections. Lipoproteins in Gram-positive bacteria are translocated across the plasma membrane and anchored by the fatty acid group. They perform critical roles, with some described as virulence determinants. The aim of this study was to explore the roles of E. faecalis lipoproteins in the stress response and virulence. We constructed a mutant affected in the predicted prolipoprotein diacylglyceryl transferase gene lgt, and examined the role of Lgt in membrane anchoring, growth, the stress response and virulence. Inactivation of lgt enhanced growth in a high concentration of Mn(2+) or under oxidative stress in vitro, and significantly decreased virulence.

Lipoproteins of Enterococcus faecalis: bioinformatic identification, expression analysis and relation to virulence

Enterococcus faecalis is a ubiquitous bacterium that is capable of surviving in a broad range of natural environments, including the human host, as either a natural commensal or an opportunistic pathogen involved in severe hospital-acquired infections. How such opportunistic pathogens cause fatal infections is largely unknown but it is likely that they are equipped with sophisticated systems to perceive external signals and interact with eukaryotic cells. Accordingly, being partially exposed at the cell exterior, some surface-associated proteins are involved in several steps of the infection process. Among them are lipoproteins, representing about 25 % of the surface-associated proteins, which could play a major role in bacterial virulence processes. This review focuses on the identification of 90 lipoprotein-encoding genes in the genome of the E. faecalis V583 clinical strain and their putative roles, and provides a transcriptional comparison of microarray data performed in environmental conditions including blood and urine. Taken together, these data suggest a potential involvement of lipoproteins in E. faecalis virulence, making them serious candidates for vaccine production.

Lgt processing is an essential step in Streptococcus suis lipoprotein mediated innate immune activation

BACKGROUND

Streptococcus suis causes invasive infections in pigs and occasionally in humans. The host innate immune system plays a major role in counteracting S. suis infections. The main components of S. suis able to activate the innate immune system likely include cell wall constituents that may be released during growth or after cell wall integrity loss, however characterization of these components is still limited.

METHODOLOGY/PRINCIPAL FINDINGS

[corrected] A concentrated very potent innate immunity activating supernatant of penicillin-treated S. suis was SDS-PAGE fractionated and tested for porcine peripheral blood mononucleated cell (PBMC) stimulating activity using cytokine gene transcript analysis. More than half of the 24 tested fractions increased IL-1β and IL-8 cytokine gene transcript levels in porcine PBMCs. Mass spectrometry of the active fractions indicated 24 proteins including 9 lipoproteins. Genetic inactivation of a putative prolipoprotein diacylglyceryl transferase (Lgt) gene resulted in deficient lipoprotein synthesis as evidenced by palmitate labeling. The Lgt mutant showed strongly reduced activation of porcine PBMCs, indicating that lipoproteins are dominant porcine PBMC activating molecules of S. suis.

CONCLUSION/SIGNIFICANCE

This study for the first time identifies and characterizes lipoproteins of S. suis as major activators of the innate immune system of the pig. In addition, we provide evidence that Lgt processing of lipoproteins is required for lipoprotein mediated innate immune activation.

Lipoproteins of bacterial pathogens

Bacterial lipoproteins are a set of membrane proteins with many different functions. Due to this broad-ranging functionality, these proteins have a considerable significance in many phenomena, from cellular physiology through cell division and virulence. Here we give a general overview of lipoprotein biogenesis and highlight examples of the roles of lipoproteins in bacterial disease caused by a selection of medically relevant Gram-negative and Gram-positive pathogens: Mycobacterium tuberculosis, Streptococcus pneumoniae, Borrelia burgdorferi, and Neisseria meningitidis. Lipoproteins have been shown to play key roles in adhesion to host cells, modulation of inflammatory processes, and translocation of virulence factors into host cells. As such, a number of lipoproteins have been shown to be potential vaccines. This review provides a summary of some of the reported roles of lipoproteins and of how this knowledge has been exploited in some cases for the generation of novel countermeasures to bacterial diseases.

Pooled protein immunization for identification of cell surface antigens in Streptococcus sanguinis

Background

Available bacterial genomes provide opportunities for screening vaccines by reverse vaccinology. Efficient identification of surface antigens is required to reduce time and animal cost in this technology. We developed an approach to identify surface antigens rapidly in Streptococcus sanguinis, a common infective endocarditis causative species.

Methods and Findings

We applied bioinformatics for antigen prediction and pooled antigens for immunization. Forty-seven surface-exposed proteins including 28 lipoproteins and 19 cell wall-anchored proteins were chosen based on computer algorithms and comparative genomic analyses. Eight proteins among these candidates and 2 other proteins were pooled together to immunize rabbits. The antiserum reacted strongly with each protein and with S. sanguinis whole cells. Affinity chromatography was used to purify the antibodies to 9 of the antigen pool components. Competitive ELISA and FACS results indicated that these 9 proteins were exposed on S. sanguinis cell surfaces. The purified antibodies had demonstrable opsonic activity.

Conclusions

The results indicate that immunization with pooled proteins, in combination with affinity purification, and comprehensive immunological assays may facilitate cell surface antigen identification to combat infectious diseases.

An ex vivo swine tracheal organ culture for the study of influenza infection

Background  The threat posed by swine influenza viruses with potential to transmit from pig populations to other hosts, including humans, requires the development of new experimental systems to study different aspects of influenza infection. Ex vivo organ culture (EVOC) systems have been successfully used in the study of both human and animal respiratory pathogens.

Objectives  We aimed to develop an air interface EVOC using pig tracheas in the study of influenza infection demonstrating that tracheal explants can be effectively maintained in organ culture and support productive influenza infection.

Methods  Tracheal explants were maintained in the air interface EVOC system for 7 days. Histological characteristics were analysed with different staining protocols and co‐ordinated ciliary movement on the epithelial surface was evaluated through a bead clearance assay. Explants were infected with a swine H1N1 influenza virus. Influenza infection of epithelial cells was confirmed by immunohistochemistry and viral replication was quantified by plaque assays and real‐time RT‐PCR.

Results  Histological analysis and bead clearance assay showed that the tissue architecture of the explants was maintained for up to 7 days, while ciliary movement exhibited a gradual decrease after 4 days. Challenge with swine H1N1 influenza virus showed that the EVOC tracheal system shows histological changes consistent with in vivo influenza infection and supported productive viral replication over multiple cycles of infection.

Conclusion  The air interface EVOC system using pig trachea described here constitutes a useful biological tool with a wide range of applications in the study of influenza infection.

Contribution of each of four Superantigens to Streptococcus equi-induced mitogenicity, gamma interferon synthesis, and immunity

Streptococcus equi is the causative agent of strangles, the most frequently diagnosed infectious disease of horses worldwide. The disease is characterized by abscessation and swelling of the lymph nodes of the head and neck, which can literally strangle the horse to death. S. equi produces four recently acquired phage-associated bacterial superantigens (sAgs; SeeH, SeeI, SeeL, and SeeM) that share homology with the mitogenic toxins of Streptococcus pyogenes. The aim of this study was to characterize the contribution of each of these S. equi sAgs to mitogenic activity in vitro and quantify the sAg-neutralizing capacity of sera from naturally infected horses in order to better understand their role in pathogenicity. Each of the sAgs was successfully cloned, and soluble proteins were produced in Escherichia coli. SeeI, SeeL, and SeeM induced a dose-dependent proliferative response in equine CD4 T lymphocytes and synthesis of gamma interferon (IFN-gamma). SeeH did not stimulate equine peripheral blood mononuclear cells (PBMC) but induced proliferation of asinine PBMC. Allelic replacement mutants of S. equi strain 4047 with sequential deletion of the superantigen genes were generated. Deletion of seeI, seeL, and seeM completely abrogated the mitogenic activity and synthesis of IFN-gamma, in equine PBMC, of the strain 4047 culture supernatant. Sera from naturally infected convalescent horses had only limited sAg-neutralizing activities. We propose that S. equi sAgs play an important role in S. equi pathogenicity by stimulating an overzealous and inappropriate Th1 response that may interfere with the development of an effective immune response.

Getting to grips with strangles: an effective multi-component recombinant vaccine for the protection of horses from Streptococcus equi infection

Streptococcus equi subspecies equi (S. equi) is a clonal, equine host-adapted pathogen of global importance that causes a suppurative lymphodendopathy of the head and neck, more commonly known as Strangles. The disease is highly prevalent, can be severe and is highly contagious. Antibiotic treatment is usually ineffective. Live attenuated vaccine strains of S. equi have shown adverse reactions and they suffer from a short duration of immunity. Thus, a safe and effective vaccine against S. equi is highly desirable. The bacterium shows only limited genetic diversity and an effective vaccine could confer broad protection to horses throughout the world. Welsh mountain ponies (n = 7) vaccinated with a combination of seven recombinant S. equi proteins were significantly protected from experimental infection by S. equi, resembling the spontaneous disease. Vaccinated horses had significantly reduced incidence of lymph node swelling (p = 0.0013) lymph node abscessation (p = 0.00001), fewer days of pyrexia (p = 0.0001), reduced pathology scoring (p = 0.005) and lower bacterial recovery from lymph nodes (p = 0.004) when compared with non-vaccinated horses (n = 7). Six of 7 vaccinated horses were protected whereas all 7 non-vaccinated became infected. The protective antigens consisted of five surface localized proteins and two IgG endopeptidases. A second vaccination trial (n = 7+7), in which the IgG endopeptidases were omitted, demonstrated only partial protection against S. equi, highlighting an important role for these vaccine components in establishing a protective immune response. S. equi shares >80% sequence identity with Streptococcus pyogenes. Several of the components utilized here have counterparts in S. pyogenes, suggesting that our findings have broader implications for the prevention of infection with this important human pathogen. This is one of only a few demonstrations of protection from streptococcal infection conferred by a recombinant multi-component subunit vaccine in a natural host.

Numerous research groups have vaccinated, using recombinant antigens, against streptococcal infections in mouse model systems and shown protection. We have here demonstrated efficient protective vaccination of the natural host, the horse, using recombinant antigens. Streptococcus equi subspecies equi (S. equi) is an equine host-adapted and highly contagious pathogen of global importance. Six out of seven Welsh mountain ponies vaccinated with a combination of seven recombinant S. equi proteins were protected from experimental infection as assessed by clinical examination, pyrexia, lymph node swelling, inflammation, bacterial recovery, and post mortem examination. The protective antigens consisted of five surface localized proteins and two endopeptidases that are specific for IgG; the latter were shown to be of major importance for efficacy. Several of the antigens used here have similarities in Streptococcus pyogenes, implying that our findings are of importance for development of a vaccine against this important human pathogen.

Contribution of lipoproteins and lipoprotein processing to endocarditis virulence in Streptococcus sanguinis

Streptococcus sanguinis is an important cause of infective endocarditis. Previous studies have identified lipoproteins as virulence determinants in other streptococcal species. Using a bioinformatic approach, we identified 52 putative lipoprotein genes in S. sanguinis strain SK36 as well as genes encoding the lipoprotein-processing enzymes prolipoprotein diacylglyceryl transferase (lgt) and signal peptidase II (lspA). We employed a directed signature-tagged mutagenesis approach to systematically disrupt these genes and screen each mutant for the loss of virulence in an animal model of endocarditis. All mutants were viable. In competitive index assays, mutation of a putative phosphate transporter reduced in vivo competitiveness by 14-fold but also reduced in vitro viability by more than 20-fold. Mutations in lgt, lspA, or an uncharacterized lipoprotein gene reduced competitiveness by two- to threefold in the animal model and in broth culture. Mutation of ssaB, encoding a putative metal transporter, produced a similar effect in culture but reduced in vivo competiveness by >1,000-fold. [(3)H]palmitate labeling and Western blot analysis confirmed that the lgt mutant failed to acylate lipoproteins, that the lspA mutant had a general defect in lipoprotein cleavage, and that SsaB was processed differently in both mutants. These results indicate that the loss of a single lipoprotein, SsaB, dramatically reduces endocarditis virulence, whereas the loss of most other lipoproteins or of normal lipoprotein processing has no more than a minor effect on virulence.

Impact of lgt mutation on lipoprotein biosynthesis and in vitro phenotypes of Streptococcus agalactiae

Although Streptococcus agalactiae, the group B Streptococcus, is a leading cause of invasive neonatal disease worldwide the molecular basis of its virulence is still poorly understood. To investigate the role of lipoproteins in the physiology and interaction of this pathogen with host cells, we generated a mutant S. agalactiae strain (A909DeltaLgt) deficient in the Lgt enzyme and thus unable to lipidate lipoprotein precursors (pro-lipoproteins). The loss of pro-lipoprotein lipidation did not affect the viability of S. agalactiae or its growth in several different media, including cation-depleted media. The processing of two well-characterized lipoproteins, but not a non-lipoprotein, was clearly shown to be aberrant in A909DeltaLgt. The mutant strain was shown to be more sensitive to oxidative stress in vitro although the molecular basis of this increased sensitivity was not apparent. The inactivation of Lgt also resulted in changes to the bacterial cell envelope, as demonstrated by reduced retention of both the group B carbohydrate and the polysaccharide capsule and a statistically significant reduction (P=0.0079) in A909DeltaLgt adherence to human endothelial cells of fetal origin. These data confirm that failure to process lipoproteins correctly has pleiotropic effects that may be of significance to S. agalactiae colonization and pathogenesis.

The Streptococcus equi prophage-encoded protein SEQ2045 is a hyaluronan-specific hyaluronate lyase that is produced during equine infection

Streptococcus equi causes equine 'strangles'. Hyaluronate lyases, which degrade connective tissue hyaluronan and chondroitins, are thought to facilitate streptococcal invasion of the host. However, prophage-encoded hyaluronate lyases are hyaluronan-specific and are thought to be primarily involved in the degradation of the hyaluronan capsule of streptococci during bacteriophage infection. To understand the role of prophage-encoded hyaluronate lyases further, we have biochemically characterized such a hyaluronate lyase, SEQ2045 from S. equi, and have shown that it is produced during equine infection. Prophage-encoded hyaluronan-specific hyaluronate lyases may therefore play a more direct role in disease pathogenesis than previously thought.

A novel streptococcal integrative conjugative element involved in iron acquisition

In this study, we determined the function of a novel non-ribosomal peptide synthetase (NRPS) system carried by a streptococcal integrative conjugative element (ICE), ICESe2. The NRPS shares similarity with the yersiniabactin system found in the high-pathogenicity island of Yersinia sp. and is the first of its kind to be identified in streptococci. We named the NRPS product 'equibactin' and genes of this locus eqbA-N. ICESe2, although absolutely conserved in Streptococcus equi, the causative agent of equine strangles, was absent from all strains of the closely related opportunistic pathogen Streptococcus zooepidemicus. Binding of EqbA, a DtxR-like regulator, to the eqbB promoter was increased in the presence of cations. Deletion of eqbA resulted in a small-colony phenotype. Further deletion of the irp2 homologue eqbE, or the genes eqbH, eqbI and eqbJ encoding a putative ABC transporter, or addition of the iron chelator nitrilotriacetate, reversed this phenotype, implicating iron toxicity. Quantification of (55)Fe accumulation and sensitivity to streptonigrin suggested that equibactin is secreted by S. equi and that the eqbH, eqbI and eqbJ genes are required for its associated iron import. In agreement with a structure-based model of equibactin synthesis, supplementation of chemically defined media with salicylate was required for equibactin production.

In the absence of Lgt, lipoproteins are shed from Streptococcus uberis independently of Lsp

The role of lipoprotein diacylglyceryl transferase (Lgt) and lipoprotein signal peptidase (Lsp) responsible for processing lipoproteins was investigated in Streptococcus uberis, a common cause of bovine mastitis. In the absence of Lgt, three lipoproteins [MtuA (SUB0473), Hap (SUB1625) and an extracellular solute-binding protein (SUB0365)] were detected in extracellular locations. All were shown by Edman degradation analysis to be cleaved on the carboxy side of the LXXC lipobox. Detection of MtuA, a lipoprotein shown previously to be essential for infectivity and virulence, was used as a surrogate lipoprotein marker to locate and assess processing of lipoproteins. The absence of Lgt did not prevent location of MtuA to the cell membrane, its location in the wild-type strain but, in contrast to the situation with wild-type, did result in a widespread location of this protein. In the absence of both Lgt and Lsp, MtuA was similarly released from the bacterial cell. In such strains, however, the cell-associated MtuA represented the full-length gene product, indicating that Lsp was able to cleave non-lipidated (lipo)proteins but was not responsible for their release from this bacterium.

Lipoprotein biogenesis in Gram-positive bacteria: knowing when to hold 'em, knowing when to fold 'em

Gram-positive bacterial lipoproteins are a functionally diverse and important class of peripheral membrane proteins. Recent advances in molecular biology and the availability of whole genome sequence data have overturned many long-held assumptions about the export and processing of these proteins, most notably the recent discovery that not all lipoproteins are exported as unfolded substrates through the general secretion pathway. Here, we review recent discoveries concerning the export and processing of these proteins, their role in virulence in Gram-positive bacteria and their potential as vaccine candidates or targets for new antimicrobials.

In vitro culture of equine respiratory mucosa explants

An in vitro model of the upper respiratory tract of the horse was developed to investigate mechanisms of respiratory diseases. Four tissues of the upper respiratory tract of three horses were collected. Explants were maintained in culture at an air-liquid interface for 96h. At 0, 24, 48, 72 and 96h of cultivation, a morphometric analysis was performed using light microscopy, scanning electron microscopy and transmission electron microscopy. The explants were judged on morphometric changes of epithelium, basement membrane and connective tissue. Viability was evaluated using a fluorescent Terminal deoxynucleotidyl transferase-mediated dUTP Nick End Labelling (TUNEL) staining. No significant changes in morphometry and viability of any of the explants were observed during cultivation. Hence, the in vitro model may be useful to study infectious and non-infectious diseases at the level of the equine respiratory tract, with potential application to the development of vaccines and treatments for diseases of the respiratory tract.

Maturation of Streptococcus pneumoniae lipoproteins by a type II signal peptidase is required for ABC transporter function and full virulence

Cell surface lipoproteins are important for the full virulence of several bacterial pathogens, including Streptococcus pneumoniae. Processing of prolipoproteins seems to be conserved among different bacterial species, and requires type II signal peptidase (Lsp) mediated cleavage of the N-terminal signal peptide to form the mature lipoprotein. Lsp has been suggested as a target for new antibiotic therapies, but at present there are only limited data on the function of Lsp for Gram-positive bacterial pathogens. We have investigated the function and role during disease pathogenesis of the S. pneumoniae Lsp, which, blast searches suggest, is encoded by the gene Sp0928. Expression of Sp0928 protected Escherichia coli against the Lsp antagonist globomycin, and proteomics and immunoblot analysis demonstrated that deletion of Sp0928 prevented processing of S. pneumoniae prolipoproteins to mature lipoproteins. These data strongly suggest that Sp0928 encodes the S. pneumoniae Lsp. However, immunoblots of membrane-associated proteins, immunoelectron microscopy and flow cytometry assays all confirmed that in the absence of Lsp, immature lipoproteins were still attached to the cell surface. Despite preservation of lipoprotein attachment to the cell membrane, loss of S. pneumoniae Lsp resulted in several phenotypes associated with impaired lipoprotein function and reduced S. pneumoniae replication in animal models of infection.

Vaccination of horses against strangles using recombinant antigens from Streptococcus equi

Strangles is an upper respiratory tract infection in horses, which is highly contagious and one of the more costly diseases of the horse. Three recombinant antigens were used to vaccinate horses, which were then experimentally challenged with Streptococcus equi, the causative agent for strangles. The vaccinated horses showed significantly reduced bacterial growth (p=0.02) and nasal discharge (p=0.0004), a typical symptom of strangles. Other clinical signs of strangles were also reduced and at post mortem examination, lower rate of empyaema or scarring of the guttural pouches was found in the vaccinated group (p=0.01). The antigens used were EAG (alpha2-macroglobulin, albumin, and IgG-binding protein), CNE (a collagen-binding protein), and SclC (a collagen-like protein). The adjuvant used was Abisco, a saponin derived matrix. No adverse effects were observed following vaccination with the antigens and adjuvant.