TroR Negatively Regulates the TroABCD System and Is Required for Resistance to Metal Toxicity and Virulence in Streptococcus suis

Streptococcus suis is an emerging zoonotic pathogen that causes severe swine and human infections. Metals are essential nutrients for life; however, excess metals are toxic to bacteria. Therefore, maintenance of intracellular metal homeostasis is important for bacterial survival. Here, we characterize a DtxR family metalloregulator, TroR, in S. suis. TroR is located upstream of the troABCD operon, whose expression was found to be significantly downregulated in response to excess manganese (Mn). Deletion of troR resulted in reduced growth when S. suis was cultured in metal-replete medium supplemented with elevated concentrations of zinc (Zn), copper (Cu), or cobalt (Co). Mn supplementation could alleviate the growth defects of the ΔtroR mutant under Zn and Co excess conditions; however, it impaired the growth of the wild-type (WT) and complemented (CΔtroR) strains under Cu excess conditions. The growth of ΔtroR was also inhibited in metal-depleted medium supplemented with elevated concentrations of Mn. Moreover, the ΔtroR mutant accumulated increased levels of intracellular Mn and Co, rather than Zn and Cu. Deletion of troR in S. suis led to significant upregulation of the troABCD operon. Furthermore, troA expression in the WT strain was induced by ferrous iron [Fe(II)] and Co and repressed by Mn and Cu; the repression of troA was mediated by TroR. Finally, TroR is required for S. suis virulence in an intranasal mouse model. Together, these data suggest that TroR is a negative regulator of the TroABCD system and contributes to resistance to metal toxicity and virulence in S. suis. IMPORTANCE Metals are essential nutrients for life; however, the accumulation of excess metals in cells can be toxic to bacteria. In the present study, we identified a metalloregulator, TroR, in Streptococcus suis, which is an emerging zoonotic pathogen. In contrast to the observations in other species that TroR homologs usually contribute to the maintenance of homeostasis of one or two metals, we demonstrated that TroR is required for resistance to the toxicity conferred by multiple metals in S. suis. We also found that deletion of troR resulted in significant upregulation of the troABCD operon, which has been demonstrated to be involved in manganese acquisition in S. suis. Moreover, we demonstrated that TroR is required for the virulence of S. suis in an intranasal mouse model. Collectively, these results suggest that TroR is a negative regulator of the TroABCD system and contributes to resistance to metal toxicity and virulence in S. suis.

Effective Antibacterial and Antihemolysin Activities of Ellipticine Hydrochloride against Streptococcus suis in a Mouse Model

Streptococcal toxic shock-like syndrome (STSLS) caused by the epidemic strain of Streptococcus suis leads to severe inflammation and high mortality. The life and health of humans and animals are also threatened by the increasingly severe antimicrobial resistance in Streptococcus suis There is an urgent need to discover novel strategies for the treatment of S. suis infection. Suilysin (SLY) is considered to be an important virulence factor in the pathogenesis of S. suis In this study, ellipticine hydrochloride (EH) was reported as a compound that antagonizes the hemolytic activity of SLY. In vitro, EH was found to effectively inhibit SLY-mediated hemolytic activity. Furthermore, EH had a strong affinity for SLY, thereby directly binding to SLY to interfere with the hemolytic activity. Meanwhile, it was worth noting that EH was also found to have a significant antibacterial activity. In vivo, compared with traditional ampicillin, EH not only significantly improved the survival rate of mice infected with S. suis 2 strain Sc19 but also relieved lung pathological damage. Furthermore, EH effectively decreased the levels of inflammatory cytokines (interleukin-6 [IL-6], tumor necrosis factor alpha [TNF-α]) and blood biochemistry enzymes (alanine transaminase [ALT], aspartate transaminase [AST], creatine kinase [CK]) in Sc19-infected mice. Additionally, EH markedly reduced the bacterial load of tissues in Sc19-infected mice. In conclusion, our findings suggest that EH can be a potential compound for treating S. suis infection in view of its antibacterial and antihemolysin activity.IMPORTANCE In recent years, the inappropriate use of antibiotics has unnecessarily caused the continuous emergence of resistant bacteria. The antimicrobial resistance of Streptococcus suis has also become an increasingly serious problem. Targeting virulence can reduce the selective pressure of bacteria on antibiotics, thereby alleviating the development of bacterial resistance to a certain extent. Meanwhile, the excessive inflammatory response caused by S. suis infection is considered the primary cause of acute death. Here, we found that ellipticine hydrochloride (EH) exhibited effective antibacterial and antihemolysin activities against S. suisin vitroIn vivo, compared with ampicillin, EH had a significant protective effect on S. suis serotype 2 strain Sc19-infected mice. Our results indicated that EH, with dual antibacterial and antivirulence effects, will contribute to treating S. suis infections and alleviating the antimicrobial resistance of S. suis to a certain extent. More importantly, EH may develop into a promising drug for the prevention of acute death caused by excessive inflammation.

Role of two-component regulatory systems in the virulence of Streptococcus suis

Streptococcus suis is an important zoonotic pathogen that causes severe infections and great economic losses worldwide. Understanding how this pathogen senses and responds to environmental signals during the infectious process can offer insight into its pathogenesis and may be helpful in the development of drug targets. Two-component regulatory systems (TCSs) play an essential role in this environmental response. In S. suis, at least 15 groups of TCSs have been predicted. Among them, several have been demonstrated to be involved in virulence and/or stress response. In this review, we discuss the progress in the study of TCSs in S. suis, focusing on the role of these systems in the virulence of this bacterium.

A Zebrafish Larval Model to Assess Virulence of Porcine Streptococcus suis Strains

Streptococcus suis is an encapsulated Gram-positive bacterium, and the leading cause of sepsis and meningitis in young pigs resulting in considerable economic losses in the porcine industry. It is also considered an emerging zoonotic agent. In the environment, both avirulent and virulent strains occur in pigs, and virulent strains appear to cause disease in both humans and pigs. There is a need for a convenient, reliable and standardized animal model to assess S. suis virulence. A zebrafish (Danio rerio) larvae infection model has several advantages, including transparency of larvae, low cost, ease of use and exemption from ethical legislation up to 6 days post fertilization, but has not been previously established as a model for S. suis. Microinjection of different porcine strains of S. suis in zebrafish larvae resulted in highly reproducible dose- and strain-dependent larval death, strongly correlating with presence of the S. suis capsule and to the original virulence of the strain in pigs. Additionally we compared the virulence of the two-component system mutant of ciaRH, which is attenuated for virulence in both mice and pigs in vivo. Infection of larvae with the ΔciaRH strain resulted in significantly higher survival rate compared to infection with the S10 wild-type strain. Our data demonstrate that zebrafish larvae are a rapid and reliable model to assess the virulence of clinical porcine S. suis isolates.

Antimicrobial susceptibility of Streptococcus suis isolated from clinically healthy swine in Brazil

Streptococcus suis is an important pathogen in the swine industry. This study is the first to report on the antimicrobial susceptibility of S. suis isolated from clinically healthy pigs in Brazil; the fourth major pork producer in the world. The antimicrobial susceptibility of 260 strains was determined by disc diffusion method. Strains were commonly susceptible to ceftiofur, cephalexin, chloramphenicol, and florfenicol, with more than 80% of the strains being susceptible to these antimicrobials. A high frequency of resistance to some of the antimicrobial agents was demonstrated, with resistance being most common to sulfa-trimethoprim (100%), tetracycline (97.69%), clindamycin (84.61%), norfloxacin (76.92%), and ciprofloxacin (61.15%). A high percentage of multidrug resistant strains (99.61%) were also found. The results of this study indicate that ceftiofur, cephalexin, and florfenicol are the antimicrobials of choice for empirical control of the infections caused by S. suis.

Antimicrobial activity of nisin against the swine pathogen Streptococcus suis and its synergistic interaction with antibiotics

Streptococcus suis serotype 2 is known to cause severe infections in pigs, including meningitis, endocarditis and pneumonia. Furthermore, this bacterium is considered an emerging zoonotic agent. Recently, increased antibiotic resistance in S. suis has been reported worldwide. The objective of this study was to evaluate the potential of nisin, a bacteriocin of the lantibiotic class, as an antibacterial agent against the pathogen S. suis serotype 2. In addition, the synergistic activity of nisin in combination with conventional antibiotics was assessed. Using a plate assay, the nisin-producing strain Lactococcus lactis ATCC 11454 proved to be capable of inhibiting the growth of S. suis (n=18) belonging to either sequence type (ST)1, ST25, or ST28. In a microdilution broth assay, the minimum inhibitory concentration (MIC) of purified nisin ranged between 1.25 and 5 μg/mL while the minimum bactericidal concentration (MBC) was between 5 and 10 μg/mL toward S. suis. The use of a capsule-deficient mutant of S. suis indicated that the presence of this polysaccharidic structure has no marked impact on susceptibility to nisin. Following treatment of S. suis with nisin, transmission electron microscopy observations revealed lysis of bacteria resulting from breakdown of the cell membrane. A time-killing curve showed a rapid bactericidal activity of nisin. Lastly, synergistic effects of nisin were observed in combination with several antibiotics, including penicillin, amoxicillin, tetracycline, streptomycin and ceftiofur. This study brought clear evidence supporting the potential of nisin for the prevention and treatment of S. suis infections in pigs.

Effect of licochalcone A on growth and properties of Streptococcus suis

Streptococcus suis (S.suis) is an important emerging worldwide pig pathogen and zoonotic agent with rapid evolution of virulence and drug resistance. In this study, we wanted to investigate the effect of licochalcone A on growth and properties of Streptococcus suis. The antimicrobial activity of licochalcone A was tested by growth inhibition assay and the minimal inhibitory concentrations (MICs) also were determined. The effect of licochalcone A on S.suis biofilm formation was characterized by crystal violet staining. The effect of licochalcone A on suilysin secretion was evaluated by titration of hemolytic activity. To understand the antimicrobial effect, gene expression profile of S.suis treated by licochalcone A was analyzed by DNA microarray. Our results demonstrated that licochalcone A showed antimicrobial activity on S.suis with MICs of 4 µg/ml for S.suis serotype 2 strains and 8 µg/ml for S.suis serotype 7 strains. Biofilm formation was inhibited by 30–40% in the presence of licochalcone A (3 µg/ml) and suilysin secretion was also significantly inhibited in the presence of licochalcone A (1.5 µg/ml). The gene expression profile of S.suis in the presence of licochalcone A showed that 132 genes were differentially regulated, and we analyzed the regulated genes in the aspect of the bacterial cell cycle control. Among the deregulated genes, the genes responsible for the mass doubling was increased expression, but the genes responsible for DNA replication and cell division were inhibited the expression. So, we think the regulation of the cell cycle genes might provide a mechanistic understanding of licochalcone A mediated antimicrobial effect against S.suis.

[Construction and virulence evaluation of the virB1-89K gene knockout mutant of type IV-like secretion system of Streptococcus suis serotype 2]

OBJECTIVE

To construct the virB1-89K gene knockout mutant and its complementary strain of Streptococcus suis serotype 2 (SS2) highly virulent strain 05ZYH33 and evaluate the role of virB1-89K in the pathogenesis of SS2.

METHODS

The virB1-89K gene was knocked out by homologous recombination, then multiple-PCR and sequence analysis were used to identify the knockout strain deltavirB1-89K. The virB1-89K gene and its upstream promoter were cloned into the E. coli-S. suis shuttle vector pSET1, and the recombinant plasmid was electrotransformed into the deltavirB1-89K mutant to generate the complementary strain CvirB1-89K. The effects of virB1-89K deletion on the basic biological characteristics and virulence of SS2 were then determined in this study.

RESULTS

The isogenic mutant deltavirB1-89K and its complementary strain CvirB1-89K were successfully constructed. No significant differences in biological characteristics were found among the three strains. However, the virulence of the deltavirB1-89K mutant was reduced to 30% of the wild-type level and functional complementation of virB1-89K restored its pathogenicity.

CONCLUSION

The virB1-89K gene plays an important role in the pathogenesis of S. suis 2 infection.

Construction and characterization of a Streptococcus suis serotype 2 recombinant expressing enhanced green fluorescent protein

Streptococcus suis serotype 2 (S. suis 2) is an important pathogen, responsible for diverse diseases in swine and humans. To obtain a S. suis 2 strain that can be tracked in vitro and in vivo, we constructed the Egfp-HA9801 recombinant S. suis 2 strain with egfp and spc^r genes inserted via homologous recombination. To assess the effects of the egfp and spc^r genes in HA9801, the biochemical characteristics, growth features and virulence in Balb/C mice were compared between the recombinant and the parent HA9801 strain. We detected the EGFP expression from Egfp-HA9801 by epifluorescence microscopy. The results showed that the biochemical characterization and growth features of the Egfp-HA9801 recombinant were highly similar to that of the parent HA9801. We did not find significant differences in lethality (50% lethal dose), morbidity and mortality between the two strains. Furthermore, the bacterial counts in each various tissues of Egfp-HA9801-infected mice displayed similar dynamic compared with the HA9801-infected mice. Our results also showed that the Egfp-HA9801 cells grown at 37°C for 36 h displayed greater green fluorescence signals than the cells grown at 28°C for 36 h and 37°C for 24 h. The fluorescence in the tissue cryosections of Egfp-HA9801-injected mice was also stronger than that of the HA9801 group. Together, these results indicate that the egfp and spc^r insertions into the Egfp-HA9801 recombinant did not significantly change the virulence when compared with HA980, and this EGFP labeled strain can be used for future S. suis 2 pathogenesis research.

Comparative proteomic analysis of Streptococcus suis biofilms and planktonic cells that identified biofilm infection-related immunogenic proteins

Streptococcus suis (SS) is a zoonotic pathogen that causes severe disease symptoms in pigs and humans. Biofilms of SS bind to extracellular matrix proteins in both endothelial and epithelial cells and cause persistent infections. In this study, the differences in the protein expression profiles of SS grown either as planktonic cells or biofilms were identified using comparative proteomic analysis. The results revealed the existence of 13 proteins of varying amounts, among which six were upregulated and seven were downregulated in the Streptococcus biofilm compared with the planktonic controls. The convalescent serum from mini-pig, challenged with SS, was applied in a Western blot assay to visualize all proteins from the biofilm that were grown in vitro and separated by two-dimensional gel electrophoresis. A total of 10 immunoreactive protein spots corresponding to nine unique proteins were identified by MALDI-TOF/TOF-MS. Of these nine proteins, five (Manganese-dependent superoxide dismutase, UDP-N-acetylglucosamine 1-carboxyvinyltransferase, ornithine carbamoyltransferase, phosphoglycerate kinase, Hypothetical protein SSU05_0403) had no previously reported immunogenic properties in SS to our knowledge. The remaining four immunogenic proteins (glyceraldehyde-3-phosphate dehydrogenase, hemolysin, pyruvate dehydrogenase and DnaK) were identified under both planktonic and biofilm growth conditions. In conclusion, the protein expression pattern of SS, grown as biofilm, was different from the SS grown as planktonic cells. These five immunogenic proteins that were specific to SS biofilm cells may potentially be targeted as vaccine candidates to protect against SS biofilm infections. The four proteins common to both biofilm and planktonic cells can be targeted as vaccine candidates to protect against both biofilm and acute infections.

[Capsular saliva acid of Streptococcus suis 2 influences virulence and host inflammatory responses]

OBJECTIVE

We clarified the pathogenic influence of the absence of Streptococcus suis type 2 capsular saliva acid on BLAB/c mice.

METHODS

The virulence of the experimental strains were compared; the distribution of strains in vivo was determined by quantitative plating. Histopathological analysis was used to qualitatively compare the different pathogenicity of wild strain and knockout strains. ELISA was used to test the levels of cytokine in whole blood cells for the stimulation of strains.

RESULTS

The virulence of mutant strains was significantly reduced, and when the genes were restored, toxicity levels were recovered to that of the wild type strain. The distribution in blood and in the brain between wild strain and knock out strains has significant difference, and Streptococcus suis type 2 strains can cause different degrees of brain damage. During the in vitro test, the mutant strains can stimulate the whole blood cells to secrete higher levels of MCP-1 and IL-6.

CONCLUSION

Capsular saliva acid affects bacterial virulence and host cell inflammation response. As an important virulence factor of Streptococcus suis type 2, it can damage the blood brain barrier and cause meningitis.

Application of a bacteriophage lysin to disrupt biofilms formed by the animal pathogen Streptococcus suis

Bacterial biofilms are crucial to the pathogenesis of many important infections and are difficult to eradicate. Streptococcus suis is an important pathogen of pigs, and here the biofilm-forming ability of 32 strains of this species was determined. Significant biofilms were completely formed by 10 of the strains after 60 h of incubation, with exopolysaccharide production in the biofilm significantly higher than that in the corresponding planktonic cultures. S. suis strain SS2-4 formed a dense biofilm, as revealed by scanning electron microscopy, and in this state exhibited increased resistance to a number of antibiotics (ampicillin, amoxicillin, ciprofloxacin, kanamycin, and rifampin) compared to that of planktonic cultures. A bacteriophage lysin, designated LySMP, was used to attack biofilms alone and in combination with antibiotics and bacteriophage. The results demonstrated that the biofilms formed by S. suis, especially strains SS2-4 and SS2-H, could be dispersed by LySMP and with >80% removal compared to a biofilm reduction by treatment with either antibiotics or bacteriophage alone of less than 20%; in addition to disruption of the biofilm structure, the S. suis cells themselves were inactivated by LySMP. The efficacy of LySMP was not dose dependent, and in combination with antibiotics, it acted synergistically to maximize dispersal of the S. suis biofilm and inactivate the released cells. These data suggest that bacteriophage lysin could form part of an effective strategy to treat S. suis infections and represents a new class of antibiofilm agents.

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.

Human Streptococcus suis outbreak, Sichuan, China

Streptococcus suis outbreak was associated with exposure to sick or dead pigs.

From mid-July to the end of August 2005, a total of 215 cases of human Streptococcus suis infections, 66 of which were laboratory confirmed, were reported in Sichuan, China. All infections occurred in backyard farmers who were directly exposed to infection during the slaughtering process of pigs that had died of unknown causes or been killed for food because they were ill. Sixty-one (28%) of the farmers had streptococcal toxic shock syndrome; 38 (62%) of them died. The other illnesses reported were sepsis (24%) and meningitis (48%) or both. All isolates tested positive for genes for tuf, species-specific 16S rRNA, cps2J, mrp, ef, and sly. A single strain of S. suis caused the outbreak, as shown by the identification of a single ribotype. The high death ratio was of concern; prohibiting backyard slaughtering ended the outbreak.

Non-encapsulated strains reveal novel insights in invasion and survival of Streptococcus suis in epithelial cells

Streptococcus suis is a porcine and human pathogen causing invasive diseases, such as meningitis or septicaemia. Host cell interactions of S. suis have been studied mainly with serotype 2 strains, but multiple capsular serotypes as well as non-typeable strains exist with diverse virulence features. At present, S. suis is considered an extracellular pathogen. However, whether or not it can also invade host cells is a matter of controversial discussions. We have assessed adherence and invasion of S. suis for HEp-2 epithelial cells by comparing 10 serotype 2 strains and four non-typeable (NT) strains. Only the NT strains and a non-encapsulated serotype 2 mutant strain, but none of the serotype 2 strains, adhered strongly and were invasive. Invasion seemed to be affected by environmental signals, as suggested from comparison of strains grown in different media. Further phenotypic and genotypic characterization revealed a high diversity among the different strains. Electron microscopic analysis of invasion of selected invasive NT strains indicated different uptake mechanisms. One strain induced large invaginations comparable to those seen in 'caveolae' mediated uptake, whereas invasion of the other strains was accompanied by formation of filipodia-like membrane protrusions. Invasion of all strains, however, was similarly susceptible to hypertonic sucrose, which inhibits receptor-mediated endocytosis. Irrespective of the uptake pathway, streptococci resided in acidified phago-lysosome like vacuoles. All strains, except one, survived intracellularly as well as extracellular acidic conditions. Survival seemed to be associated with the AdiS protein, an environmentally regulated arginine deiminase of S. suis. Concluding, invasion and survival of NT strains of S. suis in epithelial cells revealed novel evidence that S. suis exhibits a broad variety of virulence-associated features depending on genetic variation and regulation.

Response of Streptococcus suis to iron-restricted growth conditions at high and low oxygen availability

Streptococcus suis (S. suis) is an important pathogen in pigs and has to overcome strict iron limitations in its host environment. Here, we studied iron-restricted growth of a highly virulent S. suis strain in vitro at aerobic and CO2-enriched growth conditions. At both conditions, depleting of iron in the culture medium with nitrilotriacetic acid (NTA) resulted in decreased growth rates and down regulation of several proteins. Sensitivity to NTA was significantly higher at aerobic versus CO2-enriched conditions. Growth could not be restored by addition of host iron sources such as ferritin, hemin, hemoglobin, lactoferrin or transferrin. Accordingly, S. suis was not able to produce detectable amounts of siderophores. On the other hand, growth at iron-restricted conditions was fully restored by addition of Mn2+ (at aerobic and CO2-enriched conditions) or Mg2+ (only at CO2-enriched conditions). In conclusion our results suggest that, unlike many other bacteria, S. suis adapts to iron restricted conditions by a change in its metabolism in order to replace Fe2+ by Mn2+ or Mg2+ rather than by expressing specific iron uptake systems.

Porcine choroid plexus epithelial cells induce Streptococcus suis bacteriostasis in vitro

The involvement of the choroid plexus in host defense during bacterial meningitis is unclear. Aiming to elucidate possible antibacterial mechanisms, we stimulated primary porcine choroid plexus epithelial cells (pCPEC) with proinflammatory cytokines and challenged them with various Streptococcus suis strains. In the supernatant of gamma interferon (IFN-gamma)-stimulated pCPEC, streptococcal growth was markedly suppressed. Costimulation with tumor necrosis factor alpha enhanced this bacteriostatic effect, while supplementation of L-tryptophan completely eliminated it. We also demonstrate that an activation of indoleamine 2,3-dioxygenase in the pCPEC seems to be responsible for the IFN-gamma-induced bacteriostasis. This supports the hypothesis of an active role of the choroid plexus in host defense against bacterial meningitis.

Acquisition of host plasmin activity by the Swine pathogen Streptococcus suis serotype 2

In this study, the plasminogen-binding activity of Streptococcus suis serotype 2 was investigated. Bound human plasminogen was activated by purified streptokinase, urokinase, or Streptococcus dysgalactiae subsp. equisimilis culture supernatant. Both human and porcine plasminogen were bound by S. suis. Binding was inhibited by epsilon-aminocaproic acid, and the plasminogen receptor was heat and sodium dodecyl sulfate resistant. One of the receptors was identified as glyceraldehyde-3-phosphate dehydrogenase. S. suis-associated plasmin activity was capable of activating free plasminogen, which in turn could contribute to degradation of fibronectin. This is the first report on the plasminogen-binding activity of S. suis. Further studies may reveal a contribution of this activity to the virulence of S. suis.

Production and properties of bacteriocin-like inhibitory substances from the swine pathogen Streptococcus suis serotype 2

Streptococcus suis serotype 2 is a major pathogen found in the upper respiratory tract of swine. In this study, isolates of this bacterial species were tested for the production of bacteriocin-like inhibitory substances (BLIS). Of the 38 strains tested, four inhibited the growth of other S. suis isolates according to a deferred-antagonism plate assay. Interestingly, three of the strains were originally isolated from healthy carrier pigs and were considered nonvirulent. Three isolates (94-623, 90-1330, and AAH4) that produced BLIS in liquid broth were selected for further characterization. None of the inhibitory activities was related to the production of either organic acids or hydrogen peroxide. The BLIS produced by these strains were heat stable and proteinase K, pronase, and elastase sensitive but were trypsin and chymotrypsin resistant. They were stable at pH 2 and 12 and had molecular masses in the range of 14 to 30 kDa. Maximum production was observed during the mid-log phase. Following a curing procedure with novobiocin, only 90-1330 lost the ability to produce BLIS, suggesting that the BLIS might be plasmid encoded. Analysis of the inhibitory spectra revealed that the BLIS-producing strains also inhibited the growth of Actinobacillus minor, Actinobacillus porcinus, Enterococcus durans, Micrococcus luteus, Streptococcus agalactiae, Streptococcus dysgalactiae subsp. dysgalactiae, Streptococcus equi subsp. zooepidemicus, and S. dysgalactiae subsp. equisimilis. This study reports for the first time the ability of the swine pathogen S. suis serotype 2 to produce BLIS with the characteristics of classic bacteriocins. Further studies are required to investigate the possibility of using bacteriocin-producing strains to prevent swine infections caused by virulent strains of S. suis serotype 2.

Response regulator important in pathogenesis of Streptococcus suis serotype 2

In this study, we describe the first response regulator of Streptococcus suis serotype 2, designated RevS. RevS was cloned and the sequence was determined. No histidine kinase was found in the vicinity ofrevS, therefore RevS was considered to be an orphan response regulator. An isogenic knock-out mutant of RevS was shown to be attenuated in colonising the S. suis specific organs in a competitive assay in four piglets, indicating thatrevS plays a role in the pathogenesis of S. suis infections. We analysed the protein expression profiles of various fractions of the wild-type strain 10 and the mutant strain 10DeltaRevS. The expression of known virulence factors ofS. suis by wild-type and the mutant strain, was not different. However, one protein in the protoplast fraction, with unknown identity was shown to be repressed by revS in the exponential growth phase of the mutant.

Changes in the leucocyte subpopulations of the palatine tonsillar crypt epithelium of pigs in response to Streptococcus suis type 2 infection

The tonsils are portal of entry and a site of multiplication and persistence for a variety of pathogens, including Streptococcus suis (S. suis), which is a common cause of meningitis, septicemia and arthritis in pigs. Understanding the early changes that occur in the first barrier of the tonsil, i.e. the crypt epithelium, in response to S. suis infection is critical in clarifying the pathogenesis of this disease and for the future development of efficient methods of mucosal vaccination. In this study, we investigated the early changes, from 18 to 72 h, that occur in leucocyte subpopulations of the crypt epithelium of the palatine tonsils of 3-week-old pigs in response to S. suis type 2 infection. Monoclonal antibodies against leucocyte markers CD3, CD4, CD8, gammadelta T cell receptor, lambda-immunoglobulin light-chain, myeloid cells, and major histocompatibility complex class II molecule (MHC-II) were used in an avidin-biotin immunoperoxidase technique. An increase in the number of lambda-immunoglobulin light-chain positive cells (B cell subset) was noticed in crypts of S. suis-infected animals from 18 h after infection onwards. This increase was significant at 18 and 48 h after infection. The number of CD4 and CD8 cells was greater from 18 h onwards, with a significant increase at 24 and 72 h post-infection. No significant difference in numbers of CD3, gammadelta T cell receptor and MHC-II positive cells was detected in the crypts of infected animals compared to controls. Macrophages, neutrophils and crypt epithelial cells stained positively with the myeloid marker, and the area of crypt epithelium positive for this marker was increased in the crypts of infected animals, with a significant difference detected at 24 and 72 h after infection. These results suggest that there is participation of the innate immunity in the early phase of S. suis infection, represented by neutrophils, macrophages and likely epithelial cells, and that there is a potential for the initiation of both humoral and cellular responses against S. suis within the crypt epithelium of the palatine tonsil.

Otitis interna is a frequent sequela to Streptococcus suis meningitis in pigs

Twenty-eight histologically confirmed cases of porcine leptomeningitis were examined retrospectively, with focus on the pathology of the inner and middle ear, brain, and vestibulocochlear nerve. Tissues were evaluated by histology and immunohistochemistry for Streptococcus suis serotype 2 antigen, and the bacteriologic results were recorded. Exudative otitis interna was diagnosed in 20/28 pigs (71%). The lesions primarily affected the perilymphatic ducts, with consistent involvement of the scala tympani. Perineuritis of the vestibulocochlear nerve was seen in all but four of the ears affected with otitis interna. Immunohistochemically, S. suis serotype 2 antigen was demonstrated in the leptomeningeal, perineural, and labyrinthine exudates in 11 cases. Otitis media was diagnosed in 10/28 pigs (34%), but evidence of extension to the inner ear was not observed. The findings were highly similar to descriptions of meningogenic labyrinthitis in humans and in laboratory animal models. Otitis interna in pigs can also develop via the meningogenic route and is not always, as generally stated, tympanogenic.

Environmentally regulated genes of Streptococcus suis: identification by the use of iron-restricted conditions in vitro and by experimental infection of piglets

The identification of environmentally regulated genes of Streptococcus suis by the use of iron-restricted conditions in vitro and by experimental infection of piglets is described. Eighteen unique iron-restriction-induced (iri) genes and 22 unique in-vivo-selected (ivs) genes of Strep. suis were found. None of the ivs genes was exclusively expressed in vivo. Four iri genes were identical to four clones selected in piglets. Two ivs genes were similar to genes for putative virulence factors. One of these ivs genes was identical to the epf gene of virulent Strep. suis serotype 2 strains and the other showed homology to a gene encoding a fibronectin-binding protein of Streptococcus gordonii. Two additional ivs genes showed homology to environmentally regulated genes previously identified by using an in vivo expression technology (IVET) selection system in other bacterial species. One of these showed similarity to the agrA gene of Staphylococcus aureus, a key locus involved in the regulation of numerous virulence proteins. The promoter selection system described in this paper has been successfully used for the identification of many environmentally regulated genes potentially involved in the pathogenesis of Strep. suis infections in piglets.

Species selectivity of new siderophore-drug conjugates that use specific iron uptake for entry into bacteria

Siderophores selectively bind ferric iron and are involved in receptor-specific iron transport into bacteria. Several types of siderophores were synthesized, and growth-promoting or inhibitory activities when they were conjugated to carbacephalosporin, erythromycylamine, or nalidixic acid were investigated. Overall, 11 types of siderophores and 21 drug conjugates were tested against seven different bacterial species: Escherichia coli, Bordetella bronchiseptica, Pasteurella multocida, Pasteurella haemolytica, Streptococcus suis, Staphylococcus aureus, and Staphylococcus epidermidis. In some species, the inhibitory activities of the drug conjugates were associated with the ability of the bacteria to use the siderophore portion of the molecules for growth promotion in disc diffusion tests (0.04 mumol of conjugate or siderophore per disc). E. coli used catechol-based siderophore portions as well as hydroxamate-based tri-delta-OH-N-OH-delta-N-acetyl-L-ornithine ferric iron ligands for growth under iron-restricted conditions achieved by supplemental ethylenediamine di (O-hydroxyphenylacetic acid) (100 micrograms/ml) and was sensitive to carbacephalosporin conjugated to these siderophore types (up to a 34-mm-diameter inhibition zone). B. bronchiseptica used desferrioxamine B and an isocyanurate-based or trihydroxamate in addition to catechol-based siderophore portions for promotion but was not inhibited by beta-lactam conjugates partly because of the presence of beta-lactamase. P. multocida and P. haemolytica did not use any of the synthetic siderophores for growth promotion, and the inhibitory activities of some conjugates seemed partly linked to their ability to withhold iron from these bacteria, since individual siderophore portions showed some antibacterial effects. Individual siderophores did not promote S. suis growth in restrictive conditions, but the type of ferric iron ligands attached to beta-lactams affected inhibitory activities. The antibacterial activities of the intracellular-acting agents erythromycylamine and nalidixic acid were reduced or lost, even against S. aureus and S. epidermidis, when the agents were conjugated to siderophores. Conjugate-resistant E. coli mutants showed the absence of some iron-regulated outer membrane proteins in gel electrophoresis profiles and in specific phage or colicin sensitivity tests, implying that the drugs used outer membrane receptors of ferric complexes to get into cells.

Role of capsular sialic acid in virulence and resistance to phagocytosis of Streptococcus suis capsular type 2

Streptococcus suis capsular type 2 has a capsule rich in sialic acid (NANA). Sialic acid, known to be an antiphagocytic factor for many bacterial species, inhibits the activation of the alternative complement pathway. The role of capsular NANA in virulence, resistance to phagocytosis and intracellular survival of S. suis capsular type 2 was evaluated. In general, a low concentration of NANA was observed for all the S. suis strains tested. In addition, no difference could be found in NANA concentrations between strains of different virulence degrees. Sialic acid concentration increased in the virulent strain 89-1591 and the avirulent strain 90-1330 after in vivo growth with an increased capsular material thickness compared to growth in vitro. No significant difference could be found in the phagocytosis rate by porcine blood monocytes of either strain and strain 89-1591 treated with sialidase or the sialic acid-binding lectin from Sambucus nigra (SNA I). Intracellular survival of strain 89-1591 decreased after treatments with sialidase or lectin, becoming comparable to that of strain 90-1330. Finally, no difference could be seen in virulence using a murine model, even if strain 89-1591 was treated with the enzyme or the lectin. Thus, NANA does not seem to be a critical virulence factor for S. suis capsular type 2.

Characterization of Streptococcus suis capsular type 2 haemolysin

The production of a haemolysin by Streptococcus suis capsular type 2 was investigated. Human group O erythrocytes were the most susceptible, followed by horse, sheep, cow and pig red blood cells, which exhibited similar susceptibilities; rabbit erythrocytes were the least susceptible. The haemolysin was produced at the end of the exponential growth phase. The toxin described in this paper was purified by affinity chromatography using a thiopropyl-Sepharose 6B column. It is an extracellular protein with a molecular mass of 65 kDa. The haemolysin belongs to the family of toxins known as antigenically related cholesterol-binding cytolytic toxins, since it shares common characteristics with other members of this family, such as sensitivity to oxygen and oxidizing agents, activation by reducing agents, inhibition by low concentrations of cholesterol, formation of transmembrane pores and a 'multihit' mechanism of action. In addition, anti-streptolysin antibodies inhibited the haemolytic activity caused by the S. suis haemolysin. Antibodies against the haemolysin could not be detected in pigs experimentally infected with a haemolytic positive strain of S. suis capsular type 2. To our knowledge, this is the only Lancefield group D Streptococcus producing a haemolysin with these characteristics. The role of this haemolysin in the pathogenesis of S. suis infections remains to be investigated.

Relation between encapsulation and various properties of Streptococcus suis

Encapsulation is thought to be a critical virulence factor for Streptococcus suis. In this study, encapsulation of a selected S. suis culture could be stimulated by cultivation conditions. After cultivation of the S. suis culture in fluid media supplemented with serum, the culture grew with short chains and a uniform turbidity of the growth medium. Autoclave extracts of this culture reacted with capsular type-1-specific antiserum. After cultivation of the S. suis culture in fluid media without serum, the bacteria generally formed longer chains, grew in fluid medium as granular sediment with clear supernatant, and were non-typeable. In addition, the unencapsulated variant appeared to have a more hydrophobic surface and adhered significantly more to HeLa cells. In contrast to the unencapsulated variant, the encapsulated S. suis was phagocytosed to a lesser extent by polymorphonuclear leucocytes. These findings might help to elucidate the role of encapsulation in infections with this bacterial organism.

Increase of capsular material thickness following in vivo growth of virulent Streptococcus suis serotype 2 strains

Protein profile and capsular material thickness of Streptococcus suis serotype 2 strains were compared after in vitro and in vivo growth. Three virulent and one avirulent strains were used. These strains were grown in Brain Heart Infusion (BHI) broth, cells were collected by centrifugation, resuspended in a sterile saline solution and injected in diffusion chambers. The devices were then inserted in rat abdomens for 17 h. In vitro grown strains were also inoculated into fresh BHI broth and cultivated for 17 h at 37 degrees C. In vivo as well as in vitro grown bacteria were harvested by centrifugation, processed in a French pressure cell, treated with lysozyme and centrifuged to collect cell proteins for SDS-PAGE analysis. Transmission electron microscopy using polycationic ferritin labeling to stabilize capsular material was also carried out. No significant modification was noted in the protein profile for any strain after in vivo growth except for a 39 kDa protein of one virulent strain. On the other hand, an increase in thickness of capsular material was noted for the three in vivo grown virulent strains while no change was noted for the avirulent strain. This increase in capsular material thickness of virulent strains was accompanied by an increased resistance to killing by pig polymorphonuclear leukocytes. The capacity to produce more capsular material in vivo seems to be an attribute of some virulent S. suis serotype 2 strains.