Clonal expansion of a virulent Streptococcus suis serotype 9 lineage distinguishable from carriage subpopulations.

Streptococcus suis is a porcine pathogen, causing severe invasive infections. S. suis serotype 9 is increasingly causing disease in Dutch and Chinese pig herds, but it is unknown whether all serotype 9 isolates are equally virulent and markers that can identify virulent strains are not available. Therefore, discrimination between virulent isolates and carriage isolates typically not associated with disease, is currently not possible. We collected tonsillar S. suis isolates from 6 herds not previously diagnosed with S. suis infections, and clinical S. suis isolates of previously diseased pigs. We confirmed the virulence of a virulent type strain and one representative clinical isolate, and the lack of virulence of two carriage isolates, in a pig infection model. Phylogenetic analysis of whole genome sequences of 124 isolates resulted in 10 groups, of which two were almost uniquely populated by clinical isolates. The population structure of S. suis serotype 9 appears highly diverse. However, analysis of the capsule loci sequences showed variation in a single region which fully correlated with a virulent genotype. Transmission electron microscopy suggested differences in capsule thickness between carriage and clinical genotypes. In conclusion, we found that that the S. suis serotype 9 population in the Netherlands is diverse. A distinct virulence-associated lineage was identified and could be discriminated based on the capsule locus sequence. Whilst the difference in virulence cannot be directly attributed to the DNA sequence, the correlation of capsule locus sequence with virulence could be used in the development of diagnostic tests to identify potential virulent S. suis serotype 9 in pigs.

In vitro/vivo Mechanism of Action of MP1102 With Low/Nonresistance Against Streptococcus suis Type 2 Strain CVCC 3928.

Streptococcosis is recognized as a leading infectious disease in the swine industry. Streptococcus suis serotype 2 is regarded as the most virulent species, which threatens human and pig health and causes serious economic losses. In this study, multiple in vitro and in vivo effects of MP1102 on multidrug resistant S. suis was studied for the first time. MP1102 exhibited significant antibacterial activity against S. suis (minimum inhibitory concentration, MIC = 0.028-0.228 µM), rapid bacteriocidal action, a longer postantibiotic effect than ceftriaxone, and a synergistic or additive effect with lincomycin, penicillin, and ceftriaxone (FICI = 0.29-0.96). No resistant mutants appeared after 30 serial passages of S. suis in the presence of MP1102. Flow cytometric analysis and electron microscopy observations showed that MP1102 destroyed S. suis cell membrane integrity and affected S. suis cell ultrastructure and membrane morphology. Specifically, a significantly wrinkled surface, intracellular content leakage, and cell lysis were noted, establishing a cyto-basis of nonresistance to this pathogen. DNA gel retardation and circular dichroism analysis indicated that MP1102 interacted with DNA by binding to DNA and changing the DNA conformation, even leading to the disappearance of the helical structure. This result further supported the mechanistic basis of nonresistance via interaction with an intracellular target, which could serve as a means of secondary injury after MP1102 is transported across the membrane. Upon treatment with 2.5-5.0 mg/kg MP1102, the survival of mice challenged with S. suis was 83.3-100%. MP1102 decreased bacterial translocation in liver, lung, spleen, and blood; inhibited the release of interleukin-1ß and tumor necrosis factor-α; and relieved the lung, liver, and spleen from acute injury induced by S. suis. These results suggest that MP1102 is a potent novel antibacterial agent for the treatment of porcine streptococcal disease.

Better understanding of homologous recombination through a 12-week laboratory course for undergraduates majoring in biotechnology.

Homologous recombination, a central concept in biology, is defined as the exchange of DNA strands between two similar or identical nucleotide sequences. Unfortunately, undergraduate students majoring in biotechnology often experience difficulties in understanding the molecular basis of homologous recombination. In this study, we developed and implemented a 12-week laboratory course for biotechnology undergraduates in which gene targeting in Streptococcus suis was used to facilitate their understanding of the basic concept and process of homologous recombination. Students worked in teams of two to select a gene of interest to create a knockout mutant using methods that relied on homologous recombination. By integrating abstract knowledge and practice in the process of scientific research, students gained hands-on experience in molecular biology techniques while learning about the principle and process of homologous recombination. The learning outcomes and survey-based assessment demonstrated that students substantially enhanced their understanding of how homologous recombination could be used to study gene function. Overall, the course was very effective for helping biotechnology undergraduates learn the theory and application of homologous recombination, while also yielding positive effects in developing confidence and scientific skills for future work in research. © 2017 by The International Union of Biochemistry and Molecular Biology, 45(4):329-335, 2017.

A novel endolysin disrupts Streptococcus suis with high efficiency.

Streptococcus suis serotype 2 (S. suis 2) is a zoonotic pathogen that exhibits high-level resistance and multi-drug resistance to classic antibiotics and causes serious human casualties and heavy economic losses in the swine industry worldwide. Therefore, alternative therapies or novel antibacterial agents need to be developed to combat this pathogen. A novel endolysin derived from the S. suis temperate phage phi7917, termed Ly7917, was identified, which had broad lytic activity against S. suis type 1, 2, 7 and 9. Ly7917 consisted of an N-terminal cysteine, histidine-dependent amidohydrolases/peptidase catalytic domain and C-terminal SH3b cell wall binding domain. The endolysin maintained activity at high pH and its catalytic activity could be improved by addition of 10 µM 1.5 mM Ca(2+). In animal studies, 90% of BALB/c mice challenged with typical virulent strain HA9801 of S. suis 2 were protected by Ly7917 treatment. The bacterial load in the blood of HA9801-challenged mice was efficiently reduced almost 50% by Ly7917 while that of penicillin-G-treated mice kept almost unchanged. Our data suggest that Ly7917 may be an alternative therapeutic agent for infections caused by virulent S. suis strains.

First human case report of sepsis due to infection with Streptococcus suis serotype 31 in Thailand.

BACKGROUND: Streptococcus suis is a zoonotic pathogen that causes invasive infections in humans and pigs. It has been reported that S. suis infection in humans is mostly caused by serotype 2. However, human cases caused by other serotypes have rarely been reported. This is the first report of a human case of infection with S. suis serotype 31 in Thailand. CASE PRESENTATION: A 55-year-old male alcohol misuser with liver cirrhosis was admitted with sepsis to a hospital in the Central Region of Thailand. He had consumed a homemade, raw pork product prior to the onset of illness. He was alive after treatment with ceftriaxone and no complication occurred. An isolate from blood culture at the hospital was suspected as viridans group Streptococcus. It was confirmed at a reference laboratory as S. suis serotype 31 by biochemical tests, 16S rDNA sequencing, and multiplex polymerase chain reaction for serotyping, but it was untypable by the co-agglutination test with antisera against recognized S. suis serotypes, suggesting loss of capsular material. The absence of a capsule was confirmed by transmission electron microscopy. The isolate was confirmed to be sequence type 221, with 13 putative virulence genes that are usually found in serotype 2 strains. CONCLUSION: We should be aware of the emergence of S. suis infections caused by uncommon serotypes in patients with predisposing conditions. Laboratory capacity to identify S. suis in the hospital is needed in developing countries, which can contribute to enhanced surveillance, epidemiological control, and prevention strategies in the prevalent area.

Eight Novel Capsular Polysaccharide Synthesis Gene Loci Identified in Nontypeable Streptococcus suis Isolates.

Streptococcus suis is an important pathogen of pigs and may cause serious disease in humans. Serotyping is one of the important diagnostic tools and is used for the epidemiological study of S. suis. Nontypeable S. suis strains have been reported in many studies; however, the capsular polysaccharide (CPS) synthesis cps loci of nontypeable strains have not been analyzed. In this study, we investigated the genetic characteristics of cps loci in 78 nontypeable strains isolated from healthy pigs. Eight novel cps loci (NCLs) were found, and all of them were located between the orfZ-orfX region and the glf gene. All NCLs possess the wzy and wzx genes, strongly suggesting that the CPSs of these NCLs were synthesized using the Wzx/Wzy-dependent pathway. The cps genes found in the 78 isolates were assigned to 96 homology groups (HGs), 55 of which were NCL specific. The encapsulation of the 78 isolates was also examined using transmission electron microscopy. Fifty-three isolates were found to have a capsule, and these were of varied thicknesses. Our data enhance our understanding of the cps gene cluster diversity of nontypeable S. suis strains and provide insight into the evolution of the S. suis capsular genes.

Bacterial cell surface properties: role of loosely bound extracellular polymeric substances (LB-EPS).

This study investigated the effect of loosely bound extracellular polymeric substances (LB-EPS) on the comprehensive surface properties of four bacteria (Bacillus subtilis, Streptococcus suis, Escherichia coli and Pseudomonas putida). The removal of LB-EPS from bacterial surfaces by high-speed centrifugation (12,000×g) was confirmed by SEM images. Viability tests showed that the percentages of viable cells ranged from 95.9% to 98.0%, and no significant difference was found after treatment (P>0.05). FTIR spectra revealed the presence of phosphodiester, carboxylic, phosphate, and amino functional groups on bacteria surfaces, and the removal of LB-EPS did not alter the types of cell surface functional groups. Potentiometric titration results suggested the total site concentrations on the intact bacteria were higher than those on LB-EPS free bacteria. Most of the acidity constants (pKa) were almost identical, except the increased pKa values of phosphodiester groups on LB-EPS free S. suis and E. coli surfaces. The electrophoretic mobilities and hydrodynamic diameters of the intact and LB-EPS free bacteria were statistically unchanged (P>0.05), indicating LB-EPS had no influence on the net surface charges and size distribution of bacteria. However, LB-ESP could enhance cell aggregation processes. The four LB-EPS free bacteria all exhibited fewer hydrophobicity values (26.1-65.0%) as compared to the intact cells (47.4-69.3%), suggesting the removal of uncharged nonpolar compounds (e.g., carbohydrates) in LB-EPS. These findings improve our understanding of the changes in cell surface characterizations induced by LB-EPS, and have important implications for assessing the role of LB-EPS in bacterial adhesion and transport behaviors.

Streptococcus suis sorption on agricultural soils: role of soil physico-chemical properties.

Understanding pathogen sorption on natural soil particles is crucial to protect public health from soilborne and waterborne diseases. Sorption of pathogen Streptococcus suis on 10 agricultural soils was examined, and its correlations with soil physico-chemical properties were also elucidated. S. suis sorption isotherms conformed to the linear equation, with partition coefficients (Ks) ranging from 12.7 mL g(-1) to 100.1 mL g(-1). Bacteria were observed to sorb on the external surfaces of soil aggregates by scanning electron microscopy. Using Pearson correlation and linear regression analysis, solution pH was found to have significant negative correlations with Ks. Stepwise multiple regression and path analysis revealed that pH and cation exchange capacity (CEC) were the main factors influencing sorption behaviors. The obtained overall model (Ks=389.6-45.9×pH-1.3×CEC, R(2)=0.943, P<0.001) can accurately predict Ks values. However, the variability in Ks was less dependent on soil organic matter, specific surface area, soil texture and zeta potential, probably due to the internal-surface shielding phenomenon of soil aggregates. Additionally, the sorption trends cannot be interpreted by interaction energy barriers calculated using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, suggesting the limits of DLVO theory in describing pathogen sorption on natural soils. Our results also indicated soil pH and CEC should be preferentially considered when modeling S. suis sorption process.

Contribution of eukaryotic-type serine/threonine kinase to stress response and virulence of Streptococcus suis.

Streptococcus suis serotype 2 (SS2) is an important swine and human pathogen responsible for septicemia and meningitis. The bacterial homologues of eukaryotic-type serine/threonine kinases (ESTKs) have been reported to play critical roles in various cellular processes. To investigate the role of STK in SS2, an isogenic stk mutant strain (Δstk) and a complemented strain (CΔstk) were constructed. The Δstk showed a significant decrease in adherence to HEp-2 cells, compared with the wild-type strain, and a reduced survival ratio in whole blood. In addition, the Δstk exhibited a notable reduced tolerance of environmental stresses including high temperature, acidic pH, oxidative stress, and high osmolarity. More importantly, the Δstk was attenuated in both the CD1 mouse and piglet models of infection. The results of quantitative reverse transcription-PCR (qRT-PCR) analysis indicated that the expressions of a few genes involving in adherence, stress response and virulence were clearly decreased in the Δstk mutant strain. Our data suggest that SsSTK is required for virulence and stress response in SS2.

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.

Subcytolytic effects of suilysin on interaction of Streptococcus suis with epithelial cells.

Suilysin is a pore-forming cholesterol-dependent cytolysin secreted by Streptococcus suis (S. suis), an important swine and zoonotic pathogen. The role of suilysin in S. suis host-cell interaction is still unclear. We found a higher adherence and invasion rate of an unencapsulated sly-positive strain in comparison to its isogenic sly-negative mutant. Electron microscopy revealed that formation of membrane ruffles accompanying invasion of the sly-positive strain was abolished in the sly-negative mutant. Inhibition experiments showed that the actin cytoskeleton was involved in suilysin-mediated effects. Point-mutation of the domain putatively responsible for macropore-formation resulted in abolished hemolytic and cytolysin activity, but had no effect on S. suis host cell association. Concluding, our results indicate that subcytolytic suilysin promotes S. suis association with epithelial cells.

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.

Loss of capsule among Streptococcus suis isolates from porcine endocarditis and its biological significance.

Streptococcus suis, particularly serotype 2, is a pathogen of both pigs and humans associated with a wide range of diseases, including meningitis, septicaemia and endocarditis. Among the genes in the capsular polysaccharide biosynthesis (cps) locus, cps2J exists only in the serotype 2 and 1/2 strains; therefore, cps2J-positive strains are suspected to have capsules of serotype 2 or 1/2. Coagglutination using antiserotype 1 and antiserotype 2 sera and/or transmission electron microscopy analysis of 288 cps2J-positive isolates from pigs showed that 32 (100 %) isolates from meningitis were encapsulated, whereas 86 (34 %) of 256 isolates from endocarditis were unencapsulated, indicating that capsule loss often occurred in the isolates from endocarditis. To investigate the genetic backgrounds, we randomly selected 43 unencapsulated isolates and analysed their cps loci by PCR scanning. Among them, 8 and 10 isolates apparently had deletions and insertions, respectively, in cps loci. In addition, a representative unencapsulated isolate and an unencapsulated strain showed adherence to porcine and human platelets, a major virulence determinant for infective endocarditis, to a significantly greater extent than the encapsulated strains. Although the capsule is considered to be an important virulence factor in S. suis, these results suggest that loss of capsular production is beneficial to S. suis in the course of infective endocarditis.

Pleiotropic effects of polysaccharide capsule loss on selected biological properties of Streptococcus suis.

In this study, an unencapsulated Streptococcus suis mutant was used to investigate the pleiotropic effects resulting from capsule loss. The capsule deficient mutant of S. suis acquired a biofilm-positive phenotype, which was associated with significantly increased cell surface hydrophobicity. Cell-associated fibrinogen-binding and chymotrypsin-like activities were decreased in the unencapsulated mutant. The mutant did not differ significantly from the encapsulated parent strain for minimal inhibitory concentrations to penicillin G, ampicillin, and tetracycline. However, while the encapsulated strain was highly resistant to the bactericidal action of penicillin G and ampicillin, the unencapsulated mutant was approximately 60-fold more sensitive. Compared with the parent strain, the unencapsulated mutant induced a much higher inflammatory response in monocyte-derived macrophages resulting in an increased secretion of tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, IL-6, and IL-8. The capsule appears to hinder important adhesins or hydrophobic molecules that mediate biofilm formation, as well as cell wall components capable of stimulating immune cells.

Characterisation of biofilm formation by a Streptococcus suis meningitis isolate.

Biofilm formation by a strain of Streptococcus suis serotype 2 isolated from a case of meningitis in pigs was characterised. Using a polystyrene microtitre plate assay, S. suis 95-8242 produced a dense biofilm when glucose, fructose or sucrose was used as the carbohydrate source, whereas no biofilm formed in the presence of lactose. Polysaccharide production by the biofilm-forming strain was demonstrated by the Congo red agar assay. Transmission electron microscopy revealed that bacterial cells were surrounded by a thick layer of polycationic ferritin-labelled material. S. suis 95-8242 was more resistant to both penicillin G and ampicillin in biofilms than in planktonic cultures on the basis of minimal inhibitory and minimal bactericidal concentrations.

Fibrinogen induces biofilm formation by Streptococcus suis and enhances its antibiotic resistance.

In this study, we showed that supplementing the culture medium with fibrinogen induced biofilm formation by Streptococcus suis in a dose-dependent manner. Biofilm-grown S. suis cells were much more resistant to penicillin G than planktonic cells. S. suis bound fibrinogen to its surface, a property that likely contributes to biofilm formation.

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.

Cloning and characterization of the gene encoding the glutamate dehydrogenase of Streptococcus suis serotype 2.

Given the lack of effective vaccines to control Streptococcus suis infection and the lack of a rapid and reliable molecular diagnostic assay to detect its infection, a polyclonal antibody was raised against the whole-cell protein of S. suis type 2 and used to screen an S. suis gene library in an effort to identify protective antigen(s) and antigens of diagnostic importance. A clone that produced a 45-kDa S. suis-specific protein was identified by Western blotting. Restriction analysis showed that the gene encoding the 45-kDa protein was present on a 1.6-kb pair DraI region on the cloned chromosomal fragment. The nucleotide sequence contained an open reading frame that encoded a polypeptide of 448 amino acid residues with a calculated molecular mass of 48.8 kDa, in close agreement with the size observed on Western blots. A GenBank database search revealed that the derived amino acid sequence is homologous to the sequence of glutamate dehydrogenase (GDH) protein isolated from various sources, including conserved motifs and functional domains typical of the family 1-type hexameric GDH proteins, thus placing it in that family. Because of these similarities, the protein was designated the GDH of S. suis. Hybridization studies showed that the gene is conserved among the S. suis type 2 strains tested. Antiserum raised against the purified recombinant protein was reactive with a protein of the same molecular size as the recombinant protein in S. suis strains, suggesting expression of the gene in all of the isolates and antigenic conservation of the protein. The recombinant protein was reactive with serum from pigs experimentally infected with a virulent strain of S. suis type 2, suggesting that the protein might serve as an antigen of diagnostic importance to detect S. suis infection. Activity staining showed that the S. suis GDH activity is NAD(P)H dependent but, unlike the NAD(P)H-dependent GDH from various other sources, that of S. suis utilizes L-glutamate rather than alpha-ketoglutarate as the substrate. Highly virulent strains of S. suis type 2 could be distinguished from moderately virulent and avirulent strains on the basis of their GDH protein profile following activity staining on a nondenaturing gel. We examined the cellular location of the protein using a whole-cell enzyme-linked immunosorbent assay and an immunogold-labeling technique. Results showed that the S. suis GDH protein is exposed at the surface of intact cells.

Epithelial invasion and cell lysis by virulent strains of Streptococcus suis is enhanced by the presence of suilysin.

Streptococcus suis is an important pathogen of pigs causing arthritis, pneumonia and meningitis and is an occupational disease of farmers and those in the meat industry. As with other streptococci, both virulent and avirulent strains of S. suis are frequently carried asymptomatically in the tonsillar crypts and nasal cavities. Little is known about the process by which virulent strains cross the mucosal epithelia to generate systemic disease and whether this process requires expression of specific bacterial virulence factors. Although putative virulence factors have been postulated, no specific role in the disease process has yet been demonstrated for these factors. This study is the first demonstration that virulent strains of S. suis both invade and lyse HEp-2 cells, a continuous laryngeal epithelial cell line, and that at least one bacterial virulence factor, suilysin, is involved in this process.

Characterization and protective activity of a monoclonal antibody against a capsular epitope shared by Streptococcus suis serotypes 1, 2 and 1/2.

A monoclonal antibody (mAb Z3) was produced using BALB/c mice immunized with whole cells of Streptococcus suis serotype 2 reference strain S735. Screening by dot-ELISA showed that mAb Z3, of isotype IgG2b, reacted only with reference strains and field isolates of S. suis serotypes 1, 2 and 1/2. The recognized epitope was demonstrated to be polysaccharide in nature by periodate oxidation, and located in the capsule, since mAb Z3 reacted with purified capsular material by immunoblotting and was able to stabilize the capsule as shown by electron microscopy. Further characterization indicated that mAb Z3 may react specifically with the sialic acid moiety of the capsule, a common constituent of the polysaccharidic capsular material of the three capsular types, since sialidase-treated cells did not react with mAb Z3 in immunoblotting or indirect ELISA. Purified mAb Z3 was shown to significantly increase the rate of phagocytosis of S. suis cells by porcine monocytes and to activate the clearance of bacteria from the circulation in experimentally infected mice. However, mAb Z3 only offered partial protection to mice challenged with a minimal lethal dose. Thus, even though the capsule of S. suis seems to be an important virulence factor, the epitope recognized by mAb Z3 does not appear to be involved in complete protection against infection.