Agglutination of Streptococcus suis by sialic acid-binding lectins

The 35 Streptococcus suis capsular-type reference strains as well as 45 field strains of type 2 were tested with sialic acid-binding lectins from Sambucus nigra (SNA I), Triticum vulgaris, Maackia amurensis, Homarus americanus, and Limax flavus. Only types 1, 1/2, 2, 14, 15, and 16 agglutinated with SNA I and/or the T. vulgaris lectin. All field strains agglutinated only with SNA I. Reaction with SNA I was probably due to the sialic acid moiety since it disappeared after sialidase treatment. These results confirm the presence of sialic acid in S. suis with the possible terminal sequence N-acetylneuraminic acid-alpha(2,6)GalNAc.

Discrimination of virulent and avirulent Streptococcus suis capsular type 2 isolates from different geographical origins

In an effort to relate the protein profile to virulence, proteins from the cellular fractions and from culture supernatants of Streptococcus suis capsular type 2 strains from different geographical origins were compared by using Western blots (immunoblots). The protein profiles of the cellular fractions were similar for the majority of virulent and avirulent isolates studied, with the exception of three virulent Canadian strains for which a 135-kDa protein was not detected. Examination of the culture supernatants revealed the presence of a 135-kDa protein in all strains except the same three virulent Canadian isolates. In addition, a 110-kDa protein was present in 14 of 16 virulent strains and not in avirulent isolates. When injected into mice, the 110-kDa protein induced an immunoglobulin G response and protected against infection with homologous and heterologous virulent strains. Four strains (1330, 0891, TD10, and R75/S2) that were avirulent in the mouse model of infection and four other strains (1591, 999, JL590, and AAH4) that were virulent in the mouse model were injected into pigs. All virulent strains reproduced the disease, and all avirulent strains failed to reproduce the disease (with the exception of transient lameness in one case and fever in another case). The 110-kDa protein therefore appears to be a reliable virulence marker and a good candidate for a subunit vaccine.

Distribution of serotype, virulence markers and further characteristics of Streptococcus suis isolates from pigs

This study was designed to comparatively investigate 28 S. suis cultures isolated from various pathological processes of pigs. All cultures could be identified biochemically and most of the cultures could be serogrouped with specific antiserum against Lancefield's serogroup D. Serotyping of the S. suis isolates mainly revealed capsular types 2, 1, and 22. In addition, part of the cultures, mostly those of serotype 2, reacted with monoclonal antibodies to the virulence protein muraminidase-released protein, and, in parallel with monoclonal antibodies against the virulence protein extracellular factor. Independently of serotype and the occurrence of both virulence proteins, four cultures haemagglutinated erythrocytes from pigs, humans and rabbits. The haemagglutination reaction appeared to be related to the surface hydrophobicity of the isolates. However, part of the cultures with hydrophobic surfaces did not haemagglutinate the available erythrocyte preparations. The surface characteristics of the S. suis isolates shown in this investigation could be used to characterize individual isolates of S. suis that might be of importance for epidemiological studies.

Oligosaccharide-receptor interaction of the Gal alpha 1-4Gal binding adhesin of Streptococcus suis. Combining site architecture and characterization of two variant adhesin specificities

The sugar binding specificities of two groups of Streptococcus suis, a pig pathogen that causes meningitis also in man, were determined. Both the group represented by a recently characterized strain inhibitable by galactose and N-acetylgalactosamine (type PN) and the group inhibitable by galactose (type PO) were found by hemagglutination and solid-phase binding inhibition experiments to recognize the disaccharide Gal alpha 1-4Gal of the P1 and Pk blood group antigens. Both types preferred the disaccharide in terminal position. PN showed some, whereas PO showed almost no, binding to the globoside oligosaccharide containing an additional GalNAc beta 1-3 residue. The complete hydrogen bonding patterns were determined by using deoxy and other synthetic derivatives of the receptor disaccharide, and the constructed models of the interactions were compared with that of Escherichia coli PapG396 adhesin. The essential hydroxyls for binding were the HO-4', HO-6', HO-2, and HO-3 hydroxyls on the beta' alpha-side of the Gal alpha 1-4Gal molecule. Type PO adhesin also formed weak interactions with the hydroxyls HO-6 and HO-3'. The mechanism differed from that of E. coli, which binds to a cluster of five hydroxyls (HO-6, HO-2', HO-3', HO-4', and HO-6') and thus to a different part of the receptor disaccharide. These results represent the first example of the comparison of the saccharide receptor hydrogen bonding patterns of two bacterial organisms of different origin and show that the same saccharide may be recognized by two different binding mechanisms.

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.

Clonal analysis and virulence of Australian isolates of Streptococcus suis type 2

Multilocus enzyme electrophoresis was used to divide 124 Australian isolates of Streptococcus suis type 2 into 17 electrophoretic types (ETs). Isolates in ET 1 were the most frequent cause of disease amongst Western Australian pigs, but isolates of ET 8 were more commonly associated with disease in other Australian states. Multiple isolates from 10 of 19 farms all belonged to the same ET, whilst isolates from the other farms belonged to between 2 and 4 different ETs. Some isolates could be differentiated further by DNA restriction endonuclease analysis, whilst others with the same restriction pattern were located in different, but closely-related ETs. Fourteen isolates were tested for their virulence in mice. Most caused disease if given in high numbers, but isolates in ET 1 were virulent at lower dose rates. This virulent clone also was distinguished by the fact that 80% of isolates produced extracellular factor (EF).

Serotypes and putative virulence markers of Streptococcus suis isolates from cats and dogs

Thirteen isolates of Streptococcus suis from cats and dogs were characterised. Formamide extracts of 10 of the 13 cultures reacted with group D specific antiserum, and serotyping of the cultures with specific antisera against capsular types 1 to 28 revealed the serotypes 9, 20, 1/2, 22, 26 and 4; the remaining cultures were untypable. In addition one of the cultures reacted with monoclonal antibodies against muramidase-released protein and in parallel with monoclonal antibodies against the extracellular factor (EF) of S suis. The latter reacted with proteins with a molecular weight of more than 150,000 indicating that the strain exhibited EF-related proteins. One of the 13 cultures haemagglutinated erythrocytes from human beings and various animal species and adhered in large numbers to HeLa cells.

Partial characterization of Streptococcus suis type 2 hemolysin

Streptococcus suis type 2 was evaluated for hemolysin production. Supernatants of S. suis type 2 grown in Todd-Hewitt broth were assayed for hemolytic activity by a photometric assay. Twenty-two additional serotypes of S. suis (1,3 to 22, and 1/2) were evaluated for hemolysin production; nine of them (1/2, 1, 4, 5, 14, 15, 17, 19, and 20) were positive. The effects of temperature, atmosphere, centrifugation, sonication, chemicals, bovine serum albumin, fetal calf serum, and enzymes on S. suis type 2 hemolysin activity were studied. Maximum hemolysis occurred after incubation in RPMI 1640 medium at 40 degrees C in 6% CO2 and after growth in Todd-Hewitt broth at 37 degrees C under anaerobic conditions. Hemolytic activity was absent after the addition of fetal calf serum and decreased after the addition of trypsin or amylase. However, treatment of erythrocytes with amylase or trypsin prior to incubation with supernatant also resulted in a decrease in hemolytic activity. The addition of bovine serum albumin caused increased hemolytic activity. Dipyridyl and EDTA had negligible effects on hemolysis. Hemolytic S. suis type 2 culture supernatant injected intraperitoneally failed to cause death in BALB/c mice. Data from our study indicate that S. suis type 2 hemolysin is a secreted or loosely cell bound, thermolabile molecule whose activity is growth condition dependent.

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.

Repeats in an extracellular protein of weakly pathogenic strains of Streptococcus suis type 2 are absent in pathogenic strains

Streptococcus suis type 2 strains that are pathogenic for pigs produce a 110-kDa extracellular protein factor (EF). Nonpathogenic and weakly pathogenic strains do not produce EF or produce a protein (EF*) that is immunologically related to EF. To study the pathogenesis of S. suis type 2 in pigs and to develop tools and methods for the control of S. suis type 2 infections, we cloned and characterized the genes encoding EF and various EF* proteins. Analysis of the deduced amino acid sequences showed that the first 833 amino acids at the N terminus of the EF and EF* proteins were nearly identical. The proteins differed, however, at their C termini. Unlike the 110-kDa EF protein, the EF* proteins contained several repeated units of 76 amino acids. The number and arrangement of the repeats in the EF* proteins varied. The data suggest that the gene encoding EF could have evolved from an epf* gene by a specific deletion event. The lack of repeated amino acid units in the EF protein may be related to virulence.

Molecular analysis of isolates of Streptococcus suis capsular type 2 by restriction-endonuclease-digested DNA separated on SDS-PAGE and by hybridization with an rDNA probe

This study was undertaken to assess the discriminatory value of restriction endonuclease (RE) digestion patterns of Streptococcus suis chromosomal DNA using polyacrylamide gel electrophoresis (SDS-PAGE) and DNA-rDNA hybridization. For the RE digestion patterns, DNAs were digested separately with the enzymes BamHI and BglII and the resultant fragments were separated by SDS-PAGE. An Escherichia coli rDNA probe derived from pKK3535 was used for the hybridization. Twenty-three S. suis capsular type 2 isolates recovered from diseased and clinically healthy pigs, from a human case, and from a cow were compared in this study. The majority of isolates associated with septicaemia belonged to one restriction endonuclease analysis (REA) profile group. Isolates associated with pneumonia belonged either to the REA profile group of isolates associated with septicaemia or to a second REA profile group. The REA profiles of isolates from clinically healthy animals were more heterogeneous. The REA profile of the type 2 reference strain, S735, which was originally isolated from a pig, was very different from those of the porcine and bovine isolates but similar to the profile of the human isolate. The profiles obtained after rDNA hybridization were more homogeneous. Although different patterns were detected in the 23 isolates, there was no correlation between the source of the isolate and the patterns observed with this technique.

Discrimination between virulent and nonvirulent Streptococcus suis type 2 strains by enzyme-linked immunosorbent assay

Discrimination between virulent and nonvirulent strains of Streptococcus suis type 2 will allow proper diagnosis of diseased pigs and the identification of carrier pigs. To discriminate between virulent and nonvirulent strains, we developed two double antibody sandwich (DAS) enzyme-linked immunosorbent assays (ELISA) using specific monoclonal antibodies directed against two virulence markers of S. suis type 2. One mAb was directed against the 136-kilodalton (kDa) cell-wall-associated protein, designated muramidase-released-protein (MRP). The other mAb was directed against a 110-kDa extracellular protein, provisionally called extracellular factor (EF). We examined 179 strains of S. suis serotype 2, 22 strains of S. suis serotypes 1 to 22, 22 other streptococci, 20 other bacterial strains, and one yeast. The ELISA results were almost identical with western blot analysis of these strains. Visual readings of the two DAS-ELISAs were enough to discriminate accurately between the three phenotypes of S. suis type 2. We concluded that the two DAS-ELISAs are reliable, rapid, and simple assays to identify virulent strains of S. suis type 2.

Studies on a murine model for evaluation of virulence of Streptococcus suis capsular type 2 isolates

Five different parameters, time of incubation of the culture, type of culture medium, inoculum, strain of inbred mice, and age of mice, were tested using the LD50 technique to standardize a murine model for the evaluation of the virulence of Streptococcus suis capsular type 2 isolates. A model using 28 day-old mice belonging to CF1 strain appeared to give the best results. The inoculum size was the parameter most influencing the 50% lethal dose obtained with mice. Inoculation with 1-ml volume of a bacterial suspension instead of 0.1 or 0.5 ml decreased the LD50. The standardized model was used to evaluate the virulence of some isolates of known pathogenicity for pigs. The minimum lethal dose was used in the model and it appeared that the virulence of Streptococcus suis capsular type 2 isolates can be measured from highly virulent to totally avirulent.

Virulence of Streptococcus suis type 2 strains in newborn germfree pigs depends on phenotype

To determine whether the virulence of Streptococcus suis type 2 is associated with the phenotype of the strain, we infected newborn germfree pigs with 10 strains of S. suis type 2 categorized by three phenotypes. In an earlier study, the phenotypes were distinguished by the presence or absence of the muramidase-released protein (MRP) and an extracellular factor (EF) and were designated MRP+ EF+, MRP+ EF- and MRP- EF-. Pigs were first inoculated with Bordetella bronchiseptica to predispose them to infection and were then intranasally inoculated with the streptococci. Strains of the MRP+ EF+ phenotype induced fever and increased the number of polymorphonuclear leukocytes in blood. Specific clinical signs of disease such as nervous disorders and lameness were also observed. At necropsy bacteriologic and pathologic examination disclosed meningoencephalitis, polyserositis, and polyarthritis. Strains of the MRP+ EF- phenotype induced only nonspecific clinical signs of disease such as recumbency, lack of appetite, and fever; only slight pathologic changes were detected in the serosae. The four strains of the MRP- EF- phenotype induced no signs of disease. These findings indicate that the 110-kDa EF and, to a lesser degree, the 136-kDa MRP may be associated with the virulence of the bacterium. The results demonstrated that S. suis type 2 strains producing both MRP and EF are pathogenic for pigs.

Production and characterization of two Streptococcus suis capsular type 2 mutants

Two avirulent mutants of Streptococcus suis capsular type 2 (M2 and M42) were produced from a highly virulent strain. Mutant M2, obtained after serial subcultures of the parent strain in the presence of rabbit anti-capsular type 2 serum, no longer possessed the type-specific capsular antigen, as demonstrated by serotyping methods and immunoelectron microscopy. The Lancefield group D antigen could not be detected on the cell surface of this mutant using the immunogold labelling technique. SDS-PAGE of lysozyme treated cells demonstrated that a 44 kDa protein which was present in the parent strain, was absent in mutant M2. Immunoblotting using rabbit whole cell homologous anti-serum revealed that the protein was strongly immunogenic. Mutant M2 was totally avirulent in mice, and the homologous antiserum completely failed to protect mice against challenge with the parent strain. However, mutant M42, obtained after passages of the parent strain at 42 degrees C, remained capsulated but lacked the same 44 kDa protein as mutant M2. The quantity of sialic acid present in the capsule was similar to that of the parent strain. Despite the presence of antibodies against the capsule, antiserum prepared against M42 only partially protected mice against a challenge with the parent strain. The 44 kDa cell wall protein could act as a virulence factor as well as an important immunogen of S. suis capsular type 2.

Experimental infections of mice and pigs with Streptococcus suis type 2

Five inbred strains of mice were tested for their susceptibility to Streptococcus suis type 2 including the type strain, two isolates from meningitis in pigs and two isolates from tonsils of clinically healthy pigs. C57BL/6, ICR and ddY strain mice showed lower susceptibility to all strains of S. suis type 2 than BALB/c and SS strain mice. The type strain and the isolates from diseased pigs produced septicaemia and meningitis in BALB/c and SS mice inoculated with 10(8) colony forming unit of the bacteria and 60 to 100% of these infected mice died. On the other hand, mice inoculated with the isolates from healthy pigs showed mild clinical signs but none of them died. In BALB/c mice which died or developed nervous signs, the purulent meningo-encephalitis, myocarditis, ophthalmitis, labyrinthitis and otitis media were observed. S. suis type 2 antigen was demonstrated in these lesions by immunoperoxidase staining using rabbit S. suis type 2 antiserum. These results were similar to those in the experimentally infected pigs with these virulent and avirulent strains against mice. These results indicate that BALB/c and SS strains of mice are useful as an experimental model of S. suis type 2 infections in pigs, and that there are virulent and avirulent strains against mice and pigs among the strains of S. suis type 2.