Use of Ribotyping and Hemolysin Activity To Identify Highly Virulent Streptococcus suis Type 2 Isolates

Abstract

Nineteen Streptococcus suis type 2 isolates were evaluated for their virulence in pigs and mice. Of these, seven were determined to be highly virulent in pigs on the basis of clinical sign scores and gross pathology and histopathology results. Clinical sign scores correlated with gross pathology and histopathology scores atP equal to 0.004 and P equal to 0.009, respectively. The virulence of highly virulent isolates in pigs compared somewhat with virulence in mice, but the correlation was not significant. No correlation of virulence was noted among the moderately virulent and avirulent isolates in pigs and mice. Chromosomal DNAs from all S. suis isolates were evaluated by PstI,PvuII, EcoRI, and HaeIII restriction enzyme digestion followed by hybridization with a digoxigenin-11-dUTP-labeled cDNA probe transcribed from 16S and 23S rRNAs from Escherichia coli. The hybridization patterns (ribotypes) varied depending upon the enzyme used, but a significant number of isolates determined to be highly virulent in pigs had unique hybridization patterns compared with those of the moderately virulent and avirulent isolates (P = 0.002). In addition, hemolysin activity showed a high correlation to virulence (P = 0.00008) and ribotype (P = 0.002).

Hybridization analysis of the gene encoding a hemolysin (suilysin) of Streptococcus suis type 2: evidence for the absence of the gene in some isolates

“…The 63 S. suis strains used in this study were obtained from varied geographical location and their sources, cultivation and DNA extraction methods have been described previously [11,18]. Additionally, all S. suis type 2 tested were grown on sheep blood agar plates (Remel, Lenexa, KS, USA) for hemolytic phenotype.…”

“…Although all isolates were K-hemolytic on sheep blood agar plate after 48 h of incubation, the isolates producing the 4.0 kb band gave almost a L-hemolytic type phenotype after additional 96 h incubation at room temperature whereas the isolates that produced the 1.2 kb band clearly remained K-hemolytic (data not shown), supporting the notion that di¡erent types of hemolysin may exist in S. suis type 2 isolates. Staats et al [18] had correlated hemolysin activity to the virulence of S. suis type 2. We compared the hybridization pattern in this study with their report using most of their isolates to determine if highly virulent isolates could be distinguished from moderately virulent or avirulent isolates.…”

“…6, isolates 86-3977B, 1933, D930) hybridized to the suilysin gene probes to give the 4.0 kb band. The pathogenicity of isolate DH5 has been controversial [11,18]. All of the avirulent and moderately virulent isolates possessed the 1.2 kb band, suggesting that the hybridization pattern with the suilysin gene probe might be useful for identi¢cation of most highly virulent isolates.…”

Dilemma of the Virulence of Streptococcus suis Strains

“…In addition, this strain is still being used in our laboratory for the study of virulence factors in North American strains. Surprisingly, Staats et al (12) considered this strain as highly virulent for mice but of low virulence for pigs.…”

“…Experimental infection models for S. suis can be misleading. Different studies have designated field strains as being virulent or avirulent based on the clinical condition of the animal from which the strain was isolated (clinically diseased or healthy animals), the presence of virulence-related proteins, the virulence in a mouse model, and the virulence to pigs from highhealth-status herds, specific-pathogen-free pigs, and colostrum-deprived pigs (2,7,10,12,14,15). In fact, several discrepencies exist in the literature.…”