Reappraisal of the taxonomy of Streptococcus suis serotypes 20, 22 and 26: Streptococcus parasuis sp. nov.

In order to clarify the taxonomic position of serotypes 20, 22 and 26 of Streptococcus suis, biochemical and molecular genetic studies were performed on isolates (SUT-7, SUT-286(T), SUT-319, SUT-328 and SUT-380) reacted with specific antisera of serotypes 20, 22 or 26 from the saliva of healthy pigs as well as reference strains of serotypes 20, 22 and 26. Comparative recN gene sequencing showed high genetic relatedness among our isolates, but marked differences from the type strain S. suis NCTC 10234(T), i.e. 74.8-75.7 % sequence similarity. The genomic relatedness between the isolates and other strains of species of the genus Streptococcus, including S. suis, was calculated using the average nucleotide identity values of whole genome sequences, which indicated that serotypes 20, 22 and 26 should be removed taxonomically from S. suis and treated as a novel genomic species. Comparative sequence analysis revealed 99.0-100 % sequence similarities for the 16S rRNA genes between the reference strains of serotypes 20, 22 and 26, and our isolates. Isolate STU-286(T) had relatively high 16S rRNA gene sequence similarity with S. suis NCTC 10234(T) (98.8 %). SUT-286(T) could be distinguished from S. suis and other closely related species of the genus Streptococcus using biochemical tests. Due to its phylogenetic and phenotypic similarities to S. suis we propose naming the novel species Streptococcus parasuis sp. nov., with SUT-286(T) ( = JCM 30273(T) = DSM 29126(T)) as the type strain.

Development of PCR for identifying Streptococcus parasuis, a close relative of Streptococcus suis.

Streptococcus parasuis has recently been removed taxonomically from Streptococcus suis, a zoonotic pathogen. S. parasuis has been detected in healthy pigs and in diseased pigs, which suggests that S. parasuis is involved in the normal microbiota of pigs and has potential pathogenicity. However, the pathogenicity of S. parasuis in pigs is unclear because of the lack of appropriate detection methods that discriminate S. parasuis from S. suis. In this study, we developed a PCR method that is specific for S. parasuis. The detection limit of the PCR was 350 CFU per reaction. Bacteria isolated from the saliva of eight pigs were collected and examined by PCR. Sixty-four isolates positive for PCR were obtained from the samples of all pigs. Thirteen of the 64 isolates were genetically confirmed as S. parasuis, and biologically and biochemically had nearly the same features of known S. parasuis strains, which suggested that strains positive for PCR were S. parasuis. Among the 64 isolates, 28 isolates were serotypes 20, 22, or 26 in the S. suis serotyping scheme. The remaining 36 isolates were untypeable, which suggested the presence of novel serotypes or a capsule-negative form. Therefore, the PCR method described in this study is a useful tool for identifying S. parasuis, and can be used in etiological studies on this bacterium.

[Isolation and identification of Streptococcus suis serotype 2 from sick-pig samples of Sichuan province].

Streptococcus suis serotype 2 (SS2) is a major pathogen frequently associated with infections in pigs. There are presently 35 serotypes of S.suis (serotype 1 to 34 and serotype 1/2) recognized on the basis of capsular antigens. Few people were reported to infect with SS2 in the past years. However, an accidental case happened in Sichuan province of China in 2005. Some people got ill and died, and all of them were closely contacted with sick pigs. Based on clinical features and epidemiologic data, this case could be caused by SS2 infection. Liver, spleen, kidney, lung and serum samples were collected and used for pathogen isolation and identification in laboratory, three strain bacteria were isolated. The three strains of SS2 showed typical morphology of SS2 on blood agar and under microscope with Gram stain. They were also agglutinated with standard serum of SS2. Biochemical characteristics of the three bacteria were tested using API 20 strep and analyzed by API software (version 3.3), results showed they were SS2. Four pairs of primer were designed, which were exactly matched the extracellular factor gene, muraminidase released protein gene, capsular polysaccharides gene and 16S rRNA gene respectively. These primers were used on polymerase chain reaction (PCR), and the PCR products were 626bp, 885bp, 487bp and 297bp on agarose gel, respectively. Drug sensitivity test were also done and results showed that they were sensitive to cefazolin, clindamycin, erythromycin, levofloxacin, nitrofurantoin, penicillin-G, and vancomycin and resistive to tetracycline. Balb/c mice infected with the isolated SS2 strain showed swelling in stomach and intestine, cyanochroia at mouth and suggillation under skin, which were similar to the clinical features of patients. Streptococcus suis serotype 2 were also found on lung sheeting sample under microscope with Gram stain. Rabbits infected with the isolated SS2 showed the similar clinical features with mice.

Biochemical analysis, cpn60 and 16S rDNA sequence data indicate that Streptococcus suis serotypes 32 and 34, isolated from pigs, are Streptococcus orisratti.

Streptococcus suis serotypes have traditionally been identified by morphology, biochemical profiling and serotyping. Analysis of the sequences of 16S rRNA and cpn60 genes of the 35 characterized serotypes of S. suis led to the observation that two serotypes 32 and 34, are significantly distinct from other S. suis serotypes and may represent a distinct species. Here we present DNA sequence data and biochemical profiles which indicate that S. suis serotypes 32 and 34, isolated from pigs, are clustered with Streptococcus orisratti, a Voges-Proskauer negative, alpha-haemolytic, aesculin-hydrolytic, Lancefield group A streptococcus isolated from the teeth of rats.