A PCR assay was developed for the detection of Streptococcus suis serotypes 2 and 1/2. This multiplex PCR is based on the amplification of the gene coding for 16S rRNA of S. suis and on the amplification of the cps2J gene coding for the capsule of S. suis serotypes 2 and 1/2. An internal control was constructed and added in this test to monitor the efficiency of amplification in each reaction. To evaluate the specificity of the test, 31 strains of other bacterial species related to S. suis or isolated from pigs and 42 strains of S. suis serotypes 1 and 3 to 34 were analyzed. The detection threshold of the test was 28 S. suis CFU/ml. The specificity and the sensitivity of the multiplex PCR test and the presence of an internal control allowed the analysis of biological samples without a culture step. The PCR assay was then applied to the detection of 14 S. suis serotype 1/2 strains, 88 S. suis serotype 2 strains isolated from pigs, and 25 S. suis serotype 2 strains isolated from humans. This test was also applied to analyze tonsil samples of pigs experimentally infected and carrier pigs without any symptoms.

We generated by allelic replacement a Δ dltA mutant of a virulent Streptococcus suis serotype 2 field strain and evaluated the contribution of lipoteichoic acid (LTA) d -alanylation to the virulence traits of this swine pathogen and zoonotic agent. The absence of LTA d -alanylation resulted in increased susceptibility to the action of cationic antimicrobial peptides. In addition, and in contrast to the wild-type strain, the Δ dltA mutant was efficiently killed by porcine neutrophils and showed diminished adherence to and invasion of porcine brain microvascular endothelial cells. Finally, the Δ dltA mutant was attenuated in both the CD1 mouse and porcine models of infection, probably reflecting a decreased ability to escape immune clearance mechanisms and an impaired capacity to move across host barriers. The results of this study suggest that LTA d -alanylation is an important factor in S. suis virulence.