Abstract: In order to determine the cause of death of piglets in a large-scale pig farm in Guangdong province,the disease materials such as liver,spleen,heart,brain,joint fluid and lymph nodes were collected for bacterial isolation,and the isolated strains were purified and cultured.And colony morphology observation,Gram staining,morphology observation,biochemical test,PCR amplification with serotype specific primers,virulence gene test,drug sensitivity test,growth curve test and animal test were carried out.The results showed that there were round,gray white,smooth surface and neat edge colonies on the agar plate of adult bovine serum.Microscopic examination showed Gram-positive cocci which were arranged in chains.The biochemical test results of this strain were in line with the characteristics of Streptococcus suis,and the PCR amplification results showed that both the conserved gene fragment (gdh and gapdh) of Streptococcus suis and the specific cps2J gene fragment of Streptococcus suis serotype 2 were positive,indicating that the isolated strain was Streptococcus suis serotype 2.The results of virulence gene detection showed that the virulence factor genotype of the strain was gdh⁺/gapdh⁺/epf⁺/mrp⁺/sly⁺/orf2⁺/fbps⁺.The isolated strain reached the logarithmic growth stage after cultured in nutrient broth for 6 h,and the D_{600 nm} value of the bacterial solution cultured to 10 h was 0.310.The results of drug sensitivity test showed that the strain was sensitive to penicillin,amoxicillin,amoxicillin-clavulanate potassium,but resistant to doxycycline.The animal test results showed that BALB/c mice infected with the isolated strain did not die and had no obvious clinical symptoms,and its virulence was low.This study provided important reference information for the prevention and control of Streptococcus in this pig farm.

Key words: Streptococcus suis serotype 2; separation and identification; virulence genes; drug sensitive test