Objective    To construct a mutant of Streptococcus suis serotype 2 (S. suis 2) virulent strain 05ZYH33 by knocking out its cps2B gene which encoding capsular polysaccharide (CPS) synthase.     Methods    The cps2B gene in this 05ZYH33 strain was knocked out based on the principle of homologous recombination. PCR analysis and Southern hybridization were used to identify the knockout. Transmission electron microscopy was employed to observe the morphology of the established strain. The virulence of the deficient strain was tested on the murine model of infection.     Results    PCR analysis and Southern hybridization showed that cps2B was replaced completely by the Spc

r

cassette (encoding the gene resistant to spectinomycin). No capsu

lar material was observed on the surfaces of the mutant strain by transmission electron microscopy

suggesting that S. suis 2 mutant loss of ability in capsule synthesis. Furthermore

the nonencapsulated mutant was completely avirulent in murine model of infection.     Conclusion    The cps2B gene deletion mutant of 05ZYH33 was successfully constructed. The CPS synthase of S. suis 2 plays an essential role in the pathogenesis and invasiveness of S. suis 2 infection.