The capsular polysaccharides are an important virulence factor of Streptococcus iniae, protecting the bacterium from destruction and clearance by the immune system. The cpsJ gene encodes a putative UDP-glucose epimerase involved in the capsule synthesis system. To determine the role of the CpsJ protein in the production of the capsule, a ΔcpsJ mutant was generated and analyzed by comparing its growth performances and virulence with those of the wild type (WT) strain. The ΔcpsJ mutant had longer chains, smaller colonies, and a slower growth rate and decreased optical density than the WT, suggesting that the ΔcpsJ mutant produces less capsular polysaccharide. The ΔcpsJ mutant was more able to adhere to and invaded epithelioma papulosum cyprinid cells (EPCs) when its virulence in vitro was compared with that of the WT, but survived less well in the whole blood of channel catfish. When a channel catfish infection model was used to determine the virulence of the ΔcpsJ mutant in vivo, the mutant caused an increase in survival with the mutant (53.33 %) versus the WT (26.67 %). In summary, mutation of the cpsJ gene influenced both the capsule synthesis and virulence of S. iniae.
Keywords: Capsular polysaccharides; Channel catfish; Streptococcus iniae; △cpsJ mutant.