Expressions of capsular polysaccharide synthetic gene of Streptococcus agalactiae isolated from Nile tilapia(Oreochromis niloticus) and their effects on capsular sialic acid content and bacterial pathogenicity

In order to understand the expressions of capsular polysaccharide synthetic gene of Streptococcus agalactiae (GBS) isolated from Oreochromis niloticus (Nile tilapia) and their effects on capsular sialic acid content and bacterial pathogenicity, capsular polysaccharide synthetic gene cpsE, cpsK and neuA of GBS from O. niloticus were cloned in the study. The expression levels of these genes at different temperature were detected by qPCR. Capsular sialic acid content was detected by colorimetric method. The mortality rate of O. niloticusinfected with GBS cultured at different temperature was analyzed by artificial challenge test. The results showed that amino acid sequence analysis encoded by cpsE, cpsK and neuA of GBS had conservative enzyme active sites which were essential for the synthesis of capsular polysaccharide. CpsE and NeuA of GBS isolated from O. niloticus shared high homology with those of GBS isolated from human and other fish species (>97%). The identities ofcpsK of GBS isolated from O. niloticus with those of other fish species (serotype Ia and Ib) and human (serotype Ia, Ib and II~IX) were 56%–100% and 27%–100%, respectively. The expression levels ofcpsK and neuA of GBS cultivated at different temperature were consistent with that of capsular sialic acid content. Capsular sialic acid content and the mortality rate of tilapia infected with GBS cultivated at 28 and 34 °C were increased with the rise of temperature. The mortality rate of tilapia after artificial challenge at 22 °C was the lowest though the highest capsular sialic acid content was observed. The research suggested that the expression levels of CpsK and NeuA played an important role in the process of sialylation of capsular polysaccharide. Higher capsular sialic acid content of GBS cultivated at low temperature may provide protection for it to stay in the host. However, the expressions of some important virulence factors besides capsular sialic acid might be necessary for a strong bacteria pathogenicity for GBS cultivated at higher temperature.