The influence of enzyme EII of the cellobiose-phosphotransferase system on the virulence of Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus).

Streptococcus agalactiae is the main pathogen of tilapia, causing huge economic losses to the tilapia aquaculture industry in China. To elucidate the role of the EII complexes of the cellobiose-phosphotransferase system (cel-PTS) in the regulation of S. agalactiae virulence, the △ cel-EII mutants of a high virulence strain (THN0901) and a low virulence strain (TFJ0901) were constructed using homologous recombination. The results showed that after the deletion of cel-EII complexes, the biological characteristics of THN0901 and TFJ0901 were consistent, including their cellobiose metabolism. Their biofilm formation abilities were significantly reduced (P < 0.01), and their ability to adhere to tilapia brain cells and compete when mixed in vitro and in vivo were enhanced. The virulence and histopathological lesions caused by △ cel-EII -TFJ0901 were significantly enhanced relative to those of TFJ0901, while △ cel-EII -THN0901 showed no significant differences relative to THN0901. The histopathological lesions caused by △ cel-EII -TFJ0901 tend to be serious, especially in the liver, spleen, and heart. The expression levels of manA and ascA/C of PTS, ciaR and dltR of two-component signal transduction systems (TCS), and pavA , bca and spb1 of virulence genes of △ cel-EII -TFJ0901 were significantly upregulated compared with TFJ0901 (P < 0.01), whereas those of △ cel-EII -THN0901 were significantly downregulated (P < 0.01), or somewhat downregulated, compared with THN0901. Taken together, cel-EII complexes may have a negative effect on the virulence of TFJ0901 by regulating the expression of virulence-related genes, but the negative regulation mechanism of the cel-EII complex has no obvious effect on the virulence of THN0901. • Cel-EII improves the biofilm formation ability in S. agalactiae. • Cel-EII negatively regulates the ability of adhesion and mixed competition ability. • Cel-EII involves in regulating some genes of PTS, TCS and virulence proteins. • Cel-EII-mediated virulence regulation is different in low- and high-virulence strains. [ABSTRACT FROM AUTHOR]