Transcriptome analysis of tilapia streptococcus agalactiae in response to baicalin

Streptococcus agalactiae (S. agalactiae) is a highly pathogenic bacterial pathogen in aquatic animals. Our previous study has demonstrated the significant inhibitory effect of baicalin on β-hemolytic/cytolytic activity, which is a key virulence factor of S. agalactiae. In this study, we aimed to elucidate the mechanism underlying baicalin's inhibition of S. agalactiae β-hemolytic/cytolytic activity by transcriptomic analysis. Bacteria were exposed to 39.06 µg/mL baicalin for 6 h, and their β-hemolytic/cytolytic activities were assessed using blood plates. Then, the differentially expressed genes (DEGs) were identified and characterized by RNA sequencing (RNA-Seq), and further confirmed using the qRT-PCR. A total of 10 DEGs with 7 significantly up-regulated and 3 significantly down-regulated, were found to be affected significantly under baicalin treatment. These DEGs were associated with 5 biological processes, 5 cellular components, and 3 molecular functions. They were primarily enriched in 3 pathways: lacD and lacC in galactose metabolism, lrgA and lrgB in the two-component system, and ribH/rib4 in riboflavin metabolism. These suggested that baicalin might inhibit the conversion of pyruvate to acetyl-CoA and malonyl-CoA, which are crucial precursors for β-hemolysin/cytolysin synthesis, and result in the accumulation of pyruvate, suppress the expressions of pyruvate cell membrane channel protein genes lrgA and lrgB. Baicalin could compensatory up-regulate the expressions of tryptophan/tyrosine ABC transporter family genes, ABC.X4.A, ABC.X4.P, and ABC.X4.S by inhibiting the expression of cyl A/B in cyl operons. Moreover, it hinders the conversion of D-glucose 1-phosphate to the dTDP-L-rhamnose pathway and leads to a deficiency of L-rhamnose, an important precursor for β-hemolysin/cytolysin synthesis.