Baicalin attenuates Streptococcus agalactiae virulence and protects tilapia (Oreochromis niloticus) from group B streptococcal infection

Abstract

Streptococcus agalactiae (S. agalactiae) is a major piscine pathogen that causes significant morbidity and mortality among numerous fish species, especially tilapia; these deaths have resulted in huge economic losses for aquaculture. Beta-hemolysin/cytolysin and hyaluronidase are critical virulence factors of S. agalactiae, thereby suggesting that they are important targets for the development of potential treatment strategies. Hence the effects of 40 phytochemicals on β-hemolytic/cytolytic activity and hyaluronidase production were evaluated in this paper. The results showed that only bacalin simultaneously possesses significant inhibitory effects on the β-hemolytic/cytolytic activity of five S. agalactiae strains and the hyaluronidase production of S. agalactiae PBSA0901. The β-hemolytic/cytolytic activity-inhibition ratios of 4 μg/mL baicalin ranged from 22.11% to 84.54%, and the half maximum inhibitory concentrations (IC50) of baicalin to the β-hemolytic/cytolytic activity of S. agalactiae PBSA0901 was 5.39 ± 0.55 μg/mL. Additionally, the inhibitory effects of baicalin on the β-hemolytic/cytolytic activity cannot be removed by passage. Baicalin at the concentration of 4 μg/mL completely inhibited S. agalactiae PBSA0901 from producing hyaluronidase. Furthermore, baicalin significantly increased the sensitivity of S. agalactiae PBSA0901 to H2O2. However, it had a nonsignificant effect on the sensitivity of S. agalactiae PBSA0901 to lysozyme. Moreover, studies in vivo demonstrated that baicalin had a significant inhibitory effect on the lethality of S. agalactiae PBSA0901. Furthermore, baicalin exerted a significant therapeutic effect on tilapia challenged by S. agalactiae PBSA0901, and the median effect concentration (EC50) was 525.8 mg/kg. These results indicated that baicalin would be an interesting therapeutic compound against S. agalactiae infection in tilapia.