Effect of temperature on the virulence of Streptococcus agalactiae from Nile tilapia(Oreochromis niloticus)

In order to understand the influence and mechanism of temperature on the virulence of Streptococcus agalactiae,the effects of culture temperature on the virulence related parameters of S.agalactiae including bacteria growth,adhesion,invasion,expression of virulence genes and mortality rates of tilapia were studied.The results showed that the growth speeds of S.agalactiae at different temperatures(25-40 ℃)were different;37 ℃ was the optimum growth temperature and the slowest growth speed appeared at 25 ℃.The adhesive capacity of S.agalactiae reached the peak at 37 ℃ and decreased at 40 ℃.There was no significant difference between the absorbance(OD_{590 nm})of adhesive S.agalactiae incubated in the range of 25-34 ℃.The number of colony forming units in the brain of tilapia infected artificially with S.agalactiae showed a temperature-dependent growth pattern.The number of colony forming units increased along with the increase of culture temperature and reached the peak at 37 ℃,and then decreased at 40 ℃.The number of colony forming units was positively associated with its mortality rate.The expression of virulence genes was also influenced by culture temperature.The expression level of hly and cfb genes showed a temperature-dependent expression pattern similar to the number of colony forming units.The expression of hly and cfb genes significantly increased and reached the peak at 34 ℃and 37 ℃,respectively,and then dropped rapidly while the culture temperature continued to rise.The expression of sip gene was minimally influenced by culture temperature while the expression of scpB gene decreased along with the increase of the culture temperature.The mortality rates of tilapia infected with S.agalactiae cultured at different temperatures showed temperature-dependence.The mortality rate increased along with the increase of culture temperature and peaked 66.67%±6.67% at 37 ℃,and then slightly decreased at 40 ℃.In conclusion,the culture temperature influenced the growth,adhesion,invasion and expression of some virulence genes of S.agalactiae,which may contribute to its virulence.