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 grow th,adhesion,invasion,expression of virulence genes and mortality rates of tilapia w ere studied. The results show ed that the grow th 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 w as no significant difference betw een 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 w ith S. agalactiae show ed a temperature-dependent grow th pattern. The number of colony forming units increased along w ith the increase of culture temperature and reached the peak at 37 ℃,and then decreased at 40 ℃. The number of colony forming units w as positively associated w ith its mortality rate. The expression of virulence genes w as also influenced by culture temperature. The expression level of hly and cfb genes show ed 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 w hile the culture temperature continued to rise. The expression of sip gene w as minimally influenced by culture temperature w hile the expression of scpB gene decreased along w ith the increase of the culture temperature. The mortality rates of tilapia infected w ith S. agalactiae cultured at different temperatures show ed temperature-dependence. The mortality rate increased along w ith 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 grow th,adhesion,invasion and expression of some virulence genes of S. agalactiae,w hich may contribute to its virulence.