Cytotoxicity ofStreptococcus agalactiaesecretory protein on tilapia cultured cells

Streptococcus agalactiaesecrete virulence factors believed to be able of killing host tissues, especially under elevated water temperature. A direct effect ofS. agalactiaesecretory products on tilapia cells was tested on the tilapia kidney (TK-1) cell culture. The bacteria were cultured under four different temperature levels: 22, 29, 32 and 37 degrees C; the cell-free portion was processed through SDS-PAGE; and distinct bands were identified by LC-MS/MS. At least, three virulence factors were identified, Bsp, PcsB and CAMP factor, with increasing levels as the cultured temperature rose. Expressions ofbsp,pcsBandcfbwere also up-regulated with the rising of the temperature inS. agalactiaeculture. The supernatant from the bacteria cultured under specified temperatures was added into TK-1 cell-cultured wells. Morphological damage and mortality of the cultured cells, as determined by MTT method, were increased progressively from the supernatant treatment according to the rise of temperature inS. agalactiaeculture. This study suggests that the production of the three virulence factors ofS. agalactiaereported herein is temperature-dependent, and it is likely that CAMP factor directly kills the TK-1 cells since the other two types of protein are involved inS. agalactiaecell division and the bacterial adherence to host tissues.