FIG. 4.

(A) Vero cell cytotoxicity of STEC isolate P1330, which contained only stx_2dact, three STEC parent strains containing a combination of stx₁ and stx_2dact, and stx₁ knockout mutants of these strains before the treatment with elastase. (B) Average activation of cytotoxicity of the strains before and after treatment with elastase. Culture supernatants were incubated with porcine pancreatic elastase for 1 to 2 h at 37°C and transferred to Vero cells to determine the CD₅₀. The level of activation was calculated by dividing the CD₅₀ for each elastase-treated sample by the CD₅₀ for the buffer-treated control. The bars indicate means of three individual experiments.