Cytotoxic effect of sonic lysate of stx gene-positive CTEC strains on Vero (A) and CHO cells (B). Vero and CHO cells were incubated with sonic lysate of stx gene-positive CTEC strains for 72 h. The cells were then fixed and observed under microscope (magnification, 200x). STEC strain Sakai (a) and CTEC-I strain GB1371 (c) were used as positive controls for Stx and CDT, respectively. E. coli strain C600 (b) was used as negative control. The representative cytotoxicity patterns by CTEC strains positive for stx, cdt-V (d), and for stx, cdt-V, subAB (e) analyzed in this study are shown.