Cytotoxic activity of supernatants of STEC strains O157:H7 and O104:H4 treated with various antibiotics. The cell free supernatants of STEC cultures described in Figure 2 were 10-fold serially diluted and added to semi-confluent monolayers of Vero cells in microtiter plates. After incubation for 24 h, XTT-labeling reagent was added and cultures were incubated for another 24 h before measuring the viability of the Vero cells as OD450 of the samples. The cytotoxic activity of the supernatants was calculated as described in Methods. For each antibiotic, the cytotoxicity of the supernatants is plotted against the dilution of the supernatants in the upper part of the panel. In these plots, the effect of the antibiotics on the cytotoxicity of the supernatants was determined as the increment of cytotoxicity in comparison to untreated controls, as indicated exemplarily for the 1x and 4x MIC of ciprofloxacin by green dashed lines and red dashed lines, respectively. In the lower part of each panel the increments of the cytotoxicity are plotted for the various MIC of the respective antibiotic. Shown are the means and standard errors of three independent experiments. Statistical significance is indicated by asterisks: * for p < 0.05; ** for p < 0.01.