Quantification of STX in supernatants of STEC strains O157:H7 and O104:H4 treated with various antibiotics. The STEC cultures described in Figure 1 were harvested after 24 h of cultivation and cell-free supernatants were prepared by centrifugation and filtration. The contents of STX were determined with a commercial EIA specific for both STX1 and 2 in two-fold serial dilutions of the supernatants. For each antibiotic, in the upper part of the panel the OD of the STX-specific signal is plotted against the dilution of the supernatants. In the lower part of each panel, the STX-titers are shown which were determined in the plots of the OD as indicated exemplarily for the 1x (green dashed lines) and 4x (red dashed lines) MIC of ciprofloxacin. Briefly, from the OD-value of the undiluted sample of the untreated culture a horizontal dashed line was drawn until it intersected the plot of a given MIC. From this intersection a vertical line was drawn to determine the dilution at which the OD-value of the respective supe rnatant equaled the OD-value of the untreated control. The inverse of this dilution was defined as the STX-titer of the sample. 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.