Influence of antibiotics on Stx production in coculture with antibiotic-sensitive, phage-susceptible E. coli. O157:H7 strain 185 was grown overnight in the presence of Stx-phage susceptible E. coli isolate (CMUC-170) susceptible to CIP (A and B) or AZM (C and D). Cultures were plated to determine the CFU (A and C), and Stx levels were determined by using the luciferase assay (B and D). Each symbol represents an independent trial, and the horizontal dash denotes the geometric mean. Controls (O157 alone and no-antibiotic coincubations) were repeated four to eight times, and all other coincubation experiments were repeated three to five times. Statistical significance (P < 0.05 Student unpaired t test with Welch's correction for unequal variance) compared to the pure culture of E. coli O157:H7 is denoted with an asterisk, and statistical significance compared to the no-antibiotic coculture control is denoted by a pound sign (#). The MIC for pure culture of E. coli O157:H7 is noted by a gray box. The horizontal line at 107 denotes the number of O157:H7 at the visual limit of detection. Samples with CFU/ml below the limit of detection (50 CFU/ml, which is represented by the dashed line) were incorporated into the statistical analysis using the square root of the limit of detection).