Antibiogram and PFGE types of fecal E. coli isolated from feedlot cattle using MacConkey agar amended with 4 μg/ml chlortetracycline (MT), as distributed by dietary treatment, sampling day and animal of origin. Sampling days (B to E) are depicted in Figure 1. Each box represents a single isolate from a particular steer on a given sampling day. The first eight colors represent the most commonly observed antibiogram patterns with grey indicating an infrequently observed antibiogram. Unfilled boxes indicate no isolate obtained on MT. Common letters indicate isolates with >90% genetic homology. Shaded boxes without a letter indicate isolates with <90% genetic homology with antibiogram data. Dietary treatments were as follows: Control: no antibiotics; Chlortetracycline (11 ppm; denoted T); Chlortetracycline + sulfamethazine (44 ppm; denoted TS); and Virginiamycin (31 ppm; V). nc: isolates not characterized.