FIG. 2.

Mean counts (n = 3; plus standard error) of total and ampicillin- and tetracycline-resistant E. coli isolated from fecal deposits by using MacConkey agar (MAC) or MAC amended with ampicillin (MAC+AMP, 32 μg ml⁻¹) or tetracycline (MAC+TET, 16 μg ml⁻¹), respectively. The treatments were as follows: control, no antimicrobial agents added to the diets of steers from which fecal deposits originated; A44, chlortetracycline (44 ppm); AS700, chlortetracycline and sulfamethazine (each at 44 ppm).