TABLE 3.

Degrees of similarity of PFGE group patterns of resistant E. coli isolated from fecal deposits of steers as affected by duration of environmental exposure

Resistance and exposure interval groupb	Value for antimicrobial treatment groupa
	A44				AS700				Control
	No. of isolates	Similarity to “short” group	Within- group similarity	P valuec	No. of isolates	Similarity to “short” group	Within- group similarity	P value	No. of isolates	Similarity to “short” group	Within- group similarity	P value
Ampicillin
Short	20		87.9	<0.001	20		82.5	<0.001	19		81.2	<0.001
Moderate	18	60.5	58.4	0.999	21	70.6	59.1	0.759	22	89.9	89.1	<0.001
Long	18	82.1	74.3	0.050	15	70.2	66.1	0.994	8	52.6	56.5	<0.001
Tetracycline
Short	29		61.1	0.979	31		59.5	<0.001	35		66.1	0.101
Moderate	18	54.6	57.2	0.998	17	56.3	60.8	0.19	17	67.8	73.7	<0.001
Long	32	61.0	70.4	<0.001	35	53.7	59.1	0.002	23	63.6	61.1	0.868

^aControl, 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).

^bEach interval represents the duration of environmental exposure of fecal deposits prior to sampling: short, days 0 and 7; moderate, days 56 and 70; long, days 126 and 175. Ampicillin- or tetracycline-resistant E. coli was isolated from MacConkey agar amended with ampicillin (32 μg ml⁻¹) or tetracycline (16 μg ml⁻¹), respectively. The groups of resistant E. coli were analyzed separately.

^cThe effect of the exposure interval on the significance of within-group similarity (P value) was analyzed within each antimicrobial treatment group. PFGE profiles were grouped by short, moderate, and long exposure intervals and analyzed by bootstrapping (n = 1,000).