Distribution and characterization of ampicillin- and tetracycline-resistant Escherichia coli from feedlot cattle fed subtherapeutic antimicrobials

Table 1 Distribution of isolates characterized in this study

Treatment^a	Medium used for selection^b	Number of animals	Sampling day^c				Total
			B	C	D	E
CON	M^C	5	5	5	5	5	20
	M^T	50	15	19	47	30	111
	M^A	50	0	8	1	17	26
T	M^C	3	3	3	2	3	11
	M^T	30	12	10	27	25	74
	M^A	30	2	0	1	10	13
TS	M^C	3	3	3	3	3	12
	M^T	30	23	26	29	29	107
	M^A	30	15	14	7	15	51
V	M^C	3	3	3	3	3	12
	M^T	30	11	6	25	27	69
	M^A	30	2	2	5	16	25
Total			94	99	155	183	531

^a Steers were fed no antibiotics (control, CON), or chlortetracycline and sulfamethazine (44 ppm; TS); chlortetracycline (11 ppm; T) or virginiamycin (31 ppm; V) administered in two discrete periods (see Figure 1).
^b Isolates were collected by plating fecal slurries onto (i) MacConkey agar (MAC) containing no antibiotics (control, M^C), or amended with tetracycline hydrochloride (4 μg/mL; M^T) or with ampicillin (50 μg/mL; M^A).
^c Sampling days occurred during each of the four phases of the feeding trial (see Figure 1).