Table 5.

MIC of Cu towards tcrB-negative or -positive Enterococcus spp. isolates from feces of piglets fed diets with or without added Cu and/or CTC¹

Treatment groups2	tcrB-negative enterococci		tcrB-positive enterococci
	No. of isolates	MIC, µg/mL	No. of isolates	MIC, µg/mL
Control	8	3.1	8	20.5
Cu	7	3.7	7	21.4
CTC	9	3.4	9	20.7
Cu + CTC	15	3.5	15	20.1

¹A total of 320 barrows (DNA 200 × 400, DNA Genetics, Columbus, NE) weaned at approximately 21 d of age with 7.4 kg BW were used in a 28-d nursery study. Piglets were fed a common diet during an acclimation period from days −7 to −1 and treatment diets from days 0 to 28. Treatments consisted of control, with no addition of Cu or CTC in the diet; Cu, with 200 mg/kg added Cu from copper sulfate in the diet; CTC, with 440 mg/kg CTC in the diet; and Cu + CTC, with 200 mg/kg added Cu from copper sulfate and 440 mg/kg added CTC in the diet.

²Dietary treatments were arranged in a 2 × 2 factorial design with main effects of added Cu (0 vs. 200 mg/kg from copper sulfate) and chlortetracycline (0 vs. 440 mg/kg CTC). Dietary treatments had no effect on copper susceptibilities among enterococcal isolates (P > 0.05).