Table 6.

Comparison of antibiotic susceptibilities between transferable copper resistance gene (tcrB)-positive and tcrB-negative Enterococcus spp. isolates from feces of piglets fed diets with or without added Cu and/or CTC¹

Antimicrobial compounds	Mean minimum inhibitory concentration (MIC), μg/mL2		Susceptible MIC, μg/mL	Intermediate MIC, μg/mL	Resistant MIC, μg/mL
	tcrB-negative	tcrB-positive
Chloramphenicol	32	16	≤8	16	≥32
Ciprofloxacin	1.1	7.7	≤1	2	≥4
Daptomycin	5.2	3.3	≤4	N/A	N/A3
Erythromycin	42	33.8	≤0.5	1–4	≥8
Gentamicin	16.8	31.6	≤500	N/A	>500
Kanamycin	102.4	115.2	≤512	N/A	≥1,024
Lincomycin	128	37.6	≤2	4	≥8
Linezolid	16	52.8	≤2	4	≥8
Nitrofurantoin	25.6	11.2	≤32	64	≥128
Penicillin	1.3	2.9	≤8	N/A	≥16
Streptomycin	102.4	78.4	≤1,000	N/A	>1,000
Synercid	5.4	5.6	≤1	2	≥4
Tetracycline	89.6	76.8	≤4	8	≥16
Tigecycline	1.95	1.2	≤0.25	N/A	N/A4
Tylosin tartrate	102.8	45.4	≤8	16	≥32
Vancomycin	2.9	2.7	≤4	8–16	≥32

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. Dietary treatments had no effect on antimicrobial susceptibilities among enterococcal isolates (P > 0.05).

²For the purpose of averaging the transferable copper resistance gene (tcrB)-negative and tcrB-positive MIC, values that were ≥128 μg/mL were considered as 128 μg/mL.

³According to the Clinical and Laboratory Standards Institute, only a susceptible breakpoint has been established for daptomycin (≤ 4 μg/mL).

⁴According to the Clinical and Laboratory Standards Institute, only a susceptible breakpoint has been established for tigecycline (≤ 0.25 μg/mL).