Table 3.

MIC of enterococcal isolates in complete feed and feed ingredient samples, and resistant rates in animal feed and livestock.

Antimicrobial agent	Breakpoint (μg/mL)	Species	No. of isolates	Range (μg/mL)	MIC50 (μg/mL)	MIC90 (μg/mL)	No. of resistant isolates	Resistant rate (%)c
								In animal feed	In livestock
Ampicillin	16a	All Enterococcus spp.	134	≤0.12~2	1	2	0	0.0	0.1
		E. faecalis	31	0.5~2	1	1	0	0.0	0.0
		E. faecium	75	≤0.12~2	2	2	0	0.0	0.0
Vancomycin	32a	All Enterococcus spp.	134	≤0.12~8	0.5	4	0	0.0	0.0
		E. faecalis	31	0.5~4	1	2	0	0.0	0.0
		E. faecium	75	0.25~4	0.5	4	0	0.0	0.0
Tetracycline	16a	All Enterococcus spp.	134	≤0.12~>64	0.25	0.5	8	6.0	46.9*
		E. faecalis	31	0.25~>64	0.5	64	4	12.9	52.0*
		E. faecium	75	≤0.12~>64	0.25	0.5	3	4.0	27.9*
Erythromycin	8a	All Enterococcus spp.	134	≤0.12~>128	2	8	33	24.6	21.2
		E. faecalis	31	≤0.12~4	2	2	0	0.0	36.1*
		E. faecium	75	≤0.12~>128	4	8	32	42.7	11.4*
Tylosin	64b	All Enterococcus spp.	134	0.5~>128	4	8	2	1.5	20.5*
		E. faecalis	31	2~8	2	2	0	0.0	36.4*
		E. faecium	75	1~>128	4	8	2	2.7	8.1
Lincomycin	128b	All Enterococcus spp.	134	≤0.25~>512	16	32	3	2.2	26.8*
		E. faecalis	31	0.5~256	32	32	1	3.2	37.5*
		E. faecium	75	0.5~>512	16	32	2	2.7	8.1
Gentamicin	32b	All Enterococcus spp.	134	0.25~>256	8	16	1	0.8	11.3*
		E. faecalis	31	4~>256	16	16	1	3.2	22.8*
		E. faecium	75	2~16	4	8	0	0.0	8.3*
Kanamycin	128b	All Enterococcus spp.	134	2~>512	64	128	35	26.1	31.6
		E. faecalis	31	32~>512	64	64	2	6.5	43.2*
		E. faecium	75	32~512	64	128	33	44.0	46.0
Chloramphenicol	32a	All Enterococcus spp.	134	2~16	4	8	0	0.0	8.3*
		E. faecalis	31	4~8	8	8	0	0.0	21.1*
		E. faecium	75	2~16	4	8	0	0.0	6.4
Ciprofloxacin	4a	All Enterococcus spp.	134	≤0.12~4	0.5	1	1	0.8	7.8*
		E. faecalis	31	0.25~2	1	1	0	0.0	2.8
		E. faecium	75	0.25~4	0.5	1	1	1.3	10.5

^a Established by the Clinical and Laboratory Standards Institute (14).

^b Determined microbiologically (midpoint of a bimodal MIC distribution) by the Japanese Veterinary Antimicrobial Resistance Monitoring System (6).

^c All Enterococcus spp. (n = 861), E. faecalis (n = 228), and E. faecium (n = 20) in livestock. P-values were determined using Fisher’s exact test. *p < 0.05.

MIC₅₀, minimum inhibitory concentration at which 50% of isolates are inhibited; MIC₉₀, MIC at which 90% of isolates are inhibited.