TABLE 2.

Correlation of penicillin and ampicillin NCCLS susceptibility breakpoints for enterococci with imipenem FDA susceptibility breakpoints^a

Strainb	Breakpoint for penicillin or ampicillin (μg/ml)	No. of strains correlating with imipenem breakpoint (μg/ml)
		≤4	8	≥16
E. faecalis	Penicillin (≤8)	200	0	0
	Penicillin (≥16)	1	0	0
	Ampicillin (≤8)	201	0	0
	Ampicillin (≥16)	0	0	0
E. faecium	Penicillin (≤8)	1	1	0
	Penicillin (≥16)	0	0	22
	Ampicillin (≤8)	1	1	0
	Ampicillin (≥16)	0	0	22

^aNCCLS breakpoints (in micrograms per milliliter) for penicillin and ampicillin: ≤8, susceptible; ≥16, resistant. FDA breakpoints (in micrograms per milliliter) for imipenem: ≤4, susceptible; 8, intermediate; ≥16, resistant. 

^bFor E. faecalis, n = 201, including 24 vancomycin-resistant strains. For E. faecium, n = 24, including 19 vancomycin-resistant strains.