TABLE 3.

Summary of antimicrobial susceptibility testing results using three methods (agar dilution, disk diffusion, and Etest) for clinical isolates of Enterococcus faecalis, Enterococcus faecium, and Staphylococcus aureus

Pathogen (no.) and test method	MIC, μg/ml			CLSI zone diam, mm			CLSI MIC/zone diam interpretationa (%)			EUCAST zone diam, mm			EUCAST MIC interpretationb (%)
	MIC50	MIC90	Range	ZD50c	ZD90d	Range	Se	If	Rg	ZD50	ZD90	Rangeh	S	R
Enterococcus faecalisi (69)
Agar dilution	64	128	8–512				88.4	8.7	2.9
Disk diffusion				19	15	6–37	85.5	8.7	5.8	22	19	6–37
Etest	64	128	8–>512				78.3	15.9	5.8
Enterococcus faeciumj (21)
Agar dilution	64	128	32–128
Disk diffusion				12	10	6–17				17	13	10–25
Etest	128	256	32–>512
Staphylococcus aureus (156)
Agar dilution	4	8	≤1–32										100	0
Disk diffusion				27	23	15–34				27	24	18–34
Etest	8	16	≤1–>512										98.7	1.3
MSSAk (56)
Agar dilution	4	8	≤1–32										100	0
Disk diffusion				26	22	21–34				26	22	21–34
Etest	8	16	≤1–16										100	0
MRSAl (100)
Agar dilution	8	16	≤1–32										100	0
Disk diffusion				27	24	15–34				27	24	18–34
Etest	8	16	2–>512										98.0	2.0

^aMIC results were interpreted as susceptible (≤64 μg/ml), intermediate (128 μg/ml), and resistant (≥256 μg/ml) according breakpoints published in the CLSI M100 standard (3) for Escherichia coli and Enterococcus faecalis urinary tract isolates. Zone diameter results were interpreted as susceptible (≥16 μg/ml), intermediate (13 to 15 μg/ml), and resistant (≤12 μg/ml) according to breakpoints published in the CLSI M100 standard (3) for Escherichia coli and Enterococcus faecalis urinary tract isolates.

^bEUCAST MIC breakpoints (≤32 [susceptible], >32 [resistant] μg/ml [4]) apply to isolates of Staphylococcus species and intravenous use of fosfomycin.

^cZD₅₀, zone diameter at which 50% of isolates are inhibited.

^dZD₉₀, zone diameter at which 90% of isolates are inhibited.

^eS, susceptible.

^fI, intermediate.

^gR, resistant.

^hAdditional information regarding EUCAST zone diameter measurements. For Enterococcus faecalis, 88.4% (61/69) of all isolates tested that produced a zone of inhibition had ≥1 colony within the zone of inhibition, and 14.5% (10/69) of all isolates tested that produced a zone of inhibition had ≥10 colonies within the zone of inhibition. For Enterococcus faecium, 90.5% (19/21) of all isolates tested had ≥1 colony within the zone of inhibition, and 66.7% (14/21) of all isolates tested had ≥10 colonies within the zone of inhibition. For Staphylococcus aureus, 7.7% (12/156) of all isolates tested had ≥1 colony within the zone of inhibition, and 1.3% (2/156) of all isolates tested had ≥10 colonies within the zone of inhibition.

ⁱZero percent (0/69) of isolates of Enterococcus faecalis were vancomycin resistant.

^jAproportion of 33.3% (7/21) of isolates of Enterococcus faecium were vancomycin resistant.

^kMSSA, methicillin-susceptible Staphylococcus aureus.

^lMRSA, methicillin-resistant Staphylococcus aureus.