BACKGROUND: Enterococci exhibit intrinsic resistance or high-level minimum inhibitory concentration (MIC) to β-lactams than other streptococci. This appears to be due to low affinity of penicillin-binding proteins and rarely production of β-lactamase, which gives the reason of testing β-lactamase for blood and cerebrospinal fluid isolates. Ampicillin is more effective than penicillin in vitro, and MIC of ampicillin is generally 1 dilution lower than that of penicillin. The purpose of this study is to detect β-lactamase producing enterococci and to compare MICs of ampicillin and penicillin by Vitek system (bioMerieux, Hazelwood, MO, USA) with those by agar dilution method.

METHODS: We collected 110 isolates of Enterococcus faecalis and 51 isolates of E. faecium from clinical specimens in 1998. MICs of antibiotics were determined by agar dilution method and Vitek system. We also performed β-lactamase test by the Cefinase (Becton Dickinson, USA) for 512 isolates of E. faecalis and 189 isolates of E. faecium collected in 1998.

RESULTS: The most common sites of isolates were blood, bile, surgical/traumatic wounds, closed and open pus and urine. MICs of ampicillin were 1 to 2 dilution lower than those of penicillin for E. faecalis (P=0.03). But there were no significant differences in MICs for E. faecium (P=0.19). Five isolates (4 E. faecalis and 1 E. faecium) were susceptible to ampicillin but resistant to penicillin. There were no β-lactamase producing enterococci among 701 isolates tested.

CONCLUSIONS: MIC by Vitek system tends to be 1 to 2 dilution lower than MIC by agar dilution method to β-lactams, and MIC of ampicillin is 1 to 2 dilution lower than MIC of penicillin, which could result in discrepancy in interpretation of susceptibility tests. A β-lactamase test for enterococci is not recommended for routine test in Korea. (Korean J Clin Microbiol 1999;2:194-198)