Mechanism of resistance to vancomycin in Enterococcus faecium D366 and Enterococcus faecalis A256

concentration [MIC] 32 mg/L) to vancomycin. When exponential-phase cultures were exposed to half the MIC of vancomycin, a lag of 3-4 h occurred before growth resumed. Cells preexposed to \/2 MICs of vancomycin did not show any lag. Pregrowth of D366 with vancomycin caused resistance to all glycopeptides tested. Pregrowth in vancomycin resulted in synthesis of a 3.95-kDa cytoplasmic-membrane-associated protein. This pro-tein was correlated with resistance in mutants with high-level resistance, in the presence of NaCI, which inhibited the activity of vancomycin, and when several glycopeptides with varying activities were tested. Vancomycin-grown cells appeared abnormal and lysed at a much slower rate than did normal cells. We conclude that (1) vancomycin resistance in D366 is inducible; (2) resistance is correlated with the synthesis of a 39.5-kDa cytoplas-mic membrane protein; and (3) this protein may play an additional role in the inhibition of normal lytic functions. Enterococcus faecium is an occasional human pathogen that tends to be more resistant to ~-lactam aminoglycosidesynergythan isEnterococcusjaecalis [1]. For enterococcal pathogens with high levels of ~-lactam resistance, or when ~-lactams are poorly tolerated by patients, vancomycin has been used for treatment with favorable results [2]. Recently, reports of glycopeptide-resistant en-terococci have appeared [3-6]. In several cases, re-sistance has been encoded on conjugative plasmids in E. faecium [5]. We have previously reported in-ducible resistance to glycopeptides in E. jaecalis as-sociated with the synthesis of a 39-kDa membrane protein [6]. We have also shown that this resistance is transferable to a susceptible recipient, JH2-2, and remains inducible. Subsequently we have demon-Received for publication 16 August 1988and in revised for