The penicillin resistance of Enterococcus faecalis JH2-2r results from an overproduction of the low-affinity penicillin-binding protein PBP4 and doesnot involve a psr-like gene

A penicillin-resistant mutant, JH2-2r (MIC 75 mug ml(-1)), was isolated fro m Enterococcus faecalis JH2-2 (MIC 5 mug ml(-1)) by successive passages on plates containing increasing concentrations of benzylpenicillin. A comparis on of the penicillin-binding protein (PBP) profiles in the two strains reve aled a more intensely labelled PBP4 in JH2-2r. Because the sequences of the JH2-2 and JH2-2r pbp4 genes were strictly identical, even in their promote r regions, this intensive labelling could only be associated with an overpr oduction of the low-affinity PBP4. No psr gene analogous to that proposed t o act as a regulator of PBP5 synthesis in Enterococcus hirae and Enterococc us faecium could be identified in the vicinity of pbp4 in E. faecalis JH2-2 and JH2-2r. However, a psr-like gene distant from pbp4 was identified. The cloning and sequencing of that psr-like gene from both E. faecalis strains indicated that they were identical. It is therefore postulated that the PB P4 overproduction in E. faecalis JH2-2r results from the modification of an as yet unidentified factor.