Modification of penicillin-binding protein 5 associated with high-level ampicillin resistance in Enterococcus faecium

High-level ampicillin resistance in Enterococcus faecium has been shown to be associated with the synthesis of a modified penicillin-binding protein 5 (PBP 5) which had apparently lost its penicillin-binding capability (R. Fontana, M. Aldegheri, M. Ligozzi, H. Lopez, A. Sucari, and G. Satta, Antimicrob, Agents Chemother. 38:1980-1983, 1994), The pbp5 gene of the highly resistant strain E. faecium 9439 was cloned and sequenced, The deduced amino acid sequence showed 77 and 54% homologies with the PBPs 5 of Enterococcus hirae and Enterococcus faecalis, respectively, A gene fragment coding for the C-terminal part of PBP 5 containing the penicillin-binding domain was also cloned from several E. faecium strains with different levels of ampicillin resistance, Sequence comparison revealed a few point mutations, some of which resulted in amino acid substitutions between SDN and KTG motifs in PBPs 5 of highly resistant strains, One of these converted a polar residue (the T residue at position 562 or 574) of PBP 5 produced by susceptible and moderately resistant strains into a nonpolar one (A or I), This alteration could be responsible for the altered phenotype of PBP 5 in highly resistant strains.