Mutations at C-terminal domain of pbp5 gene among high level ampicillin resistant isolates of E. faecium in Iran

Abstract

ABSTRACT

Background and the purpose of the study: The low affinity penicillin-binding protein (PBP) 5 of Enterococcus faecium is responsible for intrinsic resistance to beta-lactam antibiotics. This study was conducted to determine the MICs of ampicillin against E. faecium strains cultured from Iranian patients (n=54) in Tehran hospitals and to sequence the C-terminal ends of pbp5s from selected strains (n=15) in order to determine possible mechanism of resistance to ampicllin.

Methods: Initially, the minimum inhibitory concentrations (MICs) of ampicllin against 54 isolates of E. faecium were determined using broth macro-dilution assay. A PCR was designed to target pbp5 gene. The PCR products corresponding to the C-terminal ends of PBP5s for each strains (n=15) were sequenced.

Results: Up to 44% of isolates were highly resistant to ampicillin (MIC ³ 64 µg/ml). Amino acid substitutions were found at position number of 485 (Met 485 to A(T) and also an additional serine residue inserted just after Ser 466 among the high level resistant isolates (MIC ³ 64 µg/ml). Other substitutions were also found at Q461K and V586L in these strains.

Conclusion: It appears that amino acid alternations near the SDN motif, mainly the amino acid at position 485, were responsible for high-level resistance to ampicillin. Other substitutions outside of this motif (n=7) had no observable effect on resistance.