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

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 strainscultured from Iranian patients (n=54) in Tehran hospitals and to sequence the C-terminalends of pbp5s from selected strains (n=15) in order to determine possible mechanism ofresistance to ampicllin.Methods: Initially, the minimum inhibitory concentrations (MICs) of ampicllin against 54isolates of E. faecium were determined using broth macro-dilution assay. A PCR wasdesigned to target pbp5 gene. The PCR products corresponding to the C-terminal ends ofPBP5s for each strains (n=15) were sequenced.Results: Up to 44% of isolates were highly resistant to ampicillin (MIC ≥ 64 μg/ml). Aminoacid substitutions were found at position number of 485 (Met 485 to A(T) and also anadditional serine residue inserted just after Ser 466 among the high level resistant isolates(MIC ≥ 64 μg/ml). Other substitutions were also found at Q461K and V586L in thesestrains.Conclusion: It appears that amino acid alternations near the SDN motif, mainly the aminoacid at position 485, were responsible for high-level resistance to ampicillin. Othersubstitutions outside of this motif (n=7) had no observable effect on resistance.