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Mechanisms of resistance to imipenem and ampicillin in Enterococcus faecalis

Seiji Ono1, Tetsuro Muratani, Tetsuro Matsumoto

  • 1Department of Urology, School of Medicine, University of Occupational and Environmental Health (UOEH), 1-1 Iseigaoka, Yahatanisi-Ku, Kitakyusyu 807-8555, Japan. onochan@bronze.ocn.ne.jp

Antimicrobial Agents and Chemotherapy
|June 28, 2005
PubMed

Insights

Ampicillin and imipenem resistance in Enterococcus faecalis is linked to mutations in PBP4. These genetic changes alter penicillin-binding protein affinities, leading to reduced susceptibility to beta-lactam antibiotics.

Area of Science:

  • Microbiology
  • Molecular Biology
  • Antimicrobial Resistance

Background:

  • Enterococcus faecalis is a significant cause of hospital-acquired infections.
  • Beta-lactam antibiotics, including ampicillin and imipenem, are crucial for treating enterococcal infections.
  • Mechanisms of beta-lactam resistance in E. faecalis typically involve beta-lactamase production or altered penicillin-binding proteins (PBPs).

Purpose of the Study:

  • To investigate the mechanisms of ampicillin and imipenem resistance in vanA-possessing Enterococcus faecalis isolates.
  • To identify genetic alterations in penicillin-binding proteins (PBPs) associated with beta-lactam resistance.

Main Methods:

  • Phenotypic characterization of ampicillin and imipenem resistance using minimum inhibitory concentrations (MICs).
  • Assessment of beta-lactamase production and PBP expression levels in resistant isolates.
  • Affinity studies of PBPs for beta-lactams.
  • Sequencing of pbp4 gene fragments from resistant and susceptible strains.

Main Results:

  • Resistant isolates exhibited high MICs for ampicillin (8-16 µg/ml) and imipenem (4-32 µg/ml).
  • No beta-lactamase production or PBP overproduction was detected in resistant strains.
  • Reduced affinity of PBP4 for beta-lactams was observed in resistant strains.
  • Two specific amino acid substitutions in PBP4 (Pro520Ser and Tyr605His) correlated with high-level resistance, while a single Tyr605His substitution correlated with low-level resistance.

Conclusions:

  • Point mutations in the pbp4 gene are responsible for ampicillin and imipenem resistance in these Enterococcus faecalis isolates.
  • These mutations occur in the penicillin-binding domain of PBP4, affecting its affinity for beta-lactams.
  • This study highlights a novel mechanism of beta-lactam resistance in E. faecalis mediated by PBP4 mutations, distinct from previously reported mechanisms.

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