Penicillin-Binding Protein 5 Sequence Alteration and Levels of plp5 mRNA Expression in Clinical Isolates of Enterococcus faecium with Different Levels of Ampicillin Resistance

Address correspondence to: Mondher Belhaj, PhD, Faculté de Médecine de Tunis, LR99ES09 Laboratoire de Résistance aux Antimicrobiens, Université de Tunis El Manar, 5 Rue Djebel Akhdhar, Tunis 1007, Tunisie, E-mail: [email protected]

Introduction

Enteroccoci are not only part of the normal intestinal flora of humans and animals but are also important pathogens responsible for serious nosocomial and community-acquired infections.³⁰ Ampicillin resistance in Enterococcus faecium is a worldwide therapeutic problem, especially when it is associated with high level of resistance to aminoglycosides and glycopeptides.³⁷ Aminopenicillin antibiotics normally inactivate penicillin-binding proteins (PBPs) and interfere with the process of transpeptidation because of the structural similarity between the β-lactam ring and the D-Ala-D-Ala extremity of bacterial cell precursors. The drugs act as suicide substrates of DD-transpeptidase catalytic domain of the PBPs that catalyze the last cross-linking step of cell wall assembly.¹⁴ The intrinsic low level of resistance to ampicillin (minimum inhibitory concentration [MIC] <16 mg/L) observed in E. faecium is mediated by the production of low-affinity class B penicillin-binding protein 5 (PBP5).^10,44 Acquisition of moderate-level (MICs 16-64) and high-level (MIC >64 mg/L) resistance to ampicillin in clinical isolates of E. faecium has been associated with increased levels of PBP5 expression or decreased affinity of PBP5 for this drug.^25,36,45 The latter mechanism is associated with substitutions of amino acids in the vicinity of conserved regions near or in the C-terminal part of PBP5.^11,25 Uncommonly, β-lactamase production has also been reported.⁵

Eighty-two unduplicated ampicillin-resistant E. faecium (AREF) isolates from clinical infections at the Charles Nicolle hospital of Tunis were investigated to determine their clonal relatedness, and then to study if specific amino acid variations in the C-terminal part of PBP5 could be correlated...