Beta-lactams susceptibility testing of penicillin-resistant, ampicillin-susceptible Enterococcus faecalis isolates: a comparative assessment of Etest and disk diffusion methods against broth dilution

Natália Conceição,Wellington Francisco Rodrigues,Laís Rezende Cardoso De Souza,Kessys Lorrânya Peralta De Oliveira,Adriana Gonçalves De Oliveira,Cristina ,Lucas Emanuel Pinheiro Da Silva

doi:10.1186/s12941-020-00386-8

Abstract

This study aimed to evaluate the accuracy of disk diffusion and Etest methods, compared to that of the broth dilution reference method for identifying beta-lactam susceptibilities of Penicillin-Resistant, Ampicillin-Susceptible Enterococcus faecalis (PRASEF) isolates. Fifty-nine PRASEF and 15 Penicillin-Susceptible, Ampicillin-Susceptible E. faecalis (PSASEF) clinical nonrepetitive isolates were evaluated. The effectiveness of five beta-lactams (ampicillin, amoxicillin, imipenem, penicillin, and piperacillin) was tested. All antimicrobial susceptibility tests were performed and interpreted according to the Clinical and Laboratory Standards Institute guidelines. Interpretative discrepancies, such as essential agreement, categorical agreement, and errors, were assessed. The acceptability was ≥ 90% for both categorical agreement and essential agreement. Etest proved to be an accurate method for testing beta-lactam susceptibilities of the emerging PRASEF isolates, disk diffusion presented poor performance, particularly for imipenem and piperacillin.

Highlights

Enterococci are widely distributed in the environment and are considered as normal intestinal microbes of humans and animals, during the past few decades, they have caused various infections in humans, primarily observed in hospitalized patients [1]
The current Clinical and Laboratory Standards Institute (CLSI) [9] and European Committee on Antimicrobial Susceptibility Testing (EUCAST) [10] guidelines state that the Conceição et al Ann Clin Microbiol Antimicrob (2020) 19:43 susceptibility to ampicillin may predict susceptibility to amoxicillin, piperacillin, and imipenem for E. faecalis, studies have demonstrated that this rule may not be applicable to the penicillin resistant isolates [3, 4, 6, 8]
This study aimed to assess the performance of Etest and disk diffusion methods to determine the susceptibilities of PRASEF isolates for beta-lactam antimicrobials in comparison with broth dilution using the reference method

Summary

Introduction

Enterococci are widely distributed in the environment and are considered as normal intestinal microbes of humans and animals, during the past few decades, they have caused various infections in humans, primarily observed in hospitalized patients [1]. E. faecalis, an enterococci species, is most frequently isolated from clinical specimens [2] These microorganisms are Enterococci usually present cross-susceptibility to β-lactamase-susceptible penicillin; the emergence of clinical penicillin-resistant, ampicillin-susceptible Enterococcus faecalis (PRASEF) isolates, exhibiting an unusual resistance phenotype, have been reported in various countries [4,5,6,7,8]. Various methods are applied for the in vitro evaluation of enterococci susceptibility to beta-lactams; disk diffusion and Etest are routinely used in most clinical microbiology laboratories in the developing countries [11, 12]. This study aimed to assess the performance of Etest and disk diffusion methods to determine the susceptibilities of PRASEF isolates for beta-lactam antimicrobials in comparison with broth dilution using the reference method

Objectives

Methods

Results