Ann Lab Med.  2015 May;35(3):298-305. 10.3343/alm.2015.35.3.298.

Further Modification of the Modified Hodge Test for Detecting Metallo-beta-Lactamase-Producing Carbapenem-Resistant Enterobacteriaceae

• Kim HK
• Park JS
• Sung H
• Kim MN

• KMID: 2363201
• DOI: http://doi.org/10.3343/alm.2015.35.3.298

Abstract

BACKGROUND
The modified Hodge test (MHT) was designed to detect carbapenemase-producing Enterobacteriaceae (CPE). This study evaluated variables to improve the performance of MHT.
METHODS
Carbapenem-resistant Enterobacteriaceae isolated from November 2010 to March 2013 at the Asan Medical Center, were evaluated, including 33 metallo-beta-lactamase (MBL) producers and 103 non-CPEs. MHT was performed by using two carbapenem disks (ertapenem and meropenem; Becton Dickinson, USA), three media (Mueller-Hinton agar (MHA), MacConkey agar (MAC), and zinc-enriched MHA), and two inoculums (0.5-McFarland [McF] suspension and a 10-fold dilution of it.) PCR was performed to detect beta-lactamase genes of the MBL, AmpC, and CTX-M types.
RESULTS
The sensitivity of MHT for detecting New Delhi metallo-beta-lactamase (NDM) producers was highest using ertapenem and 0.5-McF, 52.0% on MHA and 68.0% on MAC, respectively. NDM-producing Klebsiella pneumoniae (NDMKP) were detected with higher sensitivity on MAC (78.6%) vs. MHA (28.6%) (P=0.016), but VIM-producing Enterobacter, Citrobacter, and Serratia were detected with higher sensitivity on MHA (78.5%) vs. MAC (14.3%) (P=0.004). MBL producers were consistently identified with lower sensitivity using meropenem vs. ertapenem, 39.4% vs. 60.6% (P=0.0156), respectively. The effects of zinc and inoculum size were insignificant. Enterobacter aerogenes producing unspecified AmpC frequently demonstrated false positives, 66.7% with ertapenem and 22.2% with meropenem.
CONCLUSIONS
The MHT should be adjusted for the local distribution of species and the carbapenemase type of MBL producers. MAC and ertapenem are preferable for assessing NDMKP, but MHA is better for VIM. Laboratory physicians should be aware of the limited sensitivity of MHT and its relatively high false-positive rate.

Keyword

Carbapenemase; Modified Hodge test; Metallo-beta-lactamase; Enterobacteriaceae; Klebsiella pneumoniae

MeSH Terms

Anti-Bacterial Agents/pharmacology
Carbapenems/pharmacology
DNA, Bacterial/genetics/metabolism
Disk Diffusion Antimicrobial Tests
Drug Resistance, Bacterial
Enterobacteriaceae/drug effects/*enzymology/isolation & purification
Enterobacteriaceae Infections/microbiology
Humans
Multiplex Polymerase Chain Reaction
Phenotype
beta-Lactamases/genetics/*metabolism
Anti-Bacterial Agents
Carbapenems
DNA, Bacterial
beta-Lactamases

Figure

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  Fig. 1 Modified Hodge test for NDM-producing Klebsiella pneumoniae using ertapenem disks on MacConkey agar with 0.5-McFarland inoculum (A) and ertapenem disks on Mueller-Hinton agar with 0.5-McFarland inoculum (B). The tests on MacConkey agar produced positive results, and the tests on Mueller-Hinton agar produced negative results. Modified Hodge test for a NDM-producing Enterobacter cloacae using ertapenem disks on MacConkey agar with 0.5-McFarland inoculum (C), meropenem disks on MHA with 0.5-McFarland inoculum (D), and ertapenem disks on MHA with a 1:10 dilution of 0.5-McFarland inoculum (E). Meropenem (D) demonstrated unclear margins of inhibition in Escherichia coli, and the indentation was small. The 1:10 dilution (E) demonstrated similar results as the 0.5-McFarland suspension (C).

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