Evaluation of the Vitek 2 AST-N269 Card for Detection of Meropenem Resistance in Imipenem-Susceptible Meropenem-Resistant Enterobacteriaceae

LETTER

In a recent article, Harino et al. evaluated the ability of several commercial automated susceptibility testing systems to detect the meropenem resistance of imipenem-susceptible meropenem-resistant (ISMR) Klebsiella pneumoniae isolates due to the production of both CTX-M-2 and IMP-6 (1). They reported that the Vitek 2 AST-N124 card was unreliable for the detection of ISMR Klebsiella pneumoniae because of falsely low meropenem MICs.

In 2012 in Japan, bioMérieux introduced a new Vitek 2 card, AST-N269, as a replacement for AST-N124. AST-N124 will not be available after December 2013. We therefore evaluated the ability of AST-N269 to detect ISMR Enterobacteriaceae (ISMRE). The tested isolates included 3 ISMR Klebsiella pneumoniae and 6 ISMR Escherichia coli isolates, all of which were confirmed to have both CTX-M-2 and IMP-6 by PCR and sequencing (2). The susceptibility testing systems were Vitek 2 with the advanced expert system (AES), version 5.04, using the AST-N124 and AST-N269 antibiotic panels and the dry plate Eiken test (Eiken Chemical Co., Tokyo, Japan). The dry plate Eiken is a broth microdilution test. Drug MICs for the test bacteria were determined according to the respective manufacturer's recommendations.

As shown in Table 1, the meropenem MICs of all the ISMRE strains measured by AST-N124 were incorrectly reported as 1 μg/ml. In contrast, use of the replacement card, AST-N269, gave the correct carbapenem MICs of all the ISMRE isolates tested (≤0.25 and ≥16 μg/ml for imipenem and meropenem, respectively). With use of the AST-N269 card, the AES inferred a carbapenemase resistance mechanism (either metallo-beta-lactamase or KPC) in 6 of 9 (67%) of ISMRE isolates tested. Inference by the AES of the presence of ESBLs in the ISMRE isolates was not perfect with the AST-N269 card (6 of 9 [67%] correct). The AES inferred that all of the ISMRE isolates had decreased membrane permeability, causing resistance to carbapenems.

Table 1.

MICs of ISMRE measured by Vitek with AST-N124 and AST-N269 and dry plate Eiken and interpretation by AES^a

Test	No. of isolates:
	With indicated MIC (μg/ml)						With interpretation by AES
	IPM/CS		MEPM				ESBL	MBL/KPC	DMP
	≤0.25	≤1	1	4	8	≥16			Carbapenems	Cephamycins
Vitek with AST-N124		9	9				9	6	6	3
Vitek with AST-N269	9					9	6	6	9
Dry plate Eiken	9			2	7

^aAES, advanced expert system; IMP/CS, imipenem/cilastatin; MEPM, meropenem; ESBLs, extended-spectrum beta-lactamases; MBL, metallo-beta-lactamases, DMP, decreased membrane permeability; DP, dry plate. Boldface type indicates very major errors.

Our study shows that the new Vitek 2 AST-N269 card correctly detects meropenem resistance in ISMRE and is a major improvement over the AST-N124 card, which had a 100% very major error rate for the detection of ISMRE. Performance of the AES with the new card for carbapenemase inference was excellent. However, the 67% sensitivity for the inference of ESBL production suggests the need for further studies of ESBL inference by the AES when AST-N269 is used.