Evaluation of the Vitek-2 system using AST-N020 y AST-N041 cards for susceptibility testing of beta-lactam-resistant Escherichia coli and Klebsiella pneumoniae strains of clinical origin

Abstract number: p446

Calvo  J., Salas  C., Hernández  J.R., Ruiz  B., Conejo  M.C., Armengol  M., Pascual  A., Martinez-martinez  L.

Objectives: 

To evaluate the Vitek-2 (V2) system (bioMérieux, France) using AST-N020 or AST-N041 cards for susceptibility testing and recognition of phenotypes of resistance of clinical isolates of Escherichia coli (Eco) and Klebsiella pneumoniae (Kpn) resistant to beta-lactams. AST-N041 contains specific test for extended-spectrum beta-lactamase (ESBL) detection.

Methods: 

We tested 136 organisms: 44 Eco and 20 Kpn producing TEM, SHV or CTX-M ESBL, 39 Eco hyperproducing chromosomal AmpC (HAmpC), 11 Kpn ESBL(+) and lacking porins [POR(-)] and 22 Kpn with plasmid-mediated AmpC beta-lactamase (pACBL). MICs were determined by microdilution (MD, CLSI guidelines); resistance mechanisms were studied by beta-lactamase characterization (isoelectric focusing and gene sequencing) and SDS-PAGE of outer membrane proteins. MICs of amoxicillin-CLV (AMC), piperacillin-tazobactam (PTZ), cefoxitin (FOX), cefotaxime (CTX), ceftazidime (CAZ) and cefepime (FEP) determined with N020 and N041 cards were compared between them and with the reference MD. Presumed phenotypes of resistance by the V2 advanced expert system (AES), and suggested changes in clinical categories were also compared for both cards.

Results: 

For the 136 strains, MICs of AMC, PTZ, and FOX with each card were within one dilution step for 135 strains; MICs of CTX, CAZ, and FEP with N020 were >=2 dilutions higher than with N041 for 7, 2 and 0 strains, while the reverse was noted for 8, 6 and 3 strains. MICs of FEP with N020 and N041 were >=2 dilutions lower than with reference MD for 28/64 ESBL organisms, and for 10/33 Kpn POR (-) or pACBL (+), but agreed for all Eco HAmpC. Phenotypes suggested by the AES with N020 card agreed with reference results for 59/64 ESBL (+) strains, 9/11 ESBL (+)/POR(-) and 19/22 pACBL. With N041 these values were 61/64, 11/11 and 17/22, respectively. For 31/39 Eco HAmpC the AES with N020 suggested several phenotypes, including an ESBL, reporting them as resistant to CTX, CAZ and FEP; the N041 card correctly discarded the presence of an ESBL in all 39 strains.

Conclusions: 

Differences in MICs of CTX, CAZ and FEP obtained with AST-N020 and AST-N041 cards for organisms ESBL(+) or pACBL(+) were noted in comparison with reference results, but this usually did not translate into changes in clinical categories after AES suggestions. The AES shows excellent agreement with underlying mechanisms of resistance with card N041, but with N020 it suggests false presence of ESBLin most Eco HAmpC.