False susceptibility to amikacin by VITEK 2 in Acinetobacter baumannii harbouring armA

Abstract number: P813

Jung S.K., Yu J.K., Park K.G., Han K., Park Y.J., Kim S.Y., Arakawa Y.

Objectives: Amikacin (AN) is the most active aminoglycoside for treatment of infections caused by Acinetobacter baumannii. The VITEK 2 (bioMérieux Inc., Hazelwood, MO) automated system recommends an alternative susceptibility testing prior to reporting of AN susceptibility result for A. baumannii. We accidently found an A. baumannii isolate which was susceptible to AN by VITEK 2, but showed resistance by disk diffusion test and agar dilution MIC test.

Methods: On June 2008, we performed a disk diffusion test for an A. baumannii isolate which was susceptible to AN by VITEK 2. AN MIC was determined by agar dilution method according to the CLSI guideline and presence of 16S rRNA methylase genes were investigated by PCR. Also, we tested the AN susceptibility of Serratia marcescens isolate harbouring armA and its transconjugant (E. coli J53 harbouring armA) by VITEK 2, disk diffusion test and agar dilution method. To check the purity, we picked a single colony from the A. baumannii and E. coli transconjugant into MH broth, and repeated the disk diffusion test. In addition, to investigate whether this phenomenon is associated with induction, we assayed the resistant subpopulations directly for AN susceptibility.

Results:A. baumannii showed susceptibility to AN by VITEK 2 but revealed resistance (double zone of inhibition) by disk diffusion test. AN MIC was >512 mg/ml and it harboured armA. In cases of S. marcescens harbouring armA and its transconjugant (E. coli J53 harbouring armA), S. marcescens showed resistantance to AN but its transconjugant showed susceptibility to AN by VITEK 2. By disk diffusion test, the S. marcescens and its transconjugant revealed complete resistance and resistance with double zone of inhibition, respectively. Both A. baumannii and E. coli showed same phenomenon with a single colony. However, the AN MICs of the two isolates were both very high (>512 mg/ml). The direct susceptibility testing of resistant subpopulations after exposure showed identical double zone of inhibition, suggesting it was not caused by induction.

Conclusions: VITEK 2 automated system can exhibit very major error for both A. baumannii and E. coli harbouring armA. Considering widespread dissemination of armA, further study is needed to investigate the mechanism and frequency of this phenomenon in 16S rRNA methylase-producing Gram-negative bacteria.

Session Details

Date: 16/05/2009
Time: 00:00-00:00
Session name: 19th European Congress of Clinical Microbiology and Infectious Diseases
Location: Helsinki, Finland, 16 - 19 May 2009
Presentation type:
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