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CORRESPONDENCE
Year : 2011  |  Volume : 29  |  Issue : 2  |  Page : 198-199
 

VITEK 2 and PHOENIX fail to detect high-level gentamicin-resistant Enterococcus faecium isolates with aac-aph gene


1 Department of Clinical Microbiology, Selcuk University Selcuklu Faculty of Medicine, Konya, Turkey
2 Adnan Menderes Universitesi Tip Fakültesi, Tibbi Mikrobiyoloji AD and Adu Biltem Epidemiyoloji Birimi, Aydin, Turkey

Date of Submission19-Jan-2011
Date of Acceptance09-Mar-2011
Date of Web Publication2-Jun-2011

Correspondence Address:
B Bozdogan
Adnan Menderes Universitesi Tip Fakültesi, Tibbi Mikrobiyoloji AD and Adu Biltem Epidemiyoloji Birimi, Aydin
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0255-0857.81785

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How to cite this article:
Arslan U, Tuncer I, Findik D, Bozdogan B. VITEK 2 and PHOENIX fail to detect high-level gentamicin-resistant Enterococcus faecium isolates with aac-aph gene. Indian J Med Microbiol 2011;29:198-9

How to cite this URL:
Arslan U, Tuncer I, Findik D, Bozdogan B. VITEK 2 and PHOENIX fail to detect high-level gentamicin-resistant Enterococcus faecium isolates with aac-aph gene. Indian J Med Microbiol [serial online] 2011 [cited 2019 Sep 24];29:198-9. Available from: http://www.ijmm.org/text.asp?2011/29/2/198/81785


Dear Editor,

Detection of high-level gentamicin resistance is important to evaluate the use of β-lactam and aminoglycoside combination for treatment. The most common mechanism of high-level resistance to gentamicin is due to the presence of bifunctional inactivation enzyme AAC-APH. Twenty-seven vancomycin-resistant strains were tested for gentamicin susceptibility by using automated methods VITEK and PHOENIX systems as well as agar dilution MICs, E test, and disk diffusion using 120 μg disks. The presence of resistance genes was tested by PCR using specific primers. All isolates tested carried vanA and aac-aph genes. No inhibition zone was obtained with highly charged gentamicin disks as well as E test. Agar dilution MICs showed that 5, 3, and 19 strains had MIC 256, 512, and 1024 mg/L, respectively. Four of five isolates with gentamicin MICs 256 mg/L were susceptible by both VITEK and PHOENIX systems, and the remaining one was susceptible by PHOENIX and resistant by VITEK. Three isolates with MICs 520 mg/L were reported resistant by both systems. One isolate with MIC >1024 mg/L was reported susceptible by both automates which may be due to a growth problem. Gentamicin breakpoints for Enterococcus faecium of British, French, European, and American institutions have some differences [Table 1]. EUCAST accepts that there is no synergy between β-lactams and aminoglycosides for strains with MICs >128 mg/L and this MIC level should be accepted as high level gentamicin resistance, which indicates acquisition of a resistance mechanism. BSAC also agrees with EUCAST recommendation. CA-SFM accepts ≤256 mg/L as susceptible, while for CLSI <512 mg/L is susceptible. [1],[2],[3],[4] MIC testing for gentamicin is useful only to evaluate the presence of β-lactam-aminoglycoside synergy. E. faecium isolates with MIC >128 mg/L, this synergy is broken. Our study showed that some of the strains with aac-aph gene reported as susceptible by VITEK and PHOENIX. We believe that the concentration used by automates to detect gentamicin-resistant enterococci should be re-evaluated.
Table 1: Critical values accepted by British, American, French, and European organizations for high-level gentamicin resistance in enterococci

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[Additional file 1]

 
 ~ References Top

1.EUCAST Clinical breakpoints. Available from: http://www.eucast.org/clinical_breakpoints/ [Last Accessed on 2010 Dec 1].  Back to cited text no. 1
    
2.Recommandations du CASFM. Available from: http://www.sfm.asso.fr/nouv/general.php?pa=2 [Last Accessed on 2010 Dec 1].  Back to cited text no. 2
    
3.BSAC Methods for Antimicrobial Susceptibility Testing, Version 9.1. Available from: http://www.bsac.org.uk/Resources/BSAC/Version_9.1_March_2010_final.pdf. [March 2010].  Back to cited text no. 3
    
4.Clinical and Laboratory Standards Institute Performance standards for antimicrobial susceptibility testing. 15 th informational supplement. Approved standard MS100-S16. Wayne, PA: CLSI; 2006.  Back to cited text no. 4
    



 
 
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