Indian Journal of Medical Microbiology Medknow Publications on behalf of Indian Association of Medical Microbiology ISSN: 0255-0857 EISSN: 1998-3646 Vol. 29, Num. 2, 2011, pp. 198-199

Indian Journal of Medical Microbiology, Vol. 29, No. 2, April-June, 2011, pp. 198-199

Correspondence

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

U Arslan¹, I Tuncer¹, D Findik¹, B Bozdogan²

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

Correspondence Address:B Bozdogan Adnan Menderes Universitesi Tip Fakültesi, Tibbi Mikrobiyoloji AD and Adu Biltem Epidemiyoloji Birimi, Aydin Turkey bbozdogan@adu.edu.tr

Date of Submission: 19-Jan-2011
Date of Acceptance: 09-Mar-2011

Code Number: mb11051

PMID: 21654126

DOI: 10.4103/0255-0857.81785

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 2

References

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