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Antimicrobial Agents and Chemotherapy, October 2007, p. 3726-3730, Vol. 51, No. 10
0066-4804/07/$08.00+0     doi:10.1128/AAC.01406-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Comparative Evaluation of the VITEK 2, Disk Diffusion, Etest, Broth Microdilution, and Agar Dilution Susceptibility Testing Methods for Colistin in Clinical Isolates, Including Heteroresistant Enterobacter cloacae and Acinetobacter baumannii Strains{triangledown}

Jerome R. Lo-Ten-Foe,1 Anne Marie G. A. de Smet,2 Bram M. W. Diederen,1,{dagger} Jan A. J. W. Kluytmans,1,3 and Peter H. J. van Keulen1*

Laboratory for Microbiology and Infection Control, Amphia Hospital, Breda, The Netherlands,1 Perioperative and Emergency Care, Utrecht Medical Center, Utrecht, The Netherlands,2 Medical Microbiology and Infection Control, VUmc Medical University, Amsterdam, The Netherlands3

Received 10 November 2006/ Returned for modification 9 January 2007/ Accepted 17 July 2007

Increasing antibiotic resistance in gram-negative bacteria has recently renewed interest in colistin as a therapeutic option. The increasing use of colistin necessitates the availability of rapid and reliable methods for colistin susceptibility testing. We compared seven methods of colistin susceptibility testing (disk diffusion, agar dilution on Mueller-Hinton [MH] and Isosensitest agar, Etest on MH and Isosensitest agar, broth microdilution, and VITEK 2) on 102 clinical isolates collected from patient materials during a selective digestive decontamination or selective oral decontamination trial in an intensive-care unit. Disk diffusion is an unreliable method to measure susceptibility to colistin. High error rates and low levels of reproducibility were observed in the disk diffusion test. The colistin Etest, agar dilution, and the VITEK 2 showed a high level of agreement with the broth microdilution reference method. Heteroresistance for colistin was observed in six Enterobacter cloacae isolates and in one Acinetobacter baumannii isolate. This is the first report of heteroresistance to colistin in E. cloacae isolates. Resistance to colistin in these isolates seemed to be induced upon exposure to colistin rather than being caused by stable mutations. Heteroresistant isolates could be detected in the broth microdilution, agar dilution, Etest, or disk diffusion test. The VITEK 2 displayed low sensitivity in the detection of heteroresistant subpopulations of E. cloacae. The VITEK 2 colistin susceptibility test can therefore be considered to be a reliable tool to determine susceptibility to colistin in isolates of genera that are known not to exhibit resistant subpopulations. In isolates of genera known to (occasionally) exhibit heteroresistance, an alternative susceptibility testing method capable of detecting heteroresistance should be used.

* Corresponding author. Mailing address: Laboratory for Microbiology and Infection Control, Amphia Hospital, Breda, The Netherlands. Phone: 31-13-5392650. Fax: 31-13-5441264. E-mail: pvkeulen{at}amphia.nl

{triangledown} Published ahead of print on 23 July 2007.

{dagger} Present address: Regional Laboratory of Public Health, Haarlem, The Netherlands.

Antimicrobial Agents and Chemotherapy, October 2007, p. 3726-3730, Vol. 51, No. 10
0066-4804/07/$08.00+0     doi:10.1128/AAC.01406-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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