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JAC Advance Access originally published online on August 10, 2006
Journal of Antimicrobial Chemotherapy 2006 58(4):864-867; doi:10.1093/jac/dkl330
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© The Author 2006. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Comparison of three standardized disc susceptibility testing methods for colistin

Thean Yen Tan* and Lily Siew Yong Ng

Laboratory Medicine Services, Changi General Hospital 2 Simei Street 3, Singapore 529889

Received 14 May 2006; returned 15 June 2006; revised 18 July 2006; accepted 18 July 2006

*Corresponding author. Tel: +65-68504934; Fax: +65-64269507; E-mail: thean_yen_tan{at}cgh.com.sg

Objectives: With increasing antibiotic resistance in Gram-negative bacteria, the use of the polymyxins has increased in recent years. Antibiotic disc susceptibility testing remains the most widely used method in clinical laboratories, but there is very little data on the accuracy of disc testing methods for colistin. In this study, the accuracy of three standardized methods of disc susceptibility testing for colistin was compared with agar dilution.

Methods: A total of 228 clinical isolates of Acinetobacter spp., Pseudomonas aeruginosa and Enterobacteriaceae were included in the study. Isolates were tested by agar dilution for susceptibility to colistin, and results were compared with those obtained by three disc susceptibility testing methods (product insert based on CLSI methodology, British BSAC and French SFM).

Results: Colistin displayed good activity against Acinetobacter spp., Klebsiella spp. and Escherichia coli (MIC90 2 mg/L) but was less active against P. aeruginosa (MIC90 4 mg/L) and Enterobacter spp. (MIC90 ≥ 128 mg/L). Totally, 81%, 79% and 89% of colistin-resistant isolates were falsely reported as susceptible when tested by the product insert, BSAC and SFM testing methods, respectively. There were no false-resistant results.

Conclusions: Disc susceptibility testing methods are unreliable at detecting colistin resistance. Dilution methods should be the method of choice for susceptibility testing of colistin.

Keywords: disc diffusion , susceptibility , resistance , antimicrobial activity , polymyxin


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