Background: Colistin has become a last-resort antibiotic for the management of multidrug-resistant gram-negative bacteria. The disk diffusion test is cheap and easy to perform but may be unreliable for colistin susceptibility testing due to poor diffusion of the large colistin molecule. An improved agar diffusion test would increase the reliability of colistin susceptibility testing. This study aimed to modify Muller-Hinton agar (MHA) to improve colistin diffusion in agar. Methods: MHA was modified by reducing the agar concentration from 100% to 30% and supplementing with protamine. We tested 60 gram-negative clinical isolates of Pseudomonas aeruginosa (N = 27) and Acinetobacter calcoaceticus-baumannii complex (N = 33). Disk diffusion test results were interpreted based on minimum inhibitory concentrations determined by broth microdilution. Results: The modified MHA yielded the best performance metrics, including 94.7% sensitivity, 100% specificity, and an area under the curve of 0.995 (95% confidence interval, 0.982-1.000), P < 0.001, at a cut-off point of 13 mm. Conclusions: A reduction of the agar concentration from 100% to 30% and the addition of protamine improved colistin diffusion in agar and allowed routine colistin susceptibility testing in a clinical microbiology laboratory, but should be handled with caution.
Bibliographical noteFunding Information:
This work was supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute, funded by the Ministry of Health & Welfare, Korea (grant number: HI14C1324); by the Research Program funded by the Korea Centers for Disease Control and Prevention, Ministry of Health and Welfare, Korea (2019-ER5403-00); by Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries through Agricultural Microbiome R&D Program, funded by Ministry of Agriculture, Food and Rural Affairs (918003-4). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
© Korean Society for Laboratory Medicine.
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