Background: Colistin has become a last-resort antibiotic for the management of multidrugresistant 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.
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tients who receive inadequate antimicrobial treatment [1]. The case fatality rate for bacteremia is 30-40% [2]. In Korea, bacteremia caused by major antimicrobial-resistant pathogens, especially among patients hospitalized in intensive care units, has a high incidence [3, 4]. Furthermore, extended-spectrum β-lactamase-producing Escherichia coli and Klebsiella pneumoniae, vancomycin-resistant Enterococcus faecium, and imipenem-resistant Acinetobacter baumannii have been on the rise [5-7]. Rapid antimicrobial susceptibility testing (AST) results are important for the selection of suitable anti-bacterial treatments for bacteremia. The main problem with current AST methods is the long turnaround time (TAT). In most cases, conducting AST requires overnight incubation and usually requires 48-72 hours to complete, depending on the drug-organism combination [8].
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