Comparison of three standardized disc susceptibility testing methods for colistin

Area of Science:

• Clinical microbiology
• Antimicrobial resistance
• Bacterial infectious diseases

Background:

• Polymyxin use is increasing due to rising Gram-negative bacterial resistance.
• Accurate susceptibility testing is crucial for guiding antibiotic therapy.
• Limited data exists on the reliability of disc diffusion for colistin.

Purpose of the Study:

• To evaluate the accuracy of three standardized colistin disc susceptibility testing methods.
• To compare disc testing results against the gold standard agar dilution method.

Main Methods:

• Tested 228 clinical isolates (Acinetobacter spp., Pseudomonas aeruginosa, Enterobacteriaceae) for colistin susceptibility.
• Utilized agar dilution as the reference method.
• Compared results with CLSI, BSAC, and SFM disc diffusion methods.

Main Results:

• Colistin showed good activity against Acinetobacter spp., Klebsiella spp., and Escherichia coli.
• High rates of false-susceptible results were observed with all three disc testing methods (81-89%).
• No false-resistant results were reported.

Conclusions:

• Standardized disc susceptibility testing methods are unreliable for detecting colistin resistance.
• Agar dilution methods are the preferred choice for colistin susceptibility testing.
• Clinical laboratories should utilize dilution methods for accurate colistin susceptibility assessment.