Determination of Colistin Resistance by Simple Disk Diffusion Test Using Modified Mueller-Hinton Agar

Area of Science:

• Clinical microbiology
• Antimicrobial resistance
• Diagnostic development

Background:

• Colistin is a critical last-resort antibiotic for multidrug-resistant gram-negative bacteria.
• Standard disk diffusion tests for colistin are often unreliable due to poor drug diffusion.
• Improving agar diffusion tests is crucial for accurate colistin susceptibility testing.

Purpose of the Study:

• To modify Muller-Hinton agar (MHA) to enhance colistin diffusion.
• To improve the reliability of colistin susceptibility testing using an agar diffusion method.

Main Methods:

• MHA was modified by reducing agar concentration to 30% and adding protamine.
• Disk diffusion tests were performed on 60 gram-negative isolates (Pseudomonas aeruginosa and Acinetobacter calcoaceticus-baumannii complex).
• Results were validated against minimum inhibitory concentrations determined by broth microdilution.

Main Results:

• The modified MHA demonstrated high performance: 94.7% sensitivity and 100% specificity.
• An area under the curve of 0.995 indicated excellent diagnostic accuracy.
• A cut-off of 13 mm provided reliable interpretation of colistin susceptibility.

Conclusions:

• Reducing MHA agar concentration and adding protamine significantly improves colistin diffusion.
• This modified agar diffusion test enables routine, reliable colistin susceptibility testing in clinical labs.
• The method offers a valuable tool for managing infections caused by resistant gram-negative bacteria.