Colistin Susceptibility Testing 

Colistin is a polymyxin antibiotic primarily used as a last-line treatment for multidrug-resistant (MDR) Gram-negative bacteria, including Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacterales. Due to its nephrotoxicity and neurotoxicity, colistin is reserved for severe infections, necessitating accurate susceptibility testing to prevent resistance development and optimize dosing.

Testing Methods

Colistin susceptibility testing is challenging due to poor diffusion in agar-based methods. The following phenotypic and molecular techniques are recommended:

• Broth Microdilution (BMD): CLSI and EUCAST gold-standard method; precise MIC determination in Enterobacterales, Pseudomonas spp., and Acinetobacter spp..
• Colistin Agar Test (CAT): Detects colistin-resistant mcr-mediated isolates with high sensitivity.
• Rapid Polymyxin NP Test: Colorimetric assay detecting colistin resistance via bacterial membrane permeability assessment.
• E-test (Epsilometer Test): Less reliable for colistin due to poor agar diffusion.
• Disk Diffusion Assay: Not recommended due to false susceptibility results.
• Molecular Methods (PCR, Whole Genome Sequencing): Detect mcr-1 to mcr-9 genes, responsible for plasmid-mediated colistin resistance.

Clinical Applications

Colistin susceptibility testing is vital for managing MDR infections in various clinical scenarios:

• Carbapenem-Resistant Enterobacterales (CRE) Infections: Determines colistin efficacy as salvage therapy.
• Pseudomonas aeruginosa Infections: Ensures effective treatment for MDR and extensively drug-resistant (XDR) strains.
• Acinetobacter baumannii Infections: Guides colistin-based combination therapy decisions.
• Ventilator-Associated Pneumonia (VAP) and Bacteremia: Critical in nosocomial infections caused by MDR pathogens.

Colistin susceptibility testing is crucial for treating MDR and XDR Gram-negative infections, preventing resistance emergence, and guiding antimicrobial stewardship. Broth microdilution remains the gold standard, while molecular detection of mcr genes allows rapid resistance identification, enhancing treatment outcomes and infection control.