What is the recommended method to assess microbiological sensitivity to colistin (polymyxin E)?

Broth Microdilution Is the Recommended Method for Assessing Microbiological Sensitivity to Colistin

Broth microdilution (BMD) is the only recommended method for determining colistin susceptibility, as it is the most reliable technique that provides accurate minimum inhibitory concentration (MIC) values. 1, 2

Recommended Testing Method

Broth Microdilution (Gold Standard)

• BMD should be performed according to EUCAST/CLSI guidelines
• Key specifications:
  • Use polystyrene plates (not glass or polypropylene)
  • Add colistin directly to the broth (not pre-diluted)
  • Ensure proper cation-adjusted Mueller-Hinton broth
  • Use fresh (<12 hours old) bacterial suspensions

Commercial BMD Systems

Several commercial BMD systems are available with varying performance:

1. Sensititre - Highest reliability with 97.8% categorical agreement with reference BMD 3

   • Very major error rate: 3%
   • Major error rate: 0%

2. MICRONAUT Systems - Excellent performance with ≥96% essential agreement 2

3. UMIC Colistin - Good but slightly less reliable 3, 4

   • Very major error rate: 11.3%
   • Major error rate: 0%
   • Provides unitary polystyrene strips with 11 concentrations

4. MicroScan - Good for Enterobacteriales but less reliable for non-fermenters 3

   • Very major error rate: 0.8%
   • Major error rate: 26.9% (primarily with non-fermenting organisms)

Methods to Avoid

Gradient Tests (E-test, MIC Test Strip)

• NOT recommended due to poor performance 2
• Essential agreement only 43-71% with reference BMD
• Consistently underestimate colistin MICs
• High false susceptibility rates (12-24% of isolates)

Standard Disk Diffusion

• NOT recommended due to poor diffusion of large colistin molecules in agar
• Unreliable correlation between zone diameters and MICs

Alternative Methods When Standard BMD Is Not Available

1. Disk Diffusion in Broth - 100% concordance with reference BMD in Enterobacteriales 5

   • More accessible for resource-limited laboratories
   • Can be used as an initial screening method

2. Broth Macrodilution - 100% concordance with reference BMD in Enterobacteriales 5

   • Simpler alternative to microdilution
   • Requires less specialized equipment

Special Considerations

Heteroresistance Detection

• Heteroresistance (resistant subpopulations within susceptible isolates) is common with colistin
• Standard microdilution cannot adequately detect heteroresistant populations 1
• Observation of colonies in inhibition zones of disk or E-test may indicate heteroresistance
• Previous colistin use may increase risk of heteroresistance 1

Quality Control

• Essential to include appropriate quality control strains:
  • E. coli ATCC 25922 (colistin-susceptible)
  • E. coli NCTC 13846 (mcr-1 positive, colistin-resistant)
  • P. aeruginosa ATCC 27853

Interpretation of Results

• EUCAST/CLSI breakpoints: Susceptible ≤2 μg/mL, Resistant >2 μg/mL
• For Acinetobacter species, accurate MIC determination is critical for determining appropriate therapy 1
• For polymyxin-only susceptible isolates, accurate MICs guide dosing of intravenous and potentially adjunctive inhaled colistin 1

Common Pitfalls to Avoid

1. Using glass or polypropylene plates (colistin adheres to these materials)
2. Relying on automated systems not validated for colistin testing
3. Using gradient tests or standard disk diffusion methods
4. Failing to include appropriate quality control strains
5. Not considering heteroresistance in isolates with previous colistin exposure

By following these recommendations, laboratories can ensure accurate colistin susceptibility testing, which is critical for guiding appropriate therapy for multidrug-resistant gram-negative infections.