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Susceptibility Testing for the Polymyxins: Two Steps Back, Three Steps Forward?

Shawn Vasoo1

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Optimizing polymyxin susceptibility testing is crucial due to rising drug-resistant bacteria. Broth microdilution (BMD) is recommended, but commercial panels show variable performance, highlighting the need for standardized methods and lower breakpoints.

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Area of Science:

  • Clinical microbiology
  • Antimicrobial resistance
  • Pharmacology

Background:

  • The increasing prevalence of multidrug-resistant Gram-negative bacilli necessitates optimized polymyxin susceptibility testing.
  • The Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) recommend broth microdilution (BMD) without polysorbate as the reference method.

Purpose of the Study:

  • To compare the performance of commercial BMD panels and Etest against the reference BMD method for polymyxin B and colistin.
  • To evaluate the impact of lowered breakpoints on testing accuracy and detection of the mcr-1 gene.

Main Methods:

  • Performance evaluation of three commercial BMD panels and Etest against reference BMD.
  • Testing was conducted on 76 Enterobacteriaceae isolates, including 21 mcr-1 positive strains.
  • Simulated analysis using lowered breakpoints (Susceptible ≤1 mg/liter; Intermediate 2 mg/liter; Resistant ≥4 mg/liter).

Main Results:

  • No commercial BMD panels fully met FDA performance standards, though Sensititre showed >90% categorical agreement for both polymyxins.
  • Gradient diffusion testing (Etest) exhibited unacceptable error rates, supporting its abandonment.
  • Lowered breakpoints improved error rates and agreement across methods, enhancing mcr-1 detection.

Conclusions:

  • Current commercial BMD panels have limitations for polymyxin susceptibility testing, reinforcing the need for standardization.
  • Gradient diffusion methods are unreliable for polymyxins and should be discontinued.
  • Lowering polymyxin breakpoints may be feasible but requires further validation with clinical data and improved testing reliability.