Comparison of Disc Diffusion, Etest and Broth Microdilution for Testing Susceptibility of Carbapenem-Resistant P. Aeruginosa to Polymyxins

Ann Clin Microbiol Antimicrob. 2007 Aug 15;6:8. doi: 10.1186/1476-0711-6-8.


Background: Considering the increasing use of polymyxins to treat infections due to multidrug resistant Gram-negative in many countries, it is important to evaluate different susceptibility testing methods to this class of antibiotic.

Methods: Susceptibility of 109 carbapenem-resistant P. aeruginosa to polymyxins was tested comparing broth microdilution (reference method), disc diffusion, and Etest using the new interpretative breakpoints of Clinical and Laboratory Standards Institute.

Results: Twenty-nine percent of isolates belonged to endemic clone and thus, these strains were excluded of analysis. Among 78 strains evaluated, only one isolate was resistant to polymyxin B by the reference method (MIC: 8.0 microg/mL). Very major and major error rates of 1.2% and 11.5% were detected comparing polymyxin B disc diffusion with the broth microdilution (reference method). Agreement within 1 twofold dilution between Etest and the broth microdilution were 33% for polymyxin B and 79.5% for colistin. One major error and 48.7% minor errors were found comparing polymyxin B Etest with broth microdilution and only 6.4% minor errors with colistin. The concordance between Etest and the broth microdilution (reference method) was respectively 100% for colistin and 90% for polymyxin B.

Conclusion: Resistance to polymyxins seems to be rare among hospital carbapenem-resistant P. aeruginosa isolates over a six-year period. Our results showed, using the new CLSI criteria, that the disc diffusion susceptibility does not report major errors (false-resistant results) for colistin. On the other hand, showed a high frequency of minor errors and 1 very major error for polymyxin B. Etest presented better results for colistin than polymyxin B. Until these results are reproduced with a large number of polymyxins-resistant P. aeruginosa isolates, susceptibility to polymyxins should be confirmed by a reference method.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anti-Bacterial Agents / pharmacology*
  • Carbapenems / pharmacology
  • Colistin / pharmacology*
  • Drug Resistance, Bacterial
  • False Positive Reactions
  • Humans
  • Microbial Sensitivity Tests / methods*
  • Polymyxin B / pharmacology*
  • Pseudomonas Infections / microbiology
  • Pseudomonas aeruginosa / drug effects*


  • Anti-Bacterial Agents
  • Carbapenems
  • Polymyxin B
  • Colistin