Comparison of disc diffusion, Etest and broth microdilution for testing susceptibility of carbapenem-resistant P. aeruginosa to polymyxins

Inneke M van der Heijden¹^,2 , Anna S Levin¹^,2 , Ewerton H De Pedri¹^,2 , Liang Fung¹^,2 , Flavia Rossi³ , Gisele Duboc³ , Antonio A Barone³ and Silvia F Costa¹^,2^,4

¹Laboratory of Medical Investigation 54, Hospital das Clinicas, University of São Paulo, Brazil

²Department of Infectious, Diseases of University of São Paulo, Brazil

³Laboratory of Microbiology of Hospital das Clínicas, University of São Paulo, Brazil

⁴Fernão Dias 158 apt 71, Pinheiros, São Paulo, SP, Brazil

author email corresponding author email

Annals of Clinical Microbiology and Antimicrobials 2007, 6:8doi:10.1186/1476-0711-6-8


Published:	15 August 2007

Abstract

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 μg/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.