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


CONCLUSIONS
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.
van der Heijden
Inneke M
IM
Laboratory of Medical Investigation 54, Hospital das Clinicas, University of São Paulo, Brazil. inneke@usp.br
Levin
Anna S
AS
De Pedri
Ewerton H
EH
Fung
Liang
L
Rossi
Flavia
F
Duboc
Gisele
G
Barone
Antonio A
AA
Costa
Silvia F
SF
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D015780:Carbapenems / Q000494:pharmacology; D003091:Colistin / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D005189:False Positive Reactions; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods*; D011112:Polymyxin B / Q000494:pharmacology*; D011552:Pseudomonas Infections / Q000382:microbiology; D011550:Pseudomonas aeruginosa / Q000187:drug effects*
D000900:Anti-Bacterial Agents; D015780:Carbapenems; D011112:Polymyxin B; D003091:Colistin
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