In vitro testing of carbapenem-resistant Acinetobacter baumannii to colistin (polymyxin E)

Abstract number: P1165

Goic-Barisic I., Bedenic B., Tonkic M., Novak A., Punda-Polic V.

Objectives: Carbapenem-resistant strains of A. baumannii have been reported in almost all European counties. Thus more toxic agents such as polymyxins have been used as alternative therapeutic drugs against multidrug-resistant Acinetobacter baumannii infections. At present there is no agreement about how to look for colistin resistance. The Societe Francaise de Microbiologie uses a cut off of 2 mg/l by broth microdilution testing, whereas the British Society for Antimicrobial Chemotherapy sets a cut off of 4 mg/l or less as sensitive, and 8 mg/l or more as resistant. There are no US standards for measuring disk diffusion sensitivity to polymyxin so far whereas CLSI (formerly NCCLS) documents do currently provide interpretative criteria for minimal inhibitory concentration by broth microdilution testing of polymyxins in Acinetobacter spp.

Methods: Twenty-nine non-repetitive A. baumannii isolates with carbapenem resistance profile were obtained from patients hospitalised at different Intensive Care Units inside Clinical Hospital Centre Split during this year and tested by broth microdilution to colistin (polymyxin E). Isolates were recovered from blood cultures, urine samples and bronchial secretions. Although disk diffusion sensitivity to polymyxin is not standardised, the diameter of inhibition zone to colistin (10 mg) was measured.

Results: Out of total number of tested strains, twenty-two isolates (22/29) displayed minimum inhibitory concentration of 2 mg/l and six (6/29) isolates MIC of 4 mg/l. One isolate displayed MIC of 128 mg/l to polymyxin.

All isolates of A. baumannii displayed intermediate or resistant profile to imipenem (MICs 8–16 mg/L) and meropenem (MICs 8–64 mg/L). The diameter of inhibition zone to colistin by disk diffusion test was larger than 11 mm for all strains.

Conclusion: According to currently available criteria for interpretative reading for broth microdilution testing of polymixins in Acinetobacter spp., we found variability in susceptibility of our isolates to polymyxin. Against CLSI standard, most of our isolates (22/29) displayed susceptibility to polymyxin showing MIC values of 2 mg/l or less as sensitive. Regarding the results obtained with a disk diffusion test, we would recommend the confirmation by a dilution method, especially in the case of serious systemic infections.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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