Introduction: Acinetobacter baumannii is a challenging pathogen responsible for serious nosocomial infections. Colistin resistance in carbapenem-resistant A. baumannii strains is a critical health problem as it limits the available therapeutic options. The current work aimed to study the reliability of several phenotypic methods for the detection of colistin resistance among carbapenem-resistant A. baumannii isolates in Egypt.
Methods: A total of 22 carbapenem-resistant A. baumannii isolates were recovered. Colistin minimum inhibitory concentrations (MICs) were determined using broth microdilution (BMD) and compared to agar dilution (AD), automated system (VITEK-2) and gradient test (E-test) and were analyzed by statistical methods.
Results: Phenotypic testing showed that nine of 22 isolates (40.9%) were colistin-resistant by BMD and seven of them were also resistant by AD, with the categorical agreement (CA) of 72.7% and essential agreement (EA) of 90.9%. Colistin MIC results ranged from 1-8 µg/mL and 1-32 µg/mL by both AD and BMD respectively. Detection of colistin resistance by gradient test and automated system showed high very major error (VME) rates (40.9%) compared to BMD with a lack of CA between them. AD gave moderate agreement with BMD by 90.9% EA, 72.7% CA and only 9.1% VME.
Conclusions: In delineating colistin breakpoints BMD followed by AD method are defined as the only reliable phenotypic methods for colistin resistance evaluation. More rapid and reliable tests, other than BMD and AD, are required for the convenient detection of colistin resistance in the routine clinical microbiology laboratory daily workflow.
Keywords: Acinetobacter baumannii; Colistin; automated system (VITEK-2); broth microdilution; gradient test.
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