Comparison of colistin MIC values of multidrug-resistant acinetobacter baumannii complex strains isolated from various clinical samples by automatic system and microdilution method

Authors : Funda Şahin, Sadık Akgün, Gülnur Tarhan
Pages : 1035-1043
Doi:10.17826/cumj.1717609
View : 90 | Download : 295
Publication Date : 2025-12-22
Article Type : Research Paper
Abstract :Purpose: Acinetobacter baumannii has become a major nosocomial pathogen, with increasing resistance rates are making many antibiotics ineffective. Colistin remains one of the few therapeutic options for multidrug-resistant (MDR) strains; however, reliable susceptibility testing is essential. The “Broth Microdilution (BMD)” method is currently the only accepted reference standard, but its labor-intensive nature limits its routine use. Automated systems such as BD Phoenix™100 (Becton Dickinson, USA) are widely implemented, but their accuracy in colistin testing remains under scrutiny. Material and Methods: A total of 1000 MDR A. baumannii complex strains isolated from clinical samples were analyzed. Colistin susceptibility was assessed simultaneously with both the BD Phoenix™100 and BMD methods. Escherichia coli ATCC 25922 (colistin sensitive) and E. coli NCTC 13846 (colistin resistant, mcr-1 positive) strains were used as quality controls. Breakpoints were interpreted according to EUCAST version 11.0 (MIC ≤ 2 μg/ml = susceptible; >2 μg/ml = resistant). Results: Colistin resistance rates among 1000 isolates examined were found to be 64 (6.4%) and 131 (13.1%) for the BD Phoenix™100 and BMD methods, respectively. According to comparative statistical analysis, categorical agreement (CA) was 97.2%, major error (ME) rate was 6.2% and very major error (VME) rate was 13.9%.MIC₅₀ and MIC₉₀ values were found to be 1 μg/ml and 2 μg/ml and 0.5 μg/ml and 2 μg/ml for the BD Phoenix™100 and BMD assay, respectively. Conclusion: Commercial automated systems lack high sensitivity for detecting colistin resistance. Therefore, standardization and validation studies are essential. On the other hand, while the BMD test is considered a reference method, it is impractical and time-consuming for routine use. It can be used for confirmation purposes when automated systems fail to yield results.
Keywords : Acinetobacter baumannii, Antimikrobiyal duyarlılık, kolistin, sıvı mikrodilüsyon

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