Comparative Evaluation of Colistin Susceptibility Testing Using Agar Dilution and Broth Microdilution in Multidrug-resistant and Extensively Drug-resistant Gram-Negative Isolates

Abu-El-Azayem, Abeer K.; Dwedar, Reham A.; Nashaat, Nihal; Shash, Rania Y.

doi:10.21608/ejmm.2023.310119

Comparative Evaluation of Colistin Susceptibility Testing Using Agar Dilution and Broth Microdilution in Multidrug-resistant and Extensively Drug-resistant Gram-Negative Isolates

Document Type : New and original researches in the field of Microbiology.

Authors

Department of Medical Microbiology and Immunology, Faculty of Medicine, Cairo University, Cairo, Egypt

10.21608/ejmm.2023.310119

Abstract

Background: Emergence of multidrug resistance (MDR) and extensively drug resistance (XDR) among Gram-negative bacteria has obliged the clinical re-use of old antimicrobial agents, such as colistin. The increase in the use of colistin has necessitated reliable, and accurate methods for in vitro antimicrobial susceptibility testing to allow proper antimicrobial therapy. Up till now the most commonly used method in laboratories for colistin susceptibility testing is the disk diffusion test which is considered unreliable. Objectives: This study aimed to evaluate and compare two methods for colistin susceptibility testing using agar dilution and broth microdilution among MDR and XDR Gram-negative isolates. Methodology: This study included 62 Gram-negative isolates (38 MDR and 24 XDR) obtained from the Medical Microbiology and Immunology Department, Cairo University, Egypt. Colistin susceptibility was tested for these isolates by agar dilution and broth microdilution methods. Results: Our study revealed a significant statistical difference between colistin MIC by using broth microdilution and the agar dilution method. In comparison to broth microdilution, the agar dilution method had 100% sensitivity, 85.88 % specificity, 25% positive predictive value, 100% negative predictive value and 86.21% accuracy. Conclusions: The agar dilution method is unreliable for the detection of colistin resistance. Therefore, we could depend on colistin susceptibility testing by using standard broth microdilution even in case of limited resources. This accurate susceptibility testing may help us to maintain the therapeutic usefulness of colistin until newer treatment choices are available.

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