DETECTION OF COLISTIN RESISTANCE IN MULTIDRUG-RESISTANT GRAM-NEGATIVE BACILLI: PERFORMANCE OF THE BD PHOENIX AUTOMATED SYSTEM VERSUS THE BROTH MICRODILUTION METHOD

Introduction: The increased dissemination of multidrug-resistant Gram-negative bacilli along with a lag in the development of novel, broad-spectrum antimicrobials determined the reintroduction of colistin into clinical practice. In this context, antibiotic susceptibility testing for colistin using high-performance methods has become a requirement. Objective: We compared the performances of the Becton Dickinson (BD) Phoenix 50 automated system and the Micronaut MIC-Strip, a commercial broth microdilution (BMD) method, for the detection of colistin resistance in clinical multidrug-resistant Gram-negative bacilli, isolated from patients admitted in a tertiary hospital in southern Romania. Methods: 54 clinical isolates with different multidrug-resistant phenotypes (Enterobacterales and Pseudomonas aeruginosa) were tested for colistin susceptibility with the BD Phoenix 50 automated system and the Micronaut MIC-Strip, following the Clinical Laboratory Standard Institute recommendations (CLSI, 2021) and were further classified according to the interpretative categories (resistant and intermediate, respectively). The statistical data was calculated using Microsoft Excel 2007 and OpenEPI software. Results: BD Phoenix system failed to detect 11 colistin resistant isolates (false susceptibility). By contrast, all 38 clinical isolates defined as intermediate by the BMD method were also classified as intermediate by the Phoenix system. Conclusion: BD Phoenix system is a reliable technology for detecting colistin resistance. However, high rates of false susceptibility were observed for the BD Phoenix system, indicating that the intermediate / susceptible results should be confirmed with the standard BMD method. Keywords: colistin susceptibility testing, Gram-negative bacilli, multidrug resistance.