Colistin Resistance in Acinetobacter baumannii Clinical Isolates from Bahrain: Evaluation of Detection Methods and Clonal Relationships

Background: Acinetobacter baumannii (A. baumannii) is a critical-priority pathogen of major concern in healthcare settings. Colistin remains a last-resort antibiotic for multidrug-resistant (MDR) A. baumannii infections; however, resistance is increasingly reported worldwide yet remains understudied in Bahrain. Reliable detection methods and understanding clonal dissemination are essential for infection control. Objectives: This study aimed to (1) determine the rate of colistin resistance in 102 clinical A. baumannii isolates from Bahrain, (2) evaluate the diagnostic performance of the colistin agar test (CAT) and E-test against broth microdilution (BMD method), and (3) assess clonal relationships using BOX-PCR fingerprinting. Methods: 102 clinical isolates from multiple hospitals in Bahrain underwent susceptibility testing via the BMD method, CAT, and E-test; screening for mcr-1 to mcr-5 genes; and BOX-PCR DNA fingerprinting. Results: Colistin resistance was detected in 14.7% of isolates by BMD method, higher than regional and global averages. All resistant isolates were mcr-negative, suggesting chromosomally mediated resistance. CAT showed 86.7% sensitivity, 98.8% specificity, and a 13.3% very major error rate. The E-test failed to detect resistant isolates (very major error 100%). BOX-PCR revealed predominant clonal relatedness with intra- and inter-hospital spread. Conclusions: Colistin resistance in A. baumannii from Bahrain exceeds regional and global levels, likely driven by chromosomal mechanisms under selective pressure. The BMD method remains the gold standard for colistin testing, while CAT may serve as a screening tool requiring confirmation. Strengthened stewardship and infection control measures are vital to contain dissemination.