Comparison of broth microdilution and commercial microdilution methods to detect the colistin susceptibility in multidrug-resistant gram-negative bacteria

The increasing resistance to colistin, which is one of the last antibiotic options in the fight against resistant Gram-negative bacterial infections, poses a serious threat to human health. Accurate and rapid determination of colistin susceptibility of isolates is very important for the success of treatment. The joint recommendation of the European Committee for Antimicrobial Susceptibility Testing (EUCAST) and the Clinical and Laboratory Standards Institute (CLSI) for the detection of colistin susceptibility is the broth microdilution (BMD) method. Since BMD is a susceptibility test method that requires intensive labor and time to prepare, its routine use in laboratories is difficult. Today, there are many commercial microdilution tests developed for this purpose that are easy to apply. Although there are many studies investigating the reliability of automated systems and other test methods frequently used in laboratories in determining colistin susceptibility, methods other than BMD are still controversial. 25 P. aeruginosa, 25 E. coli, 25 A. baumannii and 25 K. pneumoniae isolates, which were found multi-drug resistant (MDR) and colistin MIC value was ≤1 µg/mL with the Phoenix™ M50 automated system, were included in the study. Colistin susceptibility was also studied in 100 isolates with the Sensititre™ test, and the performances of these tests were investigated by comparing the results with the BMD method. For all isolates, the results of the Phoenix™ automated system were compared with the BMD method, and 4 isolates had very major error (VME). The categotic agreement (CA) was determined as 96% and MDR 100%. For all isolates, 1 major error (ME) was found when the results of the Sensititre™ commercial microdilution method were compared with the BMD method. CA was calculated as 99%, essential agreement (EA) as 69%, VME 0% and ME 1.04%. In this study, it was observed that both susceptibility test methods compared with the reference BMD method in determining colistin susceptibility did not meet the criteria for acceptable antimicrobial susceptibility test performance. Accordingly, colistin sensitivity should be determined by BMD, even if it is difficult to apply. Keywords: Broth microdilution, Antimicrobial susceptibility testing, Colistin