Comparison of Available Methods for Investigating The in vitro Activity of Colistin Against Different Gram-Negative Bacilli

Background: The surge in the prevalence of multidrug-resistant (MDR) Gram-negative bacterial infections with limited treatment options and the decrease in the development of new antibiotics are challenges that lead to the reuse of colistin to treat infections caused by MDR pathogens. This study aimed to determine economical, simple, and reliable colistin susceptibility testing methods as an alternative to the time and effort-consuming microdilution technique and identify the colistin resistance's genetic determinants to find if it affects the testing method. Material and Methods: Seven colistin susceptibility testing methods, namely, Disk diffusion, E-test, ComASPTM SensiTest, broth disk elution, colistin agar test, CHROMagarTM COL-APSE, and BD Phoenix ID/AST, were compared to the gold standard broth microdilution. Data of the 63 studied isolates were analyzed using very major error (VME), major error (ME), categorical agreement (CA), sensitivity, specificity, Kappa, positive and negative predictive values. Whole-genome sequencing was performed on all isolates to determine if the genetic resistant factors affect the accuracy of the specific colistin susceptibility testing method. Results: Our results revealed that disk diffusion is still an ineffective method for measuring colistin susceptibility with the highest ME (31.75%), the lowest Kappa 0 (0%), and CA (68.25%) values. In contrast, the highest sensitivity, specificity, CA, kappa value, positive and negative predictive values were reported on Phoenix, ComASPTM sensitest, and E-test methods compared with the microbroth dilution reference method. Our study did not ensure any relation between the type of colistin resistance genetic determinant (chromosomal/plasmid-mediated) and the performance of the specific colistin susceptibility test Conclusions: Phoenix, E-test, and CompASPT SensiTest methods have remained superior in reproducibility, sturdiness, simplicity of use with a performance similar to the current recommended BMD procedure. These methods can be an alternative to the current laborious, impractical broth microdilution technique, especially in microbiology laboratories with a large workload.