Evaluating the drop test method in measuring colistin susceptibility of , and .

Colistin is an antibiotic used to treat Gram-negative bacterial infections, particularly those caused by multidrug-resistant bacteria. The broth microdilution (BMD) reference method is the recommended protocol for detecting colistin susceptibility; however, it is laborious and expensive and cannot be performed in all laboratories. To evaluate the colistin susceptibility in , and using an alternative method, which has been referred to as the drop test. A 16 µg ml colistin solution was deposited on a Mueller-Hinton agar plate previously swabbed with the strain and incubated overnight, and the presence or absence of an inhibition zone was observed. The categorical agreement (CA) of the drop test with respect to BMD was 100 % when 190 Enterobacterales (19 and 171 . ) were used, and no major errors (MEs) or very major errors (VMEs) were detected. The CA of the drop test with respect to the BMD was 99.2 % for 119 . isolates, while no ME was detected and only 1 VME (6.7%) was observed. The drop test is an alternative method for antimicrobial susceptibility testing of colistin against and . It is an adequate method for detecting resistant strains of , but susceptible isolates should be confirmed using BMD. The drop test is a simpler alternative to the BMD that does not require additional equipment and allows for the testing of numerous isolates in a short period of time.