Rapid Determination of Colistin Susceptibility by Flow Cytometry Directly from Positive Urine Samples-Preliminary Results.

Urinary tract infections caused by Gram-negative bacteria (GNB) are among the most common infections and a significant cause of sepsis. The increasing prevalence of multidrug-resistant (MDR) bacteria poses challenges to empirical treatment. Colistin may be used a last-resort antibiotic for treating MDR infections, but this requires the rapid determination of susceptibility to colistin. Traditional susceptibility testing methods can take up to 48 h, and there are specific challenges in determining colistin susceptibility. This study evaluates a novel, rapid method for determining colistin susceptibility directly from positive urine samples using the FASTcolistin MIC kit from FASTinov. A total of 100 urine samples positive for Gram-negative bacilli when screened by the UF-1000i system were included in this study. After a simple sample prep, the same bacterial suspension was used for identification on MALDI-TOF and inoculated in the FASTcolistin MIC panel for our AST; after incubation at 37 °C for 1 h, it was analyzed via flow cytometry using a CytoFLEX cytometer (Beckman Coulter, Brea, CA, USA). The categorical susceptibility to colistin according to EUCAST or CLSI standards as well as the MIC values were given by bioFAST software (bioFAST 2.0). The essential agreement (EA) and bias were calculated. Different species of Enterobacterales, , and spp. were correctly identified by MALDI-TOF directly from the FASTcolistin MIC sample prep. The essential agreement between the two methods was 99%, with a bias of -17%. Both identification and susceptibility were obtained in less than 2 h. This study presents a rapid and accurate method for determining colistin MIC directly from urine samples. The shortness of time required to produce a result, 2 h versus 48 h with the conventional methods, will significantly impact treatment decisions, especially in urinary tract infections difficult to treat.