MICy: a Novel Flow Cytometric Method for Rapid Determination of Minimal Inhibitory Concentration

Early initiated adequate antibiotic treatment is essential in intensive care. Shortening the length of antibiotic susceptibility testing (AST) can accelerate clinical decision-making. Our objective was to develop a simple flow cytometry (FC)-based AST that produces reliable results within a few hours. We developed a FC-based AST protocol (MICy) and tested it on six different bacteria strains (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus pyogenes, Enterococcus faecalis) in Mueller-Hinton and Luria-Bertani broth. We monitored the bacterial growth by FC to define the optimal time of AST. All bacteria were tested against 12 antibiotics and the MIC values were compared to microdilution used as reference method. McNemar and Fleiss' kappa inter-observer tests were performed to analyze the bias between the two methods. Susceptibility profiles of the two methods were also compared. We found that FC is able to detect the bacterial growth after 4-h incubation. The point-by-point comparison of MICy and microdilution resulted in exact match above 87% (2642/3024) of all measurements. The MIC values obtained by MICy and microdilution agreed over 80% (173/216) within ±1 dilution range that gives a substantial inter-observer agreement with weighted Fleiss' kappa. By using the EUCAST clinical breakpoints, we defined susceptibility profiles of MICy that were identical to microdilution in more than 92% (197/213) of the decisions. MICy resulted 8.7% major and 3.2% very major discrepancies. MICy is a new, simple FC-based AST method that produces susceptibility profile with low failure rate a workday earlier than the microdilution method. IMPORTANCE MICy is a new, simple and rapid flow cytometry based antibiotic susceptibility testing (AST) method that produces susceptibility profile a workday earlier than the microdilution method or other classical phenotypic AST methods. Shortening the length of AST can accelerate clinical decision-making as targeted antibiotic treatment improves clinical outcomes and reduces mortality, duration of artificial ventilation, and length of stay in intensive care unit. It can also reduce nursing time and costs and the spreading of antibiotic resistance. In this study, we present the workflow and methodology of MICy and compare the results produced by MICy to microdilution step by step.