A method for quantifying the sensibility of a test microorganism to a concentration of an antimicrobial agent comprising: - preparing at a two liquid samples comprising a population of the test microorganism and a viability fluorescence marker targeting the tested microorganism, one of said liquid sample comprising the antimicrobial agent at said concentration and the another of said liquid samples do not comprising the antimicrobial agent; - for each of said samples of the test microorganism, acquiring, by means of a flow cytometer, a digital set of values comprising a fluorescence distribution of the population of microorganisms in said sample; - for each of said samples, computing of: ∘ a first abscissa value of the main mode of the acquired distribution and of a first area of the acquired distribution for abscissa values greater than the first abscissa value; ∘ a second abscissa value, greater than said first abscissa value, for which a second area of the acquired distribution between said first and second abscissa values equals a predefined percentage of the first area over 50%, - computing a ratio according to the relation: Q = QT ATB − Mode ATB QT no ATB − Mode no ATB where Mode and QT are respectively the first and second abscissa values with said concentration of antimicrobial agent, and Mode ( no ATB) and QT ( no ATB) are respectively the first and second abscissa values without said concentration of antimicrobial agent, wherein the deviation of ratio Q from 1 quantifies the sensitivity of the tested microorganism to the antimicrobial agent. - preparing at a two liquid samples comprising a population of the test microorganism and a viability fluorescence marker targeting the tested microorganism, one of said liquid sample comprising the antimicrobial agent at said concentration and the another of said liquid samples do not comprising the antimicrobial agent; - for each of said samples of the test microorganism, acquiring, by means of a flow cytometer, a digital set of values comprising a fluorescence distribution of the population of microorganisms in said sample; - for each of said samples, computing of: ∘ a first abscissa value of the main mode of the acquired distribution and of a first area of the acquired distribution for abscissa values greater than the first abscissa value; ∘ a second abscissa value, greater than said first abscissa value, for which a second area of the acquired distribution between said first and second abscissa values equals a predefined percentage of the first area over 50%, ∘ a first abscissa value of the main mode of the acquired distribution and of a first area of the acquired distribution for abscissa values greater than the first abscissa value; ∘ a second abscissa value, greater than said first abscissa value, for which a second area of the acquired distribution between said first and second abscissa values equals a predefined percentage of the first area over 50%, - computing a ratio according to the relation: Q = QT ATB − Mode ATB QT no ATB − Mode no ATB where Mode and QT are respectively the first and second abscissa values with said concentration of antimicrobial agent, and Mode ( no ATB) and QT ( no ATB) are respectively the first and second abscissa values without said concentration of antimicrobial agent,