A method for determining a minimum inhibitory concentration in vitro of one or more bacteria in a sample comprising the steps of: a. determining the gram-type of the one or more bacteria in the sample, b. preparing a plurality of bacterial suspensions in a plurality of receptacles, c. adding to the receptacles containing the bacterial suspensions of step b. varying concentrations of an antimicrobial agent, thereby creating a plurality of suspensions comprising a combination of bacteria and antimicrobial agent, d. incubating the plurality of suspensions comprising a combination of bacteria and antimicrobial agent of step c. at a suitable temperature for a suitable period of time, thereby creating a plurality of incubated suspensions comprising a combination of bacteria and antimicrobial agent, e. adding to the suspensions of step d. a single membrane-associated dye, f. illuminating the suspensions of step e. with a light at a one or more excitation wavelengths, g. performing a light scatter gating analysis to discriminate between bacteria and suspension clutter, h. measuring intensity of emitted light at two emission wavelengths of individual bacterial cells in each suspension, i. determining the spectral intensity ratios based upon step h., j. determining from the respective bacterial suspension dead/live (D/L) ratios of bacterial cells based upon step h. of each suspension, k. calculating spectral-dead-live (SDL) values by taking the spectral intensity ratios of step i. and multiplying them with the D/L ratios of step j., l. plotting the SDL as a function of the antimicrobial concentration, and m. determining the minimum inhibitory concentration based upon the function of step 1. a. determining the gram-type of the one or more bacteria in the sample, b. preparing a plurality of bacterial suspensions in a plurality of receptacles, c. adding to the receptacles containing the bacterial suspensions of step b. varying concentrations of an antimicrobial agent, thereby creating a plurality of suspensions comprising a combination of bacteria and antimicrobial agent, d. incubating the plurality of suspensions comprising a combination of bacteria and antimicrobial agent of step c. at a suitable temperature for a suitable period of time, thereby creating a plurality of incubated suspensions comprising a combination of bacteria and antimicrobial agent, e. adding to the suspensions of step d. a single membrane-associated dye, f. illuminating the suspensions of step e. with a light at a one or more excitation wavelengths, g. performing a light scatter gating analysis to discriminate between bacteria and suspension clutter, h. measuring intensity of emitted light at two emission wavelengths of individual bacterial cells in each suspension, i. determining the spectral intensity ratios based upon step h., j. determining from the respective bacterial suspension dead/live (D/L) ratios of bacterial cells based upon step h. of each suspension, k. calculating spectral-dead-live (SDL) values by taking the spectral intensity ratios of step i. and multiplying them with the D/L ratios of step j., l. plotting the SDL as a function of the antimicrobial concentration, and m. determining the minimum inhibitory concentration based upon the function of step 1. <EOS>