A method of determining the susceptibility of a unicellular or multicellular microorganism to an antimicrobial agent by acoustic flow cytometry, said method comprising the following steps: (i) incubating in a culture medium (a) cells of the microorganism from a sample that has been obtained from a subject having or suspected of having an infection with said microorganism, and/or (b) a sample that has been obtained from the subject having or suspected of having an infection with said microorganism, wherein said sample comprises cells of said microorganism, wherein said incubating is for a time and under conditions sufficient for said cells to actively divide to thereby obtain an actively dividing culture of said cells of said microorganism; (ii) exposing cells of the microorganism in and/or from the actively dividing cell culture obtained in step (i) to an antimicrobial agent for a time and under conditions sufficient for said cells of the microorganism to actively divide; (iii) labelling cells of the microorganism exposed to the antimicrobial agent in step (ii) with a nucleic acid binding fluorescent compound; (iv) measuring effect of said antimicrobial agent on cellular morphology of said cells of the microorganism by acoustic flow cytometry; and (v) determining susceptibility of the microorganism to the antimicrobial agent from the acoustic flow cytometry data output, wherein step (v) comprises determining susceptibility of the microorganism to the antimicrobial agent qualitatively by determining from the acoustic flow cytometry data output that said microorganism is either susceptible or not susceptible to said antimicrobial agent; and determining susceptibility of the microorganism to the antimicrobial agent quantitatively by determining from the acoustic flow cytometry data output the minimal inhibitory concentration of said antimicrobial agent to which the microorganism is susceptible. (i) incubating in a culture medium (a) cells of the microorganism from a sample that has been obtained from a subject having or suspected of having an infection with said microorganism, and/or (b) a sample that has been obtained from the subject having or suspected of having an infection with said microorganism, wherein said sample comprises cells of said microorganism, wherein said incubating is for a time and under conditions sufficient for said cells to actively divide to thereby obtain an actively dividing culture of said cells of said microorganism; (a) cells of the microorganism from a sample that has been obtained from a subject having or suspected of having an infection with said microorganism, and/or (b) a sample that has been obtained from the subject having or suspected of having an infection with said microorganism, wherein said sample comprises cells of said microorganism, wherein said incubating is for a time and under conditions sufficient for said cells to actively divide to thereby obtain an actively dividing culture of said cells of said microorganism; (ii) exposing cells of the microorganism in and/or from the actively dividing cell culture obtained in step (i) to an antimicrobial agent for a time and under conditions sufficient for said cells of the microorganism to actively divide; (iii) labelling cells of the microorganism exposed to the antimicrobial agent in step (ii) with a nucleic acid binding fluorescent compound; (iv) measuring effect of said antimicrobial agent on cellular morphology of said cells of the microorganism by acoustic flow cytometry; and (v) determining susceptibility of the microorganism to the antimicrobial agent from the acoustic flow cytometry data output, wherein step (v) comprises determining susceptibility of the microorganism to the antimicrobial agent qualitatively by determining from the acoustic flow cytometry data output that said microorganism is either susceptible or not susceptible to said antimicrobial agent; and determining susceptibility of the microorganism to the antimicrobial agent quantitatively by determining from the acoustic flow cytometry data output the minimal inhibitory concentration of said antimicrobial agent to which the microorganism is susceptible. determining susceptibility of the microorganism to the antimicrobial agent qualitatively by determining from the acoustic flow cytometry data output that said microorganism is either susceptible or not susceptible to said antimicrobial agent; and determining susceptibility of the microorganism to the antimicrobial agent quantitatively by determining from the acoustic flow cytometry data output the minimal inhibitory concentration of said antimicrobial agent to which the microorganism is susceptible.