Among other things, motility of at least one individual microorganism or a change in motility of at least one individual microorganism or both is or are characterized. The characterized motility or change in motility is used to detect the presence or count of the at least one individual microorganism, or determine the identity of a species or strain of the at least one individual microorganism, or determine a susceptibility of the at least one individual microorganism to one or more antibiotics or other antimicrobials.