System and Method for measuring the growth of a bacterial culture and its response to one or more antimicrobials using measurement of mass of individual microbes. Methods include periodic sampling, determining change in mass and concentration, and comparing growth rates of cultures in nutrient broth vs. mixtures containing various antibiotic mixtures. A number of antimicrobials can be compared in one measurement by multiplexing or using multiple sensors to measure in parallel. Growth and antibiotic efficacy can be assessed at low concentrations at the onset of growth, typically within 1 to 2 hours.