A new diagnostic asynchronous magnetic bead rotation (AMBR) biosensor droplet microfluidic platform that can measure small cell population growth with high sensitivity has been reported to considerably reduce the time to determine antimicrobial susceptibility. With this platform, the growth, susceptibility, and minimum inhibitory concentration (MIC) of a small uropathogenic E. coli population that was confined in microfluidic droplets and exposed to concentrations above and below the MIC of gentamicin was measured. A 52% change in the sensor signal (i.e. rotational period) was observed within 15 minutes, thus allowing antimicrobial susceptibility test (AST) measurements to be performed within minutes. This reduction in the AST time may enable more appropriate therapies to be prescribed earlier, which consequently may help reduce the emergence and spread of antimicrobial resistance.