We investigate the buoyant mass of bacterial cells in real time with the suspended microchannel resonator (SMR) as the population recovers from an osmotic shock. The density of the culture medium is chosen such that the bacteria initially have a positive buoyant mass which becomes negative as they recover from the hyperosmotic stress. This behavior can be used to differentiate between an antibiotic-resistant and an antibiotic-susceptible strain of the pathogenic bacteria Citrobacter rodentium, and we propose a general approach for exploiting the high precision of the SMR for rapid detection of antibiotic resistance.