Ambient-condition air plasma produced by indirect dielectric barrier discharges can rapidly disinfect aqueous solutions contaminated with bacteria and other microorganisms. In this study, we measured key chemical species in plasma-treated aqueous solutions and the associated antimicrobial effect for varying discharge power densities, exposure times, and buffer components in the aqueous medium. The aqueous chemistry corresponded to air plasma chemistry, and we observed a transition in composition from ozone mode to nitrogen oxides mode as the discharge power density increased. The inactivation of E. coli correlates well with the aqueous-phase ozone concentration, suggesting that ozone is the dominant species for bacterial inactivation under these conditions. Published values of ozone-water antibacterial inactivation kinetics as a function of the product of ozone concentration and contact time are consistent with our results. In contrast to earlier studies of plasma-treated water disinfection, ozone-dependent bacterial inactivation does not require acidification of the aqueous medium and the bacterial inactivation rates are far higher. Furthermore, we show that the antimicrobial effect depends strongly on gas-liquid mixing following plasma treatment, apparently because of the low solubility of ozone and the slow rate of mass transfer from the gas phase to the liquid. Without thorough mixing of the ozone-containing gas and bacteria-laden water, the antimicrobial effect will not be observed. However, it should be recognized that the complexity of atmospheric pressure plasma devices, and their sensitivity to subtle differences in design and operation, can lead to different results with different mechanisms.