An antibiotic susceptibility testing device of gram-negative bacteria, as well as a corresponding method, are discussed. The device has a temperature control unit (including a constant temperature chamber) and a contactless conductivity-based measurement system. Disposable glassy or PVC tubes are used as test vessels for AST. In the performance of AST assay, appropriate kind of liquid medium containing identical amount of target bacterial cells and target antibiotics at different concentrations are loaded into test tubes, following by incubation in the device at a setup temperature. The bacterial growth profile is monitored by collecting the differential values (ΔC) of conductivity of liquid medium, which depend on the proliferation of viable cells. Outcome of ΔC indicates whether the target bacterial cells are completely inhibited by the test antibiotic or not, enabling the user to judge the value of the minimal inhibitory concentration (MIC) simply.