An efficient and accurate antibiotic susceptibility test (AST) is indispensable for measuring the antimicrobial resistance of pathogenic bacteria. A minimal inhibitory concentration (MIC) can be obtained without performing repeated dilutions of the antibiotic by forming a linear antibiotic concentration gradient in a microfluidic channel. We demonstrated a device designed to use travelling surface acoustic waves (TSAWs) to enable a rapid formation of an antibiotic gradient in a few seconds. The TSAWs produced by a focused interdigital transducer deposited on the surface of a piezoelectric (LiNbO₃) substrate generated an acoustic streaming flow inside a microfluidic channel, which mixed confluent streams of antibiotics in a controlled fashion. The growth of bacteria exposed to the antibiotic gradient was determined by measuring the MIC, which was used as an indicator of the effectiveness of the AST. The concentration gradient produced using our device was linear, a feature that enhanced the reliability of measurements throughout the microchannel. Two ASTs, namely Pseudomonas aeruginosa against gentamicin and levofloxacin were chosen for the case of slowly proliferating bacteria, and one AST, namely Escherichia coli against gentamicin, were chosen for the rapidly proliferating case. Appropriate antibiotic doses for Pseudomonas aeruginosa and Escherichia coli were each obtained in an efficient manner.