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A regenerable bulk acoustic wave (BAW) biosensor is developed for the rapid, label-free and selective detection of <i>Escherichia coli</i> in liquid media. The geometry of the biosensor consists of a GaAs membrane coated with a thin film of piezoelectric ZnO on its top surface. A pair of electrodes deposited on the ZnO film allows the generation of BAWs by lateral field excitation. The back surface of the membrane is functionalized with alkanethiol self-assembled monolayers and antibodies against <i>E. coli</i>. The antibody immobilization was investigated as a function of the concentration of antibody suspensions, their pH and incubation time, designed to optimize the immunocapture of bacteria. The performance of the biosensor was evaluated by detection tests in different environments for bacterial suspensions ranging between 10³ and 10⁸ CFU/mL. A linear dependence between the frequency response and the logarithm of <i>E. coli</i> concentration was observed for suspensions ranging between 10³ and 10⁷ CFU/mL, with the limit of detection of the biosensor estimated at 10³ CFU/mL. The 5-fold regeneration and excellent selectivity towards <i>E. coli</i> detected at 10⁴ CFU/mL in a suspension tinted with <i>Bacillus subtilis</i> at 10⁶ CFU/mL illustrate the biosensor potential for the attractive operation in complex biological media.