A tool that can screen bacteria for β-lactamases against a panel of various antibiotics is desirable. A biosensor incorporating an indicator molecule into β-lactamases may achieve this purpose, but it requires that the attached indicator molecule must not impair the binding affinity of the protein to a great extent to provide a higher sensitivity. A modified β-lactamases with a residue on the Ω-loop or outside the Ω-loop but close to the active site of β-lactamase being replaced by a reactive residue is developed in this invention.