Abstract: The present invention relates to a method for detecting the presence of expanded-spectrum ß-lactamase (ß-lactamase hydrolyzing expanded-spectrum cephalosporin)-producing bacteria in a sample, said method comprising the steps of: a) performing cell lysis on a test sample in order to obtain an enzymatic suspension; b) reacting a fraction of the enzymatic suspension obtained in step a) with a reagent kit, said reagent kit comprising —expanded-spectrum ß-lactamase substrate selected from the group consisting of cephalosporins, aztreonam, and cephamycins, —a pH color indicator which will change color when the pH of the reaction mixture is comprised between 6.4 and 8.4, wherein a color change after step b) indicates the presence of expanded-spectrum ß-lactamase-producing bacteria in the test sample. The invention also relates to a reagent kit, to a microtiter plate and to their uses in detecting the presence of expanded-spectrum ß-lactamase producers in a test sample.