Abstract: It is essential to have efficient, simple, quick and transportable tools for reliably identifying bacteria that are multiresistant to antibiotics, more specifically extended spectrum ?-lactamase (ESBL)-producing Enterobacteriaceae, which are the most widespread among Enterobacteriaceae. The present invention meets this requirement through its ease of use and its speed. The invention is based on detecting the enzyme activity of ?-lactam hydrolysis using an antibody capable of discriminating between the intact form of the ?-lactam ring of a ?-lactam and its hydrolysis product. This antibody can be used in kits and methods enabling for rapidly detecting (in less than one hour), without using expensive equipment (a small strip visible to the naked eye), the presence of bacteria producing penicillin-type, plasmid-mediated or hyper-produced AmpC enzymes, of ESBL or carbapenemase from colonies or in a sample.