Abstract: The invention provides a method for detecting bacteria. The method utilises a peptide that forms a complex with a conjugated reporter polymer and is susceptible to cleavage by one or more proteases on the surface of a bacteria. Presence or absence of the bacteria can be determined by assessing the optical absorption and/or colour and/or photoluminescence (e.g. fluorescence) of the conjugated reporter polymer, which may undergo a conformational change after binding with the to the cleaved peptide substrate. Specifically, the peptide substrate may comprise a cleavage site for digestion by the protease, and the protease may be an omptin protease. The conjugated reporter polymer may be selected from a polythiophene, a poly(1,4-phenylene vinylene) (PPV), a poly(1,4-phenylene) (PPP), a polyfluorenes (PFO), a nitrogen-containing polymer such as polyquinoline, poly(2,5-pyridinevinylene) (PPyV), 1,3,4-oxadiazole, and poly(9-vinylcarbazole) (PVK), and a polypyrrole.