Abstract: The invention provides a method for analyzing for bacteria in a liquid sample comprising the following steps: a) contacting said sample with a substrate capable of retaining bacteria or bacterial fragments; b)staining any bacteria retained on said substrate with at least one solution comprising a first dye, said staining being conducted at a pH at which the dye has an overall positive charge and the bacteria has an overall negative charge; and c) inspecting the substrate for the presence of a stain whereby to indicate the presence of bacteria in said sample; with the proviso that where said liquid sample is other than a milk sample and substrate is a filter then said filter comprises a polysulfone or a derivative of a polysulfone.