Abstract: The device includes a first body including an inlet orifice, an outlet orifice (21), a filter membrane (3), a support member (4) for the membrane (3) that is removable from said first body (2) and a second body (6) removably fixed to said first body (2); the outlet orifice (21) is formed at the periphery of said first body (2) and a second body (6) is primarily formed of a detachable wall, mounted by adhesion to said first body (2). The test assembly includes a device of the above kind and a clamp adapted to grip said device n the manner of a vice. The incubation assembly includes a device of the above kind and a gel growth medium cassette. The method includes steps of mounting a device of the above kind in a clamp, filtering a volume of liquid in the device, nesting said device to a gel growth medium cassette and waiting for incubation to take place.