The assembly comprises a filtration unit and a gel growth medium cassette, the unit and the cassette being adapted to cooperate in order to occupy a nested position in which a membrane covers the gel growth medium, the unit and the cassette each being equipped with an intermediate stop, it being possible for at least one of the two stops to move clear so as to allow the unit to move from an intermediate position, in which the membrane is spaced away from the said gel growth medium, into the nested position. The method comprises selecting such an assembly, of engaging the unit on the cassette until the intermediate stop of the unit comes into contact with the intermediate stop of the cassette, and the step of moving clear the stop able to move clear so as to allow the unit to move from the intermediate position into the nested position.