The invention relates to a mechanical support for a drainage device of a filter unit for microbiological testing of liquid substances. Said filter unit is provided with a membrane and adapted to be mounted on said mechanical support so that said membrane faces a reception surface of said mechanical support. The mechanical support comprises: a passage one end of which discharges externally of said reception surface and the other end of which discharges internally said reception surface; and selectively operable means for closing said passage, movable between a first position in which they close said passage and a second position in which they open said passage.