Sample preparation unit, preferably for sterility testing: a housing body including at least two ports adapted to serve as fluid inlet and/or fluid outlet, a membrane support, and a lid part such that a membrane chamber is defined adjacent said membrane support. One of at least two ports is arranged so as to allow a fluid transfer to/from a first volume of the membrane chamber at a position upstream of a membrane to be placed on the membrane support, and the other ports arranged to allow fluid transfer to/from a second volume of said membrane chamber at a position downstream of a membrane to be placed on said membrane support. A movable part is provided on housing body such that the movable part and housing body are movable relative to each other, selectively interrupting/establishing fluid transfer between at least one of two ports and the membrane chamber.