Fixation filter assembly

A filter assembly for flow of filtrate comprising:

• a. an inlet and outlet for flow of first fluid filtrate containing a biological specimen containing microorganisms having a density D1 and a reservoir containing a second fluid having a density D2;
• b. an interior space for filter media, wherein said filter media divides said interior space into a first interior space proximal to said inlet and a second interior space proximal to said outlet, said first interior space opposing said second interior space relative to said filter media wherein said filter media collects said biological specimen;
• c. a reservoir plate to partition said reservoir from said second interior space wherein said reservoir plate includes at least two openings in communication with said reservoir and said second interior space, wherein said second fluid in said reservoir is a preservative for said biological specimen and is capable of exchange with said first fluid in said interior space due to differential density of said first and second fluids, wherein:
• (1) when said filtrate has a density D1 that is greater than D2, said filtrate flows out of said interior space into said reservoir and said second fluid preservative replaces said filtrate in said interior space;
• or
• (2) when said filtrate has a density D1 that is less than D2, said second fluid preservative flows from said reservoir into said interior space and said second fluid replaces said filtrate in said interior space.