Explain the Polycarbonate Nucleopore Filtre?

A given sample is filtered through a membrane filter, usually polycarbonate nucleopore filter with a pore size that will retain bacteria (generally 0.22 µm or 0.45 µm). Viable counts are made by placing the filter on agar medium plate or on an absorbent pad saturated with the culture medium. Alternatively, direct microscopic counts are made, the efficiency of which can be improved by using fluorescent dyes and epifluorescence microscope. Membrane filter technique can be used only for small samples of dilute homogenates.