The invention provides promoter specific and promoter non-specific screening assays for identifying an inhibitor of a pathogenic nucleic acid polymerase activity, e.g., an RNA polymerase derived from a phatogenic infectious organism such as a bacterium, protozoan or fungus. Generally, the methods involve forming a mixture of nucleotide, a polynucleotide template, a pathogenic polymerase, and a candidate inhibitor of polymerase activity, where at least one of the nucleotide comprises a detectable label. In a preferred embodiment, a membrane apparatus (10) having two filters extending transversely across a well, and positioned between the top, tube containing half (11) of the apparatus (10) and the bottom, drip director containing half (15) of the apparatus (10). The top filter (13) is initially hydrophobic and capable of retaining liquid media and is rendered water permeable by contacting it with an organic solvent. The bottom filter (14) is hydrophilic water permeable and efficiently passes liquid media while retaining micro-beads during vacuum filtration.