Rapid Assays for the Detection and Determination of Sparse Populations of Bacteria and Bacteriophage T7 with Radioactively Labelled Homologous Antibodies

SUMMARY: Small numbers of several bacterial species were rapidly and specifically detected with the 125I-labelled antibody method described previously. Multibacterial species detection was achieved in one operation with 125I-labelled mixtures of type-specific anti-bacterial globulins but the level and variability of the blank value increased, and sensitivity decreased, with the number of globulin components in the mixture. An indirect radio-assay that involved successive treatments of bacteria with unlabelled rabbit anti-bacterial serum and 125I-labelled immunopurified goat anti-rabbit globulin was less sensitive and reproducible than the direct radio-assay. A modified assay, developed to detect and determine bacteria filtered on to a membrane filter, allowed the detection of small numbers of bacteria in large volumes of aqueous sample. The feasibility of rapidly and specifically detecting small numbers of virus particles with 125I-labelled anti-viral globulin was investigated with bacteriophage T7 as a model particle. Methods for the rapid separation of phage-125I-labelled globulin complex were developed and a minimum of about 5 × 105 total phage particles was detected.