The resulting filtrate, called tested filtrate, was examined for the presence of bacteriophage activity against its specific host by spot test method. Briefly, a log phase culture of S. Typhimurium was mixed gently with BHI sloppy agar (0.4% agar) and plated as a thin top layer on a BHI agar plate (1.2% agar). The plate was left at room temperature for at least 30 min to allow the top agar to solidify. A 10 ìl aliquot of the tested filtrate was spotted onto the top agar. After incubation at 37°C for 24 h, the presence or absence of a lysis zone was observed and recorded. In all cases, positive tests were confirmed by plaque assay.