Plaque assay was performed by using the double layer agar plate method as described by Lu et al. with some modifications. Briefly, a bacteriophage containing sample was subjected to ten-fold dilution with BHI broth. Each bacteriophage dilution (0.1 ml) along with an equal volume of the log phase host cell (108 colony forming unit (CFU)/ml) were added to a tube containing 4.8 ml of sloppy BHI agar (pre-warmed to 50°C). The mixture was mixed thoroughly and overlaid onto the surface of a BHI agar plate. After incubation at 37°C for 24 h, lysis plaques on host bacterial lawn were observed. For calculation of bacteriophage titer, plaques were counted in the plate containing 50-300 plaques and expressed as plaque forming unit per milliliter (PFU/ml).