**6.5 Bacteriophage as therapeutic agent**

The lytic effects of bacterial deposition on *Staphylococcus aureus* isolates in milk have been investigated in vitro, and their possible applications in the treatment of udder infections caused by different bacteria have been discussed. The host range of the sequenced lytic phage was determined for 92 strains of *Staphylococcus aureus*. These isolates were taken from a quarter of the forehead samples in cases of clinical and subclinical mastitis. A point test followed by plaque analysis is used to determine the range of phage hosts. Three bacterial products STA1, ST29, EB1, ST11 and EB1, ST27 were selected according to host range, reproductive properties and storage properties to prepare a phage mixture (1: 1: 1) and tested for their lytic activity against *Staphylococcus aureus* in cold sterilized raw milk. It has been found that at least two-thirds of the phage can lyse almost two-thirds of the isolate. The phage mixture can reduce the density of *Staphylococcus aureus* bacteria in cold sterilized milk and retain their regenerative capacity in raw milk. Compared to pasteurized milk, the regenerative capacity is only moderately reduced. The significant decreasing capacity of the mixture of phages in raw milk facilitated further in vivo studies [62].
