Potential of Bacteriophage as Therapeutic Agent against Experimentally Streptococcus agalactiae Infection in Tilapia (Oreochromis niloticus)

A lytic bacteriophage, PUB 01-1 was isolated from tilapia rearing pond water using Streptococcus agalactiae UB 01 as a host bacterium. The genome of the phage was double stranded DNA with a size of about 58 kb. Transmission electron microscopy revealed that the phage had an icosahedral head of 65 nm in diameter and a long non-contractile tail of about 270 nm long and 12 nm wide. It was classified as a member of the family Siphoviridae. All 6 strains of S. agalactiae examined were sensitive to the phage but the rest of the tested bacteria were not. The median lethal dose (LD50) of S. agalactiae UB 01 for tilapia (Oreochromis niloticus) by intraperitoneal injection was 1.58 x 104 CFU/fish. The titer and retention time of the phage in kidney of uninfected tilapia receiving diet supplemented with 103 PFU/g of diet (diet 1) were lesser than those of uninfected fish receiving diet supplemented with 108 PFU/g of diet (diet 2). The presence of S. agalactiae UB 01 in fish increased the titer and retention time of the phage in fish kidneys. Diets 1 and 2 could reduce mortality of tilapia infected with S. agalactiae UB01 at a LD50. Diet 2 resulted in no mortality while 30% mortality was observed in the group receiving diet 1. These results indicate that phage PUB 01-1 has the potential as a therapeutic agent for controlling fish disease caused by S. agalactiae.