Evaluation of therapeutic potential of Streptococcus phage-1A against Streptococcus agalactiae infection in Nile tilapia (Oreochromis niloticus)

Streptococcus agalactiae is a highly invasive pathogen causing streptococcosis in aquacul-ture, globally. The potential use of lytic bacteriophage is considered as safe antimicrobi-als against the milieu of global antibiotic resistance in treating bacterial infections. Hence, in this study, a novel bacteriophage, Streptococcus phage-1A was isolated from infected tilapia farm against S. agalactiae. In the morphometric analysis by HR-TEM, Strepto-coccus phage-1A exhibited an isometric head (101.52 +/- 2 nm) and short contractile tail (62.60 +/- 1 nm), of the family Myoviridae. The phage has a short latent period (20 min) and high burst size of about 119 PFU/cell. PCR-based screening revealed its obligate lytic nature and could reveal the genomic diversity of the phage by RAPD. The phage was found to be stable at varying range of temperature, pH, and salinity. Protective efficiency of phage-1A was evaluated against S. agalactiae by challenge experiment in tilapia by 3 modes of application viz. IP, oral, and bath treatments. During the challenge experiment, it was found that there was a significant decrease in the count of S. agalactiae in phage treated fish organs compared to that of controls. Among the treatment trials, bath ther-apy was found to be more effective with 80.01% RPS comparing to IP (70.02%) and oral (50.04%) mode of administration. This is the first report on isolation, characterization, and application of bacteriophage against S. agalactiae from India and the Streptococcus phage-1A isolated in this study could demonstrate its high protective ability against streptococcal infection in tilapia.