Identification And Characterization Of A Streptococcus Iniae Strain SF1 And Its Subunit Vaccine

Streptococcus iniae is a Gram-positive bacterium and a severe aquaculture pathogen that can infect a wide range of farmed fish species. A strain named SF1 was identified from a dying Janpanese flounder. SF1, which exhibited apparent virulence in a Japanese flounder infection model and conformed to the description of S. iniae by 16S rRNA sequence analysis and API 20 Strep test. Sip11 was by a previously established exported proteins system pBU from SF1. Recombinant Sip11 was purified from Escherichia coli BL21(DE3) and tested to be an immunoprotective protein agaist SF1 in Janpanese flounder. Sip11 exhibited highly RPS (Relative Percentage Survival)≈69.7% aganist SF1. To enhance the RPS, one antigen delivery vector expressed in DH5αwas constructed, which expresses and secrets recombinant Sip11 covalently linked to a carrier protein in the form of a chimera. Vaccination of Japanese flounder with live Sip11-secreting E. coli afforded complete protection upon the fish following lethal SF1 challenge. Specific antibody production was observed in fish immunized with both Sip11 and DH5α/pTAS11, which detected from 5th week to 8th week after vaccination. These results indicate that Sip11, especially when delivered by a live bacterial carrier, is an effective vaccine candidate against SF1 infection.