Identification of protective protein antigens for vaccination againstStreptococcus dysgalactiaein cobia (Rachycentron canadum)

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Streptococcus dysgalactiae is considered a causative agent of severe infection and economic loss for the cobia industry in Taiwan. In this study, protective antigens of this pathogenic bacterium were identified and screened in cobia (Rachycentron canadum). Outer surface proteins (OMPs) of this pathogen were extracted using mutanolysin digestion. Immunogenic targets were detected by western blot and then subjected to peptide sequencing using NanoLC-MS/MS. Two surface proteins, namely phosphoenolpyruvate protein phosphotransferase (PtsA) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), showed strong reactions with cobia antisera against S. dysgalactiae. Recombinant proteins were produced in Escherichia coli cells and their protective efficacies were investigated in cobia. Fish immunised with recombinant proteins, rPtsA + ISA (ISA 763 AVG) and rGAPDH + ISA, elicited higher levels of specific antibody responses against the recombinant proteins and had high levels of lysozyme activity. Notably, vaccinated fish were protected from lethal challenge with relative percentage of survival (RPS) values for rPtsA + ISA and rGAPDH + ISA groups being 91.67% and 83.33%, while 0% RPS value was found in both ISA injected and control groups. The results presented in the study demonstrate that the GAPDH and PtsA are promising vaccine candidates for preventing S. dysgalactiae disease in cobia.HighlightsSurface proteins of Streptococcus dysgalactiae were isolated using mutanolysin digestion.Two surface immunogenic proteins, PtsA and GAPDH, were identified by western blot and NanoLC-MS/MS.PtsA and GAPDH recombinant proteins were produced using E. coli expression system.Cobia immunised with rPtsA and rGAPDH induced higher lysozyme activity, antibody titers, and protected from S. dysgalactiae lethal challenge with 91.67% and 83.33% RPS values, respectively.

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