Screening vaccine candidate strains against Streptococcus agalactiae of tilapia based on PFGE genotype

Abstract

The immunogenicity identification of epidemic strain is important for the development and application of vaccine. In this study, 85 Streptococcus agalactiae prevalent strains from the tilapia main cultured areas of China were distributed among 10 distinct PFGE genotypes (A–J). For each genotype, one representative strain (S.aA–S.aJ) was selected to develop an inactivated whole-cell bacterial vaccine (VA–VJ), which then underwent a protective immunity test. VA–VJ showed similar relative percent survival (RPS) to the homologous or heterologous strains with the identical genotype, while the average RPS among VA–VJ protecting against itself genotype strains showed large differences (44.71–98.81%). The RPS of VA–VJ vaccinated fish against infections by the mixture of S.aA–S.aJ at 15 days post vaccination (dpv) was ranged from 13.33% to 60.00%, and VB, VD, VF, and VG showed the highest RPS of 60.00%, 46.67%, 53.33% and 60.00% respectively. VB, VD and VG have their own specific protection scope, VB showed strong protective immunity to infections caused by A–D, F, G and J (53.57–100%), and VG showed strong protective immunity to C–H and J (50.00–100%), whereas VD showed weak protective immunity to all non-self genotype strains (14.81–36.67%). The results of the combined vaccination showed that VG+VB group had wider protection scope and higher RPS value than VG+VD group. Our results demonstrated that the protective immunity of S. agalactiae from tilapia was not only associated with their serotypes, but also related to their PFGE genotypes. It is difficult to acquire a single vaccine candidate strain that can protect against all genotype strains from the same serotype.