Construction of Streptococcus agalactiae sialic acid mutant and evaluation of its potential as a live attenuated vaccine in Nile tilapia (Oreochromis niloticus)

BNC-rSS, a bivalent subunit nanovaccine affords the cross-protection against Streptococcus agalactiae and Streptococcus iniae infection in tilapia

Streptococcal disease has severely restricted the development of global tilapia industry, which is mainly caused by Streptococcus agalactiae (S. agalactiae) and Streptococcus iniae (S. iniae). Vaccination has been proved to be a potential strategy to control it. In this study, a multi-epitope subunit vaccine Sip-Srr (SS) was prepared based on the B-cell antigenic epitopes prediction and multiple sequence alignment analysis of Sip and Srr sequences. Furthermore, the BNC-rSS nanocarrier vaccine system was constructed by connecting the rSS protein with modified bacterial nanocellulose (BNCs) and characterized by Fourier Transform Infrared Spectroscopy and Scanning Electron Microscope, the immersion immune effect against S. agalactiae and S. iniae infection was evaluated. The results showed that compared with the control group, BNC-rSS significantly enhanced serum antibody production, related enzyme activities and immune-related genes expression. It was noteworthy that BNC-rSS vaccine improved immune protection of tilapia, with survival rates of 66.67 % (S. agalactiae) and 60.00 % (S. iniae), respectively, compared with those of rSS vaccine (30 % and 33.33 %, respectively). Our study indicated that the BNC-rSS nanovaccine could elicit robust immune responses in tilapia by immersion immunization, and had the potential to offer cross-protection against S. agalactiae and S. iniae infection in tilapia.

Attenuated Streptococcus agalactiae WC1535 ∆Sia perturbs the gut microbiota of Oreochromis niloticus, massively colonizes the intestine, and induces intestinal mucosal immunity after intraperitoneal inoculation

We previously developed and assessed the effectiveness of the attenuated Streptococcus agalactiae (Group B Streptococcus, GBS) strain WC1535 ∆Sia (with neuA-D gene cluster deletion) vaccine in tilapia (Oreochromis niloticus). In this study, we characterized the bacterial communities of the tilapia intestines by 16S rRNA high-throughput sequencing and assessed the serum antibody response, expression of immune-related genes, and histological changes following formalin-killed GBS vaccine (FKV) and the live attenuated vaccine ∆Sia (LAV). Results showed that FKV and LAV induced robust systemic and intestinal mucosal immune responses in tilapia without causing obvious pathological changes in the hindgut, spleen, and head kidney but exerted different effects on intestinal bacterial communities. The richness or diversity of the intestinal bacterial community of FKV tilapia showed no significant changes compared with that of the control fish (p > 0.05) at either day 21 post-initial vaccination (21 dpiv) or day 35 (day 14 after the second immunization) (35 dpiv). The community composition of FKV tilapia and controls was significantly similar, although the relative abundance of some genera was significantly altered. Relative to control fish, the gut ecosystem of LAV tilapia was significantly disturbed with a substantial increase in community diversity at 21 dpiv (p < 0.05) and a significant decrease at 35 dpiv in fish with high serum antibody response (ΔSia35H) (p < 0.05). However, there was no significant difference between ΔSia35H and ΔSia35L (low serum antibody response) fish (p > 0.05). Moreover, the community composition of LAV tilapia at 21 dpiv or 35 dpiv was considerably different from that of the controls. Particularly, GBS ∆Sia was found to be abundant in the intestine at 21 and 35 dpiv. This result suggested that the parenteral administration of the LAV (∆Sia) may also have the effect of oral vaccination in addition to the immune effect of injection vaccination. In addition, a significant correlation was found between the expression of immune-related genes and certain bacterial species in the intestinal mucosal flora. Our findings will contribute to a better understanding of the effects of inactivated and attenuated vaccines on gut microbiota and their relationship with the immune response.

Current status and development prospects of aquatic vaccines

Diseases are a significant impediment to aquaculture's sustainable and healthy growth. The aquaculture industry is suffering significant financial losses as a result of the worsening water quality and increasing frequency of aquatic disease outbreaks caused by the expansion of aquaculture. Drug control, immunoprophylaxis, ecologically integrated control, etc. are the principal control strategies for fish infections. For a long time, the prevention and control of aquatic diseases have mainly relied on the use of various antibiotics and chemical drugs. However, long-term use of chemical inputs not only increases pathogenic bacteria resistance but also damages the fish and aquaculture environments, resulting in drug residues in aquatic products, severely impeding the development of the aquaculture industry. The development and use of aquatic vaccines are the safest and most effective ways to prevent aquatic animal diseases and preserve the health and sustainability of aquaculture. To give references for the development and implementation of aquatic vaccines, this study reviews the development history, types, inoculation techniques, mechanisms of action, development prospects, and challenges encountered with aquatic vaccines.