Virulence genes contributing to Aeromonas veronii pathogenicity in Nile tilapia (Oreochromis niloticus): approaching the development of live and inactivated vaccines

Characterization of Streptococcus agalactiae 1a isolated from farmed Nile tilapia (Oreochromis niloticus) in North America, Central America, and Southeast Asia

Streptococcosis caused by Streptococcus agalactiae 1a in Nile tilapia (Oreochromis niloticus) is a severe disease challenge for the global supply of tilapia. Currently, the extensive use of antibiotics is the primary curative tool used to minimize the impact of the disease. Vaccination is a prophylactic measure that has been shown to reduce antibiotic use in the aquaculture sector substantially. However, no commercially licensed vaccine against Streptococcus agalactiae 1a is currently available. This study aimed to investigate, through molecular and immunological methods, if Streptococcus agalactiae 1a isolates collected from North America (NAM), Central America (CAM), and Southeast Asia (SEA) were similarly suitable for the development of a potentially effective vaccine to serve the global tilapia farming industry. Our comparative data showed that the Streptococcus agalactiae 1a isolates from NAM, CAM and SEA had similar biochemical profiles, and genetic multi-locus sequence typing (MLST) analysis showed that the NAM and CAM isolates belonged to sequence type 7 (ST-7) and clonal complex 1, while isolates from SEA grouped into three sequence types (ST-1650, ST-500, and ST-7) and two distinct clonal complexes (CC1 and CC12). Isolates from NAM, CAM, and SEA displayed similar antigenic profiles determined by western blotting with polyclonal rabbit antisera, which was supported by in vivo cross-protection studies, showing that fish immunized with vaccines based on SEA and CAM isolates with different genetic MLST profiles were highly protected against cross-challenge using the same bacterial strains for challenge. Overall, the data obtained from our investigations provide strong indications that Streptococcus agalactiae 1a distributed in NAM, CAM, and SEA are serologically uniform pathogens, and vaccines based on isolates of Streptococcus agalactiae 1a from these regions may be suited for vaccination of tilapia worldwide.

Silica nanoparticles alleviate the immunosuppression, oxidative stress, biochemical, behavioral, and histopathological alterations induced by Aeromonas veronii infection in African catfish (Clarias gariepinus)

In the aquaculture industry, silica nanoparticles (SiNPs) have great significance, mainly for confronting diseases. Therefore, the present study aims to assess the antibacterial efficiency of SiNPs as a versatile trial against Aeromonas veronii infection in African catfish (Clarias gariepinus). Further, we investigated the influence of SiNPs in palliating the immune-antioxidant stress biochemical, ethological, and histopathological alterations induced by A. veronii. The experiment was conducted for 10 days, and about 120 fish were distributed into four groups at random, with 30 fish each. The first group is a control that was neither exposed to infection nor SiNPs. The second group (SiNPs) was vulnerable to SiNPs at a concentration of 20 mg/L in water. The third group was experimentally infected with A. veronii at a concentration of 1.5 × 107 CFU/mL. The fourth group (A. veronii + SiNPs) was exposed to SiNPs and infected with A. veronii. Results outlined that A. veronii infection induced behavioral alterations and suppression of immune-antioxidant responses that appeared as a clear decline in protein profile indices, complement 3, lysozyme activity, glutathione peroxidase, and total antioxidant capacity. The kidney and liver function biomarkers (creatinine, urea, alkaline phosphatase, and alanine aminotransferase) and lipid peroxide (malondialdehyde) were substantially increased in the A. veronii group, with marked histopathological changes and immunohistochemical alterations in these tissues. Interestingly, the exposure to SiNPs resulted in a clear improvement in all measured biomarkers and a noticeable regeneration of the histopathological changes. Overall, it will establish that SiNPs are a new, successful tool for opposing immunological, antioxidant, physiological, and histopathological alterations induced by A. veronii infection.

Comparison of the efficacy of Aeromonas veronii ΔhisJ vaccine in Carassius auratus via different immunization routes

Introdction

Aeromonas veronii is a significant pathogen to various aquatic life. Infections in fish can lead to high mortality rates, causing substantial economic losses in aquaculture. Vaccination is proposed as a substitute for antibiotics in aquaculture to decrease disease-related mortality and morbidity. Our study previously constructed a hisJ-deleted strain of A. veronii, which provided protective effect to Loach.

Methods

To further assess the vaccine's applicability, this study evaluated its genetic stability and safety, and the immune protective effects in Carassius auratus through four distinct administration routes: intraperitoneal injection, intramuscular injection, oral administration, and immersion, to determine the efficacy of these administration routes.

Results

The results showed that the vaccine remained genetically stable after 45 generations. Immunization via these administration routes was safe for Carassius auratus, with intraperitoneal and intramuscular injections causing stronger adverse reactions. Immersion immunization resulted in mild adverse reactions, and no significant adverse reactions were observed following oral immunization. Immunizing Carassius auratus at safe concentrations via these routes enhanced the phagocytic activity in serum, increased the levels of non-specific immune-related enzymes (ACP, AKP, C3, C4, LZM, SOD, and IgM), and improved specific serum antibody levels. It also elevated levels of cytokines related to inflammatory responses (IL-1β, IL-10, TNF-α, TGF-β) in organ tissues (liver, spleen, kidney, mid-post intestine, and gills). The survival rates of Carassius auratus were measured after challenging with the virulent strain A. veronii TH0426, resulting in the relative survival rates of 64% for Intraperitoneal vaccine group, 56% for Intramuscular vaccine group, 52% for oral vaccine group, and 48% for immersion vaccine group. Analysis of bacterial load in the liver, spleen, and kidney post-challenge showed a decreasing trend in the control group, indicating that the vaccine strain ΔhisJ could gradually restrict the rapid proliferation of bacteria in these tissues, thereby providing a certain level of immune protection against A. veronii.

Discussion

In brief, the vaccine strain ΔhisJ can serve as a safe live attenuated vaccine for Carassius auratus, and this study lays the foundation for the development of live attenuated vaccines against Aeromonas veronii.

Cloning and characterization of an aerolysin gene from a marine pathogen Vibrio splendidus

Vibrio splendidus is one of the main pathogens caused diseases with a diversity of marine cultured animals, especially the skin ulcer syndrome in Apostichopus japonicus. However, limited virulence factors have been identified in V. splendidus. In this study, one aerAVs gene coding an aerolysin of V. splendidus was cloned and conditionally expressed in Escherichia coli. The haemolytic activity of the recombinant AerAVs was analyzed. Western blotting was used to study of the secretion pathway of proaerolysin, and it showed that the proaerolysin was secreted via both outer membrane vehicles and classical secretion pathways. Since no active protein of aerolysin was obtained, one aerolysin surface displayed bacterium DH5α/pAT-aerA was constructed, and its haemolytic activity and virulence were determined. The results showed that the AerAVs displayed on the surface showed obvious haemolytic activity and cytotoxic to the coelomocyte of A. japonicus. Artificial immerse infection separately using the DH5α/pAT or DH5α/pAT-aerA was conducted. The result showed that the mortality percent of sea cucumber A. japonicus challenged with DH5α/pAT-aerA was 38.89 % higher than that challenged with the control strain DH5α/pAT, and earlier death occurred. Combined all the results indicates that aerolysin with the haemolytic activity and cytotoxic activity is a virulence factor of V. splendidus.