Lactococcus garvieae is one of the primary pathogens causing the severe disease to golden pompano ( Trachinotus ovatus ). Vaccination is the most effective measure of preventing the occurrence and spread of infectious diseases. However, no commercially available vaccine protects T. ovatus against L. garvieae infection. In the present study, the formalin-inactivated L. garvieae vaccines, consisting of bacterin alone (aqueous vaccine) and combined with oil adjuvant Montanide™ ISA 763 A VG (vaccine+763A), were prepared and administered to T. ovatus by intraperitoneal injection. The protective efficacy was evaluated by testing the relative percent survival (RPS), serum enzyme activity, and expression levels of immune-related genes in the tissues of T. ovatus after immunization. The results indicated that the relative percentage survival (RPS) values of aqueous vaccine and vaccine+763A were 80% and 90.8% at 8 weeks post-vaccination. The level of antibody titer, lysozyme activities, superoxide dismutase, and catalase in the two vaccinated fish groups increased significantly compared to that in the control group. Moreover, the expression levels of immune-related genes IgM , TNF-α , IL-8 , MHCI-α , MHCII , CD4 , CD8α, and IL-1β in all the tested tissues from T. ovatus were significantly enhanced after inoculation with aqueous vaccine and vaccine+763A. These findings demonstrated that the developed vaccine combined with oil adjuvant ISA 763 A VG improved immune responses and provided good protective efficacy against L. garvieae infection in the golden pompano culture.

Japanese seabass (Lateolabrax japonicus) is an important species of cultured marine fish with high economic value in China. Nevertheless, from May to November 2019, mass mortality among cultured Japanese seabass occurred in Zhuhai City, Guangdong Province of China. Approximately 0.2–0.5% mortality was recorded daily, and the cumulative mortality was up to 30% during this disease outbreak. In this study, the clinical signs and pathological characteristics of diseased fish were investigated. Furthermore, the pathogenicity and antibiotic sensitivity of identified pathogenic bacteria from diseased fish were analyzed. The infected fish showed clinical signs of uncoordinated swimming; anorexia; pigment changes; and a number of 1–5 mm grayish-white nodules in the liver, spleen, and kidney tissues was also found. A bacterial strain, which was designated as ZHLJ2019, was isolated from the diseased fish. To ensure that ZHLJ2019 isolate was the causative agent, a Koch postulate trial was performed. Healthy Japanese seabass were infected by the intraperitoneal injection of 5 × 10⁴, 5 × 10⁵ and 5 × 10⁶ CFU/fish, and cumulative mortalities within 42 days were 75%, 90%, and 100%, respectively. The bacteria colony had traditional morphological and biochemical characteristics similar to that of Mycobacterium marinum. Phylogenetic molecular analyses of 16S rRNA, rpoB, hsp65, erp, and ITS genes confirmed that the isolated strain ZHLJ2019 was M. marinum. The granulomatous inflammation in internal organs of Japanese seabass naturally and experimentally infected with ZHLJ2019 isolate was consistent with the classic pathological features of mycobacteriosis. Drug susceptibility of ZHLJ2019 isolate to 11 antibiotics was determined by broth dilution method in vitro. The minimum inhibitory concentrations (MICs) of minocycline, rifampicin, ethambutol, isoniazid, streptomycin, doxycycline hydrochloride, kanamycin sulfate, levofloxacin, roxithromycin, and prothionamide against the strain ZHLJ2019 were 4, 2, 8, 4, 16, 8, 8, 8, 4, and 8 μg/mL, respectively. The results of this study suggest that M. marinum is the causal agent responsible for the morbidity and mortality of Japanese seabass cultured in intensive brackish water dirt ponds in southern China.