The objective of this study was to determine the causative agent of an outbreak with clinical signs similar to those of piscine streptococcosis in farmed snakeskin gourami ( Trichopodus pectoralis). Initial microscopic examination revealed the predominance of a Gram-positive, cocci bacteria in the brain and kidney of the diseased fish. This bacterium was successfully isolated and identified as Streptococcus suis based on nucleotide homology of 16S rDNA and species-specific PCR. This isolate tested negative for serotype 2, one of the major zoonotic serotypes. Experimental infection was then performed to investigate the pathogenicity of the bacterium and its histopathological manifestation. Naïve juvenile and adult snakeskin gourami were injected intraperitoneally with a low dose (1.2×10 5 CFU/fish) and a high dose (1.2×10 7 CFU/fish) of S. suis. Cumulative mortality appeared to be dose- and size-dependent. Experimentally diseased fish exhibited clinical signs consistent with naturally diseased fish. Severe histopathological changes in multiple organs were observed in both juvenile and adult fish, including meningitis, severe congestion in the brain and eyes, thickened stromal layers of the retina, severe hepatic lipidosis and tissue degeneration. Notably, numerous granulomas containing massive bacterial cells in the necrotic core were observed in the infected fish. Relatively pure colonies of S. suis were recovered from tissues of experimentally diseased fish. Taken together, this study fulfilled Koch’s postulates, indicating that S. suis is a new piscine pathogen. Although this is a case report, public awareness and biosecurity measures should be considered to prevent the spread of the disease. Further surveillance of the pathogen’s distribution and research into the underlying causes of fish-host adaptation will provide insights into the genuine impact and appropriate disease control strategies.

Bacteriophage is considered an alternative to antibiotics and environmentally friendly approach to tackle antimicrobial resistance (AMR) in aquaculture. Here, we reported isolation, morphology and genomic characterizations of a newly isolated lytic bacteriophage, designated pAh6.2TG. Host range and stability of pAh6.2TG in different environmental conditions, and protective efficacy against a pathogenic multidrug-resistant (MDR) Aeromonas hydrophila in Nile tilapia were subsequently evaluated. The results showed that pAh6.2TG is a member of the family Myoviridae which has genome size of 51,780 bp, encoding 65 putative open reading frames (ORFs), and is most closely related to Aeromonas phage PVN02 (99.33% nucleotide identity). The pAh6.2TG was highly specific to A. hydrophila and infected 83.3% tested strains of MDR A. hydrophila (10 out of 12) with relative stability at pH 7 ­ 9, temperature 0 ­ 40 °C and salinity 0 ­ 40 ppt. In experimental challenge, pAh6.2TG treatments significantly improved survivability of Nile tilapia exposed to a lethal dose of the pathogenic MDR A. hydrophila, with relative percent survival (RPS) of 73.3% and 50% for phage multiplicity of infection (MOI) 1.0 and 0.1, respectively. Significant reduction of bacterial counts in rearing water at 3 h (6.7 ± 0.5 to 18.1 ± 6.98 folds) and in fish liver at 48 h post-treatment (2.7 ± 0.24 to 34.08 ± 26.4 folds) was observed in phage treatment groups while opposite pattern for bacterial counts was observed in untreated control. Interestingly, the surviving fish provoked specific antibody (IgM) against the challenged A. hydrophila. These results might explain the higher survival in phage treatment groups. In summary, the findings suggested that the lytic bacteriophage pAh6.2TG is an effective alternative to antibiotics to control MDR A. hydrophila in tilapia and possibly other freshwater fish.