Characteristics of bacteria strain <em>Streptococcus iniae</em> SiTH1 causing pop-eye disease in Asian seabass (<em>Lates calcarifer</em>) in Vietnam: a candidate for inactivated vaccine production.

Abstract

Bacterial strain Streptococcus iniae SiTH1 was isolated from Asian seabass cage-cultured in Vietnam with clinical signs such as pop-eye, hemorrhage, and skin ulcers. The strain SiTH1 was identified as S. iniae by PCR analysis, using the two specific primer pairs SP1/SP2 and LOXF/LOXR, which target the ITS rDNA and lctO, respectively. Six virulence genes were identified in S. iniae SiTH1, including simA, scpI, pdi, pgm, cpsD, and sagA. High mortality was obtained following experimental challenge of Asian seabass with S. iniae SiTH1, with the 50% lethal dose (LD50) being 104.4 CFU/fish. For evaluating immune responses, fish were immunized by i.p. injection (0.1 mL/fish) with inactivated bacteria by formalin at three concentrations: 109, 1010, and 1011 CFU/mL. Expression analysis of five immune-related genes (IL-1β, IL-6, IL-10, Mx, and TNF-α) following injection revealed that at 24 hours after immunization, all five genes were regulated at different levels, related to the antigen concentration. Specifically, bacterial concentrations of 109 and 1010 CFU/mL tended to stimulate immune-related genes more compared to a higher concentration of 1011 CFU/mL. Serum antibody titers in fish tended to increase gradually with antigen concentration. The effect of vaccination, tested by challenge of fish with live bacteria, showed that 21 days after immunization, the RPS of fish vaccinated with antigen concentrations of 109, 1010, 1011 CFU/mL were 63.3%; 76.7%; 78.3%, respectively. Based on these results, the S. iniae SiTH1 strain is a good candidate for inactivated vaccine production against streptococcosis in seasbass in Vietnam.