A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Aquaculture and Aquatic Resources Management

Thesis (M. Sc.) - Asian Institute of Technology, 2024

Streptococcus agalactiae is a highly pathogenic bacterium that causes significant mortality and severe economic losses in tilapia aquaculture. The current understanding of S. agalactiae infection in tilapia fry remains limited. Therefore, this research aimed to investigate the phenotypic characteristics of S. agalactiae, its serotypes, and the antimicrobial susceptibility of bacterial isolates recovered from tilapia broodstock and fry. Subsequently, we assessed the pathogenicity of the different serotypes in tilapia fry through an immersion challenge experiment. Phenotypic characterization showed that all ten isolates (6 from fry and 4 from broodstock) displayed Þ-hemolysis. Antimicrobial susceptibility test revealed that the isolates were resistant to Neomycin and Nalidixic acid, but were susceptible to Amoxicillin, Doxycycline hydrochloride, Erythromycin, Norfloxacin, and Ciprofloxacin. Molecular serotyping identified serotypes Ia and III, with all isolates from broodstock classified as serotype III and those from fry as serotype Ia. The immersion challenge experiment demonstrated that serotype Ia exhibited significantly (p<0.01) higher virulence compared to serotype III, resulting in acute mortality in tilapia fry, while serotype III caused a delayed mortality pattern. This research offers a better understanding of the pathogenicity of S. agalactiae serotypes in tilapia fry, which may contribute to the development of bivalent vaccines to combat streptococcosis in the future.