Comparative Pathogenicity of spp. in Cultured Red Hybrid Tilapia ().

The genus has been recognised as an important pathogenic species in aquaculture that causes motile septicaemia (MAS) or less severe, chronic infections. This study compares the pathogenicity of the different spp. that were previously isolated from freshwater fish with signs of MAS. A total of 124 isolates of spp. were initially screened for the ability to grow on M9 agar with -inositol as a sole carbon source, which is a discriminatory phenotype for the hypervirulent (vAh) pathotype. Subsequently, LD of six selected spp were determined by intraperitoneal injection of bacterial suspension containing 10, 10, and 10 CFU/mL of the respective sp. to red hybrid tilapias. The kidneys, livers and spleens of infected moribund fish were examined for histopathological changes. The screening revealed that only 1P11S3 was able to grow using -inositol as a sole carbon source, and no vAh strains were identified. The LD of 1P11S3 was 10 CFU/mL, while the non--inositol utilizing 4PS2 and 8TK3 was lower at 10 CFU/mL. Similarly, tilapia challenged with the -inositol 1P11S3 showed significantly ( < 0.05) less severe signs, gross and histopathological lesions, and a lower mortality rate than the non--inositol 4PS2 and 8TK3. These findings suggested that -inositol utilizing 1P11S3 was not a hypervirulent sp. under current experimental disease challenge conditions, and that diverse spp. are of concern in aquaculture farmed freshwater fish. Therefore, future study is warranted on genomic level to further elucidate the influence of -inositol utilizing ability on the pathogenesis of spp., since this ability correlates with hypervirulence in strains.