Molecular characterization of Streptococcus iniae isolated from hybrid tilapia (Oreochromis niloticus × Oreochromis aureus)

Streptococcus iniae is a hemolytic, Gram-positive bacterium that causes significant disease problems in freshwater and marine fish species throughout the world. Since first diagnosed in March 1992, S. iniae has become one of the most common bacterial diseases and causes significant economic losses in cultured hybrid tilapia (Oreochromis niloticus × Oreochromis aureus) in Saudi Arabia. The specificity of the two sets of oligonucleotide primers used for PCR detection was conformed to generate specific amplicons to S. iniae. Polymerase chain reaction amplification was specific, giving rise to products of expected size for S. iniae. With the 16S rRNA gene primers (Sin-1 and Sin-2) the PCR assay confirmed and resulted in the amplification of 300 bp specific band detected among the local strain tested (AH1) and the reference strain S. iniae ATCC 29178. The primers combination of the lactate oxidase gene (lctO) (Lox-1 and Lox-2) gave the expected amplification product of 870 bp with local S. iniae (AH1) only which was identical to that given by the type strain S. iniae ATCC 29178. No specific band was amplified in other type strains, such as S. agalactiae, and Lactococcus garvieae ATCC 43921. The sequence analysis of the 16S rDNA product clearly revealed that the fish isolate was S. iniae identical and showed 99.8% similarity to the ATCC 29178 S. iniae type strain.