Occurrence of rare genotypes of Streptococcus agalactiae in cultured red tilapia Oreochromis sp. and Nile tilapia O. niloticus in Thailand—Relationship to human isolates?

Streptococcus agalactiae (Group B streptococcus, GBS) is an important pathogen affecting humans and animals as well as aquatic species. Between April 2003 and March 2006, bacteria were isolated from infected red tilapia Oreochromis sp. and Nile tilapia O. niloticus cultured in Thailand. They were identified as GBS by API 20 STREP, polymerase chain reaction (PCR) and multiplex PCR-based reverse line blot hybridization (mPCR/RLB). Genotyping indicated that they belonged to two genotypes. One of these genotypes has not been identified in any of 1792 GBS isolates genotyped by mPCR/RLB; it belongs to serotype Ia and contains genes encoding proteins Cα (bca) and Cβ (bac), three insertions sequences (IS1381, IS861 and ISSag2) and the group II intron GBSi1. The other has recently been identified among invasive human isolates in Hong Kong; it belongs to an uncommon subtype of serotype III (III-4), contains bca, three insertion sequences (IS1381, ISSag1 and ISSag2) and a tetracycline resistance gene (tetM). Pathogenicity studies in red tilapia by experimental infection established that GBS is a pathogen of fish. Histopathological changes indicated that infection of tilapia with GBS produced disease with systemic involvement characterized by multiple necrotic foci in various tissues. The possible epidemiology and epizoology of these bacteria among human, mammal and aquatic species are discussed in this report.