Occurrence of rare genotypes of Streptococcus agalactiae in cultured red tilapia Oreochromis sp. and Nile tilapia O. niloticus in Thailand—Relationship to human isolates? Naraid Suanyuk a , Fanrong Kong b , Danny Ko b , Gwendolyn L. Gilbert b , Kidchakan Supamattaya a, ⁎ a Aquatic Animal Health Research Center, Department of Aquatic Science, Faculty of Natural Resources, Prince of Songkla University, Hat Yai, Songkhla, 90110, Thailand b Centre for Infectious Diseases and Microbiology, Institute of Clinical Pathology and Medical Research, Westmead Hospital, Westmead, New South Wales, 2145, Australia abstract article info Article history: Received 14 June 2007 Received in revised form 5 June 2008 Accepted 16 July 2008 Keywords: Streptococcus agalactiae Tilapia Genotyping Pathogenicity 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. © 2008 Elsevier B.V. All rights reserved. 1. Introduction In Thailand, intensive fish culture has been established over the past decade. At present, more than ten species of fish are cultured, with Nile tilapia Oreochromis niloticus as the major species produced (Fishery Information Technology Center, 2004). However, the success of aquaculture may be impeded by the prevalence of infectious diseases. Among Gram-positive bacteria, Streptococcus spp. infect sand goby Oxyeleotris marmoratus (Kasornchan et al., 1986) and Asian sea bass Lates calcarifer (Danayadol et al., 2000) causing serious damage to fish farming in Thailand. Traditional classifications of streptococci have been based on the serogrouping of the carbohydrate antigens of the cell wall and their haemolytic activities. Lancefield serogroup B (GBS) corresponds to a single streptococcal species, S. agalactiae (Evans et al., 2002). GBS has been reported as an important fish (Baya et al., 1990; Duremdez et al., 2004; Evans et al., 2002; Rasheed and Plumb, 1984; Rasheed et al., 1985), bovine (Martinez et al., 2000, 2001; Meiri-Bendek et al., 2002) and human (Hickman et al., 1999; Ip et al., 2006; Persson et al., 2004; Straková and Motlová, 2004) pathogen. Based on the composition of the capsular polysaccharide, GBS can be divided into ten serotypes (Ia, Ib and II to IX) (Chaffin et al., 2000; Persson et al., 2004; Slotved et al., 2007). Only serotype Ib has been previously reported in fish (Vandamme et al., 1997). In Thailand, GBS infection in fish was reported by our group (Suanyuk et al., 2005) from infected tilapia cultured in Surat Thani province, southern Thailand. Subsequently, mortalities of cultured red tilapia Oreochromis sp. and Nile tilapia O. niloticus showing typical signs of streptococcal infection occurred in earthen ponds and cages throughout the country. Our previous study reported the biochemical characteristics and pathogenicity of these bacteria (Suanyuk et al., 2005). In this study, the distribution of GBS and their genetic diversity were investigated. GBS strains used in this study were isolated from infected tilapia cultured in Thailand including the isolates reported in our previous study. 2. Materials and methods 2.1. Fish Infection chronology, sources of isolation and a list of fish hosts are summarized in Table 1 . The first case occurred in 2003 in a stock of red tilapia from a commercial fish farm in Surat Thani province, southern Aquaculture 284 (2008) 35–40 ⁎ Corresponding author. Tel.: +6674 465102. E-mail addresses: snaraid@hotmail.com (N. Suanyuk), kidchakan.s@psu.ac.th (K. Supamattaya). 0044-8486/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2008.07.034 Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online