PCR detection and PFGE genotype analyses of streptococcal clinical isolates from tilapia in China

Large-scale streptococcal outbreaks occurred continuously in tilapia farms of China from 2009 to 2011. The objective of this study was to characterize the prevalent strains of tilapia streptococci from the main cultured areas of China through species specific PCR and pulse field gel electrophoresis (PFGE). A total of 105 prevalent strains were isolated from Guangdong, Guangxi, Hainan and Fujian provinces between 2006 and 2011, 85 of which were identified as Streptococcus agalactiae while the rest were all identified as Streptococcus iniae. The prevalent stains in 2006 and 2007 were S. iniae (94.7%, 18/19), with S. agalactiae account for only 5.3% (1/19); The prevalent strains in 2009 and 2011 however changed to S. agalactiae (97.7%, 84/86), with only 2.3% (2/86) was S. iniae. Of these 105 strains, a total of 13 PFGE types (A-M) were characterized, among which D, F, G and K genotypes were predominant, accounting for 81.90% (86/105). The cluster analysis of PFGE electropherograms separated S. iniae and S. agalactiae to two distinctive branches, 20 strains of S. iniae exhibiting 3 types of PFGE band patterns with a similarity of 94.8-100%, and the 85 strains of S. agalactiae producing 10 types of PFGE band patterns with a similarity between 48.4% and 100%. Data suggested that the prevalent strains of tilapia streptococci in China have shifted from the former (before 2008) dominant strains of S. iniae to the current (2009-2011) dominant strains of S. agalactiae. Moreover, PFGE genotypes of the prevalent strains demonstrated geographic differences and temporal changes.