Identification and characterization of Streptococcus iniae from farmed American Bullfrogs (Aquarana catesbeiana)

The American bullfrog (Aquarana catesbeiana) is an economically important aquaculture species in China. Intensive high-density breeding of American bullfrogs has caused a variety of diseases. In this study, we isolated a dominant strain, STR202108, from the viscera of American bullfrogs raised on a bullfrog farm in Guangzhou, Guangdong Province, China, and affected by a disease outbreak that has led to many deaths. We identified the strain as Streptococcus iniae, on the basis of morphological, biochemical and phylogenetic analyses. Regression infection experiments using strain STR202108 indicated that the clinical signs in artificially infected bullfrogs were the same as those in naturally diseased bullfrogs. The typical clinical signs were hepatomegaly with white necrosis, and renal and intestinal bleeding. Histopathological examination indicated that strain STR202108 damaged multiple organs, such as the liver and intestines, in bullfrogs. Drug sensitivity tests showed that STR202108 was sensitive to tetracyclines, macrolides and β-lactam antibiotics. Challenge with high concentrations of the isolated strain STR202108 resulted in a high mortality rate in zebrafish, but no clear clinical signs or death in mice. This study is the first report of Streptococcus iniae infection in American bullfrogs. These results may aid in elucidating the pathogenicity of Streptococcus iniae in a variety of animals, and in controlling and preventing Streptococcus iniae infections in American bullfrogs.