Isolation of an unusual strain of Edwardsiella tarda from turbot and establish a PCR detection technique with the gyrB gene

Aims: The aim of this study was to report an unusual Edwardsiella tarda and develop an effective method to identify this bacterium. Methods and Results: During the spring and summer of 2006, an epizootic occurred among cultured turbot (Scophthalmus maximus) in Qingdao, China. A gram-negative, rod-shaped bacterium (designated as LTB-4) was isolated from the infected fish, and was proved to be virulent to turbot. Based on the 16S rDNA sequencing and phenotypic tests, the bacterial pathogen was identified as E. tarda. Unlike those commonly described E. tarda strains, no flagellate was observed. Partial gyrB genes were amplified from E. tarda using the universal primers of gyrB genes and sequenced. The polymerase chain reaction (PCR) primers for the gyrB gene were designed specific to E. tarda. It revealed positive amplification of the gyrB fragment in E. tarda, whereas other bacterial species were negative. In addition, the technique enabled the recognition of E. tarda from diseased fish. Conclusions: The isolate was identified as E. tarda without flagellate and an effective method was developed to identify E. tarda based on using the gyrB gene as a taxonomic marker. Significance and Impact of the Study: The unusual E. tarda was first reported in China and the PCR allowed the rapid and sensitive detection of E. tarda.