Development of a real-time PCR for detection of the oyster pathogen Nocardia crassostreae based on its homogeneous 16S-23S rRNA intergenic spacer region

Nocardia crassostreae, the causative agent of Pacific oyster nocardiosis (PON), is a Gram-positive actinomycete bacterium associated with Pacific oyster (Crassostrea gigas) mortalities. Oysters infected with this bacterium have been reported previously from the west coast of North America and Japan. More recently, N. crassostreae was reported in oyster culture areas in the Netherlands. In this study, a sensitive real-time PCR for specific detection of N. crassostreae was developed, and the intra-species divergence of N. crassostreae from different geographical locations was studied. The 16S-23S rRNA intergenic spacer (ITS) region of N. crassostreae was sequenced for a number of infected oysters originating from the Netherlands, Japan and Canada. The sequence analyses showed an absence of genetic variation in the ITS region between N. crassostreae from different geographical locations. Based on these ITS sequences a species-specific and highly sensitive SYBR Green real-time PCR assay was developed to facilitate detection of N. crassostreae in oyster tissue. To evaluate this new detection tool for N. crassostreae a preliminary validation was carried out and real-time PCR results were compared with other detection methods (histology, conventional PCR and bacterial isolation) using field samples from Lake Grevelingen, the Netherlands. The genetic homogeneity in the ITS region between N. crassostreae from different geographical locations might be explained by the recent spread of the organism via the international trade in Pacific oysters for aquaculture purposes. However, the lack of genetic variation could also suggest that N. crassostreae is a genetically monomorphic species.