Advantages of selective medium for surveillance of Tenacibaculum species in marine fish aquaculture

Abstract

We evaluated the suitability of three diagnostic techniques to detect Tenacibaculum maritimum in farmed Chinook salmon (or ‘king’ salmon), Oncorhynchus tshawytscha, exhibiting clinical signs of tenacibaculosis with skin spots and ulcers. A selective bacterial culture technique using Marine Shieh's Selective Medium (MSSM) found 100% detection of T. maritimum in sampled farmed O. tshawytscha. In comparison, pathogen detection using a non-selective culture medium (Marine Agar) revealed 33.3% detection of T. maritimum, while a droplet digital polymerase chain reaction (ddPCR) specific assay provided 81.5% positive detection. Skin abnormality type was a significant predictor of ddPCR detection, with spots less likely to be found positive compared to ulcers (p = 0.01). This study also reports the first isolation of co-occurring Tenacibaculum soleae and Tenacibaculum dicentrarchi from skin abnormalities of farmed O. tshawytscha in New Zealand. The 16S ribosomal RNA sequences amplified from representative isolates shared 100% and 99% identities with T. soleae and T. dicentrarchi 16S rRNA sequences, respectively. T. soleae and T. dicentrarchi colonies were morphologically distinct from T. maritimum. The MSSM culture technique facilitated rapid recovery of all three Tenacibaculum species within 48 h when incubated at room temperature (20–23 °C). In this study we discuss the advantages of a selective medium for surveillance of pathogenic Tenacibaculum species that infect fish when used in combination with confirmatory identification by molecular techniques, in contrast to relying only on PCR based detection methods. We suggest that culture dependent diagnostic techniques provide sufficiently sensitive, rapid, and cost-effective on-farm screening as this method detects only viable organisms which pose immediate risk to fish. The rapid identification and management of emerging fish-pathogenic species of Tenacibaculum in New Zealand is essential for the diagnosis of tenacibaculosis outbreaks which can impact farm production and animal welfare.