Cytotoxicity Profiling of Tenacibaculum Extracellular Products on Chinook Salmon (Oncorhynchus tshawytscha) Gill Cell...

Tenacibaculosis is an important disease of marine fishes caused by bacteria of the genus Tenacibaculum. Pathogenic Tenacibaculum species or serotypes, including Tenacibaculum maritimum, T. dicentrarchi, T. finnmarkense and T. soleae, have been isolated from farmed Chinook salmon (Oncorhynchus tshawytscha) during disease outbreaks in Aotearoa-New Zealand. We investigated the cytotoxic effects of extracellular products (ECP) produced by these bacteria on the Chinook salmon gill epithelial cell line, OTgill1PFR. ECP from T. maritimum had the greatest cytotoxic effect on confluency and metabolism of OTgill1PFR following 24-h exposure. ECP of T. dicentrarchi also caused significant but more moderate effects, while T. finnmarkense and T. soleae had minimal effects on the cell confluency or metabolism. These in vitro results were consistent with whole-animal challenge findings, where T. maritimum caused the most severe clinical disease and mortality. A finding of note was that OTgill1PFR exposure to ECP from all Tenacibaculum species resulted in minimal detectable membrane disruption as assessed by end-point assay and continuous live-cell imaging, suggesting the observed cytotoxic effects on confluency and metabolic activity may occur via mechanisms not primarily associated with acute membrane lysis. The OTgill1PFR cell model was sensitive enough to detect both interspecific and intraspecific differences in Tenacibaculum ECP activity, proving its utility as an in vitro platform for investigating host-pathogen interactions and virulence mechanisms in salmonid tenacibaculosis.