Comparison of the host-pathogen interactions between Pseudomonas plecoglossicida and Nocardia seriolea in a cell line derived from head kidney of yellow large croaker

Nocardia seriolae and Pseudomonas plecoglossicida are two important pathogenic bacterial species in aquaculture, causing visceral granulomatous disease in large yellow croaker (Larimichthys crocea) and other economically important fish species. However, the difference of host-pathogen interactions between these two bacteria have not been well defined. In present study, ultrastructural changes, cell cytotoxicity, production of reactive oxygen species (ROS), apoptosis, ferroptosis, and transcriptomic profiles induced by P. plecoglossicida and N. seriolae were evaluated in a head kidney originating cell line from yellow large croaker (LYC-hK). Transmission electron microscope showed that both P. plecoglossicida and N. seriolae were able to invade into and multiply intracellularly, but display different intracellular ultrastructural changes upon bacterial infection. Lactate dehydrogenase assay showed that both two bacteria had low cell cytotoxicity toward LYC-hK cell, compared to the cytotoxic positive bacterium, Photobacterium damselae subsp. damselae. Flow cytometry detection showed that N. seriolae had significant stronger ability to trigger ROS production and apoptosis in LYC-hK than that of P. plecoglossicida. Intracellular reduced glutathione (GSH), total glutathione (GSH + GSSG) and Fe²⁺ measurements showed that N. seriolae infection led to an extremely low GSH or GSH + GSSG level and significant higher Fe²⁺ level in LYC-hK. Moreover, comparative transcriptomes by RNA-sequencing revealed some same and different cellular and molecular responses in LYC-hK after N. seriolae or P. plecoglossicida infection. These findings will provide new insight into understanding the pathogenesis of fish visceral granulomatous disease caused by these two bacterial pathogens.