Toxin Secretion Systems Used by Marine Pathogenic Bacteria: A Review

Diseases caused by marine bacteria pose a significant challenge for aquaculture and related activities, resulting in high mortality rates and substantial economic losses. This review presents a list of bacterial pathogens found in cultured aquatic organisms, along with the secretion systems they use to export virulence factors or toxins. We focus on the principal secretion systems employed by pathogenic marine bacteria, particularly those belonging to the genera Vibrio, Aeromonas, Photobacterium and Tenacibaculum. We describe the structural organization, functional mechanisms and toxin-associated substrates of secretion systems from types I, II, III, IV, VI and IX. Furthermore, we explain the mechanisms of action of the most important marine bacterial toxins, focusing on hemolysins, pore-forming toxins, enzymes and protein toxins, with particular emphasis on the PirAB toxin, which causes acute hepatopancreatic necrosis disease (AHPND). Additionally, we discuss the regulatory networks that control toxin expression in response to environmental conditions and quorum-sensing signals, as well as the impact of the toxins secreted by marine organisms on aquaculture systems. Understanding the mechanisms of toxin secretion in aquaculture is essential for developing effective prevention measures and improving overall health management in aquatic production systems.