Highly pleomorphic strains of the Vibrio predator Pseudoalteromonas piscicida and their outer membrane vesicles: A scanning electron micrographic study

Authors: Richards, Gary; Uknalis, Joseph; Watson, Michael

Submitted to: Microorganisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2025
Publication Date: 2/7/2025
Citation: Richards, G.P., Uknalis, J., Watson, M.A. 2025. Highly pleomorphic strains of the Vibrio predator Pseudoalteromonas piscicida and their outer membrane vesicles: A scanning electron micrographic study. Microorganisms. 13/365. https://doi.org/10.3390/microorganisms13020365.
DOI: https://doi.org/10.3390/microorganisms13020365

Interpretive Summary: Pseudoalteromonas piscicida are marine bacteria which have a beneficial predatory function in reducing competing bacteria, including pathogenic human and fish Vibrio bacteria in the marine environment. The predatory state relies on the development of vesicles (sacks which contain digestive compounds) on the surface of the Pseudoalteromonas piscicida. These vesicles are transferred to the surface of their prey, where they digest holes, effectively killing the prey in the process. Disease-causing Vibrio species commonly serve as prey. The mechanisms and triggers by which Pseudoalteromonas develop these vesicles are unknown. This study was performed to determine if the presence of vesicles, which are an indicator of the predatory state of the Pseudoalteromonas, are induced by the level of nutrients in the seawater and if the presence of prey bacteria is required to induce vesicle formation. This was accomplished using scanning electron microscopy where samples were magnified up to 100,000 times and photographed. Results showed that the Pseudoalteromonas could produce vesicles even in the absence of prey bacteria. They also produced vesicles in both high and low nutrient seawater. Consequently, the predatory function of P. piscicida may make them useful to reduce the levels of vibrios and other bacteria in aquaculture systems to lessen disease and mortalities in farm-raised seafoods. They may also be useful in the development of commercial processing interventions to enhance the safety of seafoods destined for human consumption.

Technical Abstract: Pseudoalteromonas species are recognized for their probiotic roles in reducing pathogens in aquaculture products by secreting a broad range of antimicrobial compounds. Some species, like P. piscicida, are also predators that attack susceptible prey bacteria, including V. parahaemolyticus, by transferring outer membrane vesicles (OMV) containing digestive compounds to the surface of their prey. These vesicles digest holes in the prey’s cell wall releasing nutrients upon which the Pseudoalteromonas feed. In the present study, scanning electron microscopy was performed on two P. piscicida strains grown in sterile seawater and in nutrient-enriched seawater, without the presence of bacterial prey, to determine if the presence of prey or low nutrient media were required to induce vesicle formation. Micrographs revealed OMV formation and high pleomorphism of P. piscicida in the absence of prey cells and regardless of the nutrient levels of the seawater. Phenotypic characteristics ranged from the presence of: i) vesiculated and non-vesiculated bacteria, ii) large bulbous OMV versus small OMV, iii) pilus-like connectors of widely varying lengths to which vesicles were attached, iv) highly elongated (10-µm long) Pseudoalteromonas cells, and v) cells which appeared to extend to 50-µm long and to be septating and dividing into short chains and individual cells. The possible contribution of these novel phenotypes to Pseudoalteromonas predation is discussed.