An opportunistic pathogenic Pseudomonas isolated from healthy brown alga Saccharina japonica

Abstract

Commercially farmed Saccharina japonica suffers from various diseases during its cultivation. Isolation and identification of the causative agents are crucial for the diagnosis and control of the diseases. In this study, an opportunistic pathogenic bacterium was isolated from healthy matured S. japonica and was identified as Pseudomonas aeruginosa MSLj6–25. The pathogenicity of P. aeruginosa MSLj6–25 was tested by the infection assay. Microscopic observations indicated that P. aeruginosa MSLj6–25 can cause green-rotten disease. Ultrastructural changes showed that the chloroplasts and the nucleus became condensed at 7 h after infection, while at 24 h, the chloroplasts and the nucleus were deformed. Colonies of P. aeruginosa MSLj6–25 were found to be flat and bright yellow. The bacterial cells were rod shaped with a single polar flagellum. MSLj6–25 had the highest similarity of 99.93% with Pseudomonas aeruginosa strain CZ-45 by comparing 16S rRNA gene sequences. Combining biochemical characterization, we identified the taxonomic classification of MSLj6–25. To the best of our knowledge, this is the first time that P. aeruginosa was identified as an opportunistic pathogen in S. japonica. These results contribute to the baseline knowledge of pathogenic bacteria in S. japonica and lay the foundation for disease diagnosis and control.