Isolation and Identification of Opportunistic Pathogenic and Antagonistic Bacteria of Saccharina japonica Using Modified Culture Media

To explore the composition of epiphytic bacterial communities on Saccharina japonica and to isolate as well as to identify both the opportunistic pathogenic and antagonistic bacteria, four modified culture media (1/10 ZoBell 2216E, 1/10 ZoBell 2216E supplemented with alginate, 1/10 ZoBell 2216E supplemented with S. japonica extract solution, and S. japonica extract medium) and one traditional culture medium (ZoBell 2216E) were used to isolate the epiphytic bacteria. Combining the full-length 16S rDNA amplicon sequencing, infection assay and the Burkholderia agar diffusion assay, this study analyzed the composition of epiphytic bacteria of S. japonica and screened for pathogenic and antagonistic bacteria. Analysis of full-length 16S rDNA amplicon sequencing data showed that dominant bacterial species in the epiphytic bacterial community were Mariniblastus fucicola, Granulosicoccus coccoide, Litorimonas cladophorae. A total of 195 bacterial strains were isolated using the four modified culture media and one traditional culture medium, and the dominant species were Cobetia amphilecti and Bacillus altitudinis. Among them, the highest number and variety for isolated bacteria strains were the modified culture medium M2 (1/10 ZoBell 2216E medium). Cobetia was the dominant genus (63.6%) of 11 strains of the opportunistic pathogenic bacteria, which can cause green-rotten diseased symptom in S. japonica within 24 h in lab condition. While, Bacillus was the dominant genus (62.5%) of 8 strains of the antagonistic bacteria, showing antagonistic activity to the pathogenic bacteria of bleaching disease of S. japonica (Pseudoalteromonas piscicida X-8).