Comparison of Bacterial and Potentially Pathogenic Bacterial Communities in Ballast Water from Different Source Waters

Ballast water can transport microorganisms, including potentially pathogenic bacteria (PPB), between aquatic environments. This study examined bacterial communities and PPB assemblages in 11 ballast water samples using 16S rRNA gene amplicon sequencing. Samples were classified into six groups based on the country of uptake and water type. Total bacterial community composition differed significantly among groups (PERMANOVA, p = 0.037), whereas no significant differences were detected in PPB community composition (p = 0.138). Pseudomonadota was the dominant bacterial phylum across all samples. A total of 51 species-level PPB-associated amplicon sequence variants (ASVs) were identified using sequence identity and pathogenicity information from reference databases and the published literature. Several PPB-associated ASVs showed significant correlations with environmental variables, particularly dissolved oxygen. An Integrated Risk Index (IRI) was applied to compare the relative characteristics of PPB assemblages across source groups. The Australian seawater group was classified in the Moderate IRI category, while the remaining five groups were classified in the Low category. Given the limited sample size and the absence of direct validation of pathogenicity, these findings should be regarded as exploratory and provide baseline information for future ballast water microbial monitoring.