Vessel transport of antibiotic resistance genes across oceans and its implications for ballast water management

The emergence and spread of antibiotic resistance are major threats to ecosystems and human health. Transoceanic channels (e.g., ship ballast water) can transfer harmful aquatic organisms across geographically isolated waters. However, the occurrence of antibiotic resistance genes (ARGs) in ship ballast water and their relationship with microbial communities and environmental factors remain unknown. In this study, ballast water from 28 vessels sailing to Shanghai and Jiangyin (China) were collected, and the ARGs in these water samples were investigated. Considerable levels of ARGs and integrase of the class-I integrons (intI1) were detected in all ballast water samples. sul1 and tetQ were the most and least abundant ARGs in ballast water samples, respectively. The ARGs were strongly correlated with those of the 16S rRNA and intI1 genes. Ballast water exchange can reduce the absolute abundance of some kinds of ARGs while increasing the relative abundance of several ARGs (e.g., mefA, mexF, strB, sul1, and tetQ). Moreover, the bacterial hosts of ARGs were generally different in the unexchanged ballast water (UEBW) and exchanged ballast water (EBW). In particular, Leisingera and unclassified_Erythrobacteraceae were the main ARGs-associated genera in the EBW, while Pseudohongiella, Cycloclasticus, OM43_clade, norank_f_Rhodospirillaceae, and norank_f_Rhodobacteraceae were the dominant ARGs hosts in the UEBW. Overall, ship ballast water is an effective moving carrier for the global transference of ARGs, and its sufficient management is required for mitigating ARGs propagation across oceans.