Megaplasmids of the enteropathogenic species Vibrio parahaemolyticus, Vibrio cholerae and Vibrio vulnificus represent a group of novel genetic elements unrelated to other plasmids of Vibrionaceae

Vibrio (V.) parahaemolyticus is a marine-associated bacterium that has previously been linked to foodborne illness associated with seafood consumption. Various plasmids harbouring antimicrobial resistance and virulence genes have been described for V. parahaemolyticus. By whole genome sequencing, we found two V. parahaemolyticus strains harbouring a large additional circular genomic element of 0.882 Mbp. NCBI database search revealed that this element represents a rare but globally distributed megaplasmid detected in four additional Vibrio strains spanning distinct species (V. parahaemolyticus, V. cholerae and V. vulnificus), geographical origins, and hosts. In addition to the two megaplasmid-harbouring V. parahaemolyticus strains from our study, we also identified two megaplasmid-free isogenic strains in our strain collection, confirming that the megaplasmid is indeed a plasmid encoding non-obligatory functional traits. The divergent GC content and codon usage of the megaplasmid suggest a non-Vibrio origin. The genetic diversity found in the six investigated megaplasmid sequences indicates adaptation within the different Vibrio hosts. Although many of the megaplasmid genes could not be categorised through Cluster of Orthologous Genes (COG) classification, genes coding for partitioning systems, type IV secretion systems, defence systems, and toxin-antitoxin modules were identified. These contribute to a high plasmid stability, as we demonstrated by the curing experiments performed with our strains. Consequently, the megaplasmid described here represents a highly stable but adaptive genetic element that is characterised by as yet unexplored genetic traits.