Plasmids are often described as small, independently replicating DNA molecules that carry a handful of accessory functions. A study published October 20 in Nature Communications challenges that simplified picture by characterizing 21 complete megaplasmid genomes associated with Enterobacteriaceae, some reaching 1.58 million base pairs.
Three of the elements were reconstructed from isolates of Escherichia coli and Shigella flexneri. The researchers also found related sequences in hundreds of gut and other microbiomes, while genomic evidence suggested possible associations with additional Enterobacteriaceae genera. These findings expand the known scale and diversity of mobile DNA in organisms relevant to clinical microbiology and human gut ecology.
The megaplasmids encode machinery consistent with conjugative transfer, along with potential secreted effectors, multidrug efflux systems and unusually large collections of transfer RNAs. Some contain as many as 208 tRNA genes. The authors propose that this coding capacity could influence host competitiveness, interactions with other mobile elements and defence against processes that target translation. Such functional explanations remain hypotheses unless supported by appropriate laboratory experiments.
For antimicrobial-resistance surveillance, the discovery is a reminder that short-read assemblies and plasmid databases may miss unusual, highly repetitive or exceptionally large elements. Even when a resistance-related gene is found, determining whether it is chromosomal or plasmid-borne can materially change the interpretation of mobility risk. Long-read sequencing and careful assembly validation are therefore particularly valuable for characterizing atypical replicons.
The study does not show that every megaplasmid is transferable in nature or that all identified hosts carry the same functional traits. Instead, it provides new reference material and testable hypotheses about how microbial genomes exchange and retain large blocks of DNA. The findings may help refine plasmid-detection pipelines and the broader study of bacterial genome evolution.