Horizontal gene transfer (HGT) makes a powerful contribution to microbial evolution, enabling microbes to rapidly adapt to environmental changes by acquiring clinically- and ecologically-important traits like antibiotic resistance, bioremediation, or virulence. HGT is mainly driven by the activity of mobile genetic elements (MGEs), such as plasmids, transposons, and bacteriophages, which can have distinct fitness ‘interests’ and evolutionary trajectories as semi-autonomous entities living in symbiosis with microbes. Besides accelerating HGT, MGEs interact with one another in multifarious ways both competitive and collaborative, while the functions encoded by MGEs represent a powerful molecular toolkit which has been repurposed by microbes for various services including intraspecific competition. In this theme issue, we take an ‘MGE-eye view’ of microbiology, reflecting the increasing appreciation of the ubiquity, impact, versatility, and diversity of MGEs.

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