by Mechas and Roberto
We recently prepared a lecture that included a brief discussion of bacterial evolution. Of course, we covered horizontal gene transfer (HGT), which we consider a key pillar in the evolutionary process. We dutifully began our presentation by listing, perforce: transformation, conjugation, and transduction. In doing so we were following the path likely followed by most teachers of microbiology. This way, the knowledge of HGT mechanisms itself has been diligently transferred from one generation of microbiologists to the next. But this time, as we completed the lecture, we both had the same thought: our list needed updating!
Science is continually evolving, expanding as we explore novel ecosystems and organisms other than those well-known laboratory model bacteria, such as E. coli and Salmonella. Among the many exciting developments of recent decades are the discoveries of additional mechanisms for transferring DNA among bacterial cells. We found the recent review by Arnold, Huang and Hanage, "Horizontal gene transfer and adaptive evolution in bacteria," to be an excellent read on HGT and its evolutionary implications. We reproduce their first figure here as it does justice to the more recently discovered mechanisms of HGT. When we see these new pathways of HGT, we feel certain that the time is ripe to revise our long-held practice of presenting only "the classic three."
We now know that extracellular vesicles (EVs) can deliver DNA to recipient cells and thus mediate HGT. While this phenomenon remains somewhat underappreciated, this occurs in diverse environments, from guts to seawater. As we recently posted in STC, these vesicles are not only abundant in the ocean but they also promote gene transfer in the marine bacterium Prochlorococcus. In discussing this fourth mechanism of HGT, Soler and Fortere recently proposed to name this process "vesiduction." But there are yet other novel HGT pathways. DNA transfer can be mediated by pilus-like "nanotubes" and phage-like "gene transfer agents" or GTAs. Rest assured; we will be covering those in future STC posts. In the meantime, if any of our readers still teach only the classic three, we hope that like us, you'll update your list!