Bacteria evolve fast – and that could be bad news for those of us who don't want to see easily-treatable diseases make a roaring comeback. Resistance to antibiotics is a growing concern, but if scientists can see how the microbes evolve, they might be able to intervene. Now researchers at Indiana University have peeked into that tiny world, producing the first direct images of bacteria extending "harpoons" to snare and absorb bits of DNA.

One of the main ways bacteria evolve new traits is through DNA uptake, otherwise known as horizontal gene transfer. This process allows them to latch onto fragments of DNA from their surroundings and incorporate it into their own genome, teaching themselves new tricks such as antibiotic resistance. Bacteria can then share these snippets with each other, spreading the ability throughout the population.

"Horizontal gene transfer is an important way that antibiotic resistance moves between bacterial species, but the process has never been observed before, since the structures involved are so incredibly small," says Ankur Dalia, senior author of the study. "It's important to understand this process, since the more we understand about how bacteria share DNA, the better our chances are of thwarting it."

While past research has focused on blocking bacteria from sharing these DNA fragments, the new study looked to find ways to potentially prevent them from absorbing them in the first place. It's long been known that bacteria use extendable appendages known as pili to help them gather DNA from around them, but direct evidence showing this mechanism has eluded scientists until now.

To highlight that process – literally – the IU researchers painted the bacteria and DNA fragments with fluorescent dyes. The team was able to observe and capture on video bacteria sending out their pili like harpoons to catch the fragments of DNA, before reeling them back in through tiny pores in the cell wall of the bacteria.

"It's like threading a needle," says Courtney Ellison, first author on the study. "The size of the hole in the outer membrane is almost the exact width of a DNA helix bent in half, which is likely what is coming across. If there weren't a pilus to guide it, the chance the DNA would hit the pore at just the right angle to pass into the cell is basically zero."

The next steps for the researchers are to closely examine the other end of the process – namely, how pili latch onto DNA. The team says that the protein used by the pili seems to interact with DNA in a completely new way. Further down the track, the fluorescent dye technique could be used to get a closer look at other bacterial functions.

The research was published in the journal Nature Microbiology.

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