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Clockwise from upper left: This series of four still images shows a pilus stretch out from a bacterium, in green, to catch a piece of DNA in the environment, in red. This is the first step in the DNA uptake process. Photo: Ankur Dalia, Indiana University

Bacteria have been known to play a kind of genetic leapfrog, swapping DNA between one another to boost their evolution exponentially.

The process was dubbed horizontal gene transfer, and surface structures known as competence pili were somehow involved, but little else about the process was firmly established.

The pili reach out, poke a hole in fellow bacteria and drag folded lengths of DNA out like grappling hooks, according to a new study in the journal Nature Microbiology.

The researchers observed the process by tagging key amino acids with dye, and watching the process in real time, report the scientists from Indiana University and CUNY Brooklyn.

“Using the model naturally transformable species Vibrio cholera and a pilus-labelling method, we define the mechanism for type IV competence pilus-mediated DNA uptake during natural transformation,” the scientists said. “To directly observe pilus-DNA binding, we fluorescently labelled both pili and DNA and tracked them by microscopy in real time.”

The cholera bacteria were cultivated, and mutant strains with key fluorescent genes, that were amplified and spliced to order by PCR reactions, according to the paper.

While electron microscopes had previous shown static images implicating the pili in some kind of DNA transfer process, the fluorescence microscopy showed its dynamics. That includes the precision extraction of the DNA, through the drilled pore that is much smaller than the DNA snippets. The pili thus act like harpoons, and drag the targeted DNA through by a kind of folding.

“The binding of DNA to the pilus tip as observed here is also compelling because the secretin pore through which the pilus extends and retracts is only wide enough to accommodate the pilus fiber and would probably exclude DNA bound along the pilus length,” the researchers said. “Thus retraction of tip-bound DNA could allow for the threading of DNA through the secretin pore in the wake of a retracting pilus fiber.”

Further understanding of the process could lead to pivotal gains in the battle against antibiotic resistance, the scientists said. Part of the quick spread of resistant genes is due to this horizontal gene transfer DNA swapping.

“The process has never been observed before, since the structures involved are so incredibly small,” Ankur Dalia, one of the authors from IU Bloomington, said. “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.”

More weapons in the arsenal against resistant germs are perceived as vital by world health officials.

Complete resistance has been spreading worldwide. For instance, the first “superbug” with complete antibiotic resistance appeared in a Pennsylvania woman’s urinary tract infection in April 2016.

Even last-line antibiotics have begun to show limitations against some of the rising genetic adaptations in the bacteria, like the colisin resistance found in the urine of two patients in Atlanta, as reported by the CDC in March.

 

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