Bacteria like to have sex just like anyone else. Of course, in their world, sex is the acquisition or donation of any amount of DNA, rather than a genome-scrambling full-contact sport.

When I was in school 20 years ago, there were three ways it was thought bacteria could accomplish this: by direct uptake of naked DNA from the environment; through accidental transfer of bacterial DNA by viruses shuttling between them; and through conjugation, in which bacteria engage in something most people would recognize as conjugal visits. They transfer small loops of DNA through a long thin tunnel called a pilus(!).

When I imagined that first method -- direct uptake of naked DNA, also called transformation -- I had an idea that in a moment of weakness, the bacterial cell wall somehow parted and the DNA snuck or got sucked in. The way it was taught, it was all rather vague. Consulting my old microbiology textbook, I see that even then it was known that there was some sort of protein – embedded in the cell membrane, it was thought -- that acted as a gatekeeper for DNA picked up off the street.

What I have just learned, thanks to a study published last week by scientists at Indiana University in Nature Microbiology, is that what I should have been envisioning was more like the frame at right in this video, in which free-floating DNA is labeled red:

Bacteria snatch naked DNA from their environment using a harpoon-like competence pilus and then haul it back into their cells, like partiers spearing cocktail weenies with toothpicks. That analogy is not as big of a stretch as you’d think, either, because DNA floating in the environment can be food as well as sex partner (as odd as it is to wrap our brains around that).

The organism the scientists chose to study for this research was Vibrio choleraepurveyor of death to tens of millions of humans over the last two centuries -- but also a bacterium that likes to mix things up, if you know what I’m saying. Scientists were curious about how exactly that happens.

Why should we care about how bacteria have sex? Voyeurism aside, bacterial sex is how the organisms spread antibiotic resistance genes, change their camouflage, and acquire new tactics in the war on hosts. Knowing how they “do it” could help keep future death purveying to a minimum.

In particular, scientists were interested in the role of the pilus. Pili are tubes built of proteins called pilins. Pili probably evolved multiple times for multiple uses. In addition to their “mating” function in conjugation, some pili mobilize bacteria by racheting them along surfaces.

In the last decade or so, scientists have realized that the competence pilus is an agent – perhaps the agent -- of bacterial transformation. One scenario is that the pilus grabs DNA and pulls it inside the bacterium as the pilus retracts.

The alternative hypothesis is that the pilus opens a cell membrane pore protein called secretin, allowing DNA to diffuse inside, more like the scenario I imagined back in school.

The major distinction between the two, the scientists say, is whether the pilus can bind DNA. Still images of this process show DNA hanging out at the tip of a pilus, but it’s hard to know if that’s a coincidence. Video footage would be more conclusive, the authors reasoned.

So, they decided to attempt something obvious that hadn’t been tried before because no one knew how to do it. They figured out a way to paint the pilus of living Vibrio cells and environmental DNA with fluorescent molecules. Then they turned off the room lights and on the black lights and watched to see what happened. The result was the video you already watched.

How do they believe this is accomplished?

Bacteria extend and retract pili with the help of two ATP-powered motors embedded in the inner cell membrane, PilB and PilT. These enzymes snap pilins together or pull them apart at the base like pop beads.

Pilus building or demolition in Vibrio happens quickly. About half of cells grown under favorable conditions made at least one pilus one micrometer long in about a minute. As you saw in the video, they can get much longer too! (Other bacteria may use pili that are much shorter. Long pili appear to be useful for snatching DNA off of biofilms rather than stuff floating freely in the environment).

Model of pilus-mediated DNA uptake proposed by authors. OM = outer membrane, IM = inner membrane, PG = peptidoglycan, ComEA = rachet protein, PilQ = secretin pore. Purple ovals are pilin subunits. Credit: Ellison et al. 2018

Using multiple methods, including the fluorescence microscopy video showing the pilus pulling labeled DNA toward the cell, this team demonstrated that competence pili bind to double-stranded DNA at their tips. The pilus is then retracted by removal of pilin subunits by PilT at its base. When they physically blocked the retraction of the pilus DNA uptake delcined 100-fold, even though DNA was still bound to the pilus.

When the DNA reaches the cell membrane, it faces a seeming dilemma: the pore, called PilQ, that it has to fit through is much smaller than the width of a folded strand of DNA. No problem. It appears, the authors say, that ATP-motorized PilT threads the needle by yanking the DNA through the tight aperture. Then rachet proteins called ComEA floating in the space between the inner and outer membranes latch on to the DNA and finish hauling the prize the rest of the way through the porthole with some lusty heave-hos, like a salvage crew on a ship at sea.


Ellison, Courtney K., Triana N. Dalia, Alfredo Vidal Ceballos, Joseph Che-Yen Wang, Nicolas Biais, Yves V. Brun, and Ankur B. Dalia. "Retraction of DNA-bound type IV competence pili initiates DNA uptake during natural transformation in Vibrio cholerae." Nature Microbiology (2018): 1.