After the switch is flipped, bacteria have a ‘just in case’ mechanism to confirm identities

Chris stands in front of shelves of glass beakers.
Chris Vallasso

Those people at Google think they’re sooooo smart. So, too, the Apple and Microsoft wunderkinds.

Their software (and many others) use two-factor authentication in the digital world to verify identity, but they’re a little behind. More down to earth, a one-celled soil bacterium beat them to it by who-knows-how-many millions of years.

Ph.D. student Chris Vassallo in molecular biologist Dan Wall’s laboratory found the bacterium Myxococcus xanthus performs its equivalent of a secret handshake after an initial meet-and-greet encounter in its social world. The second-level of verification is important. They die if not recognized.

Earlier research in Wall’s lab (see related story) found these bacteria recognize kin through the cell surface receptor called TraA and transfer cellular goods to one another when touching via a process the lab calls outer membrane exchange (OME). This current research, published recently in the open-access journal eLife, is about the cargo that’s exchanged.

M. xanthus social lifestyle requires them to cooperate with their kin or close family members.

“It’s very important these cells know who they are cooperating with,” says Vassallo, from Cheyenne. “They don’t want to give beneficial treatment to another cell they are competing with if it’s not their self. One way they do this is through toxin exchange.”

Swap Potent Potions

The cells exchange potentially toxic proteins during OME. The process takes a couple minutes.

“If their identities don’t match, they’ll kill each other with the toxins,” said Vassallo.

The toxic cocktail of proteins moves from cell to cell, chewing up DNA or RNA if the cell is not immune. Vassallo says these bacteria don’t die immediately. Although sick, they are able to infect other cells, similar to humans with a transmittable disease.

Wall’s laboratory found the bacteria use a receptor that is unique to M. xanthus. In the wild, underfoot outdoors, there are hundreds of different recognition receptors within the myxobacteria group.

Using the TraA receptor for identity verification is not enough, though.

“We think this offers a fair amount of specificity, but by no means does this provide the level of specificity that would be needed in the environment,” Wall says.

A few grams of soil might contain a hundred distinct M. xanthus social groups, all living together but not necessarily wanting to cooperate with one another, says Wall.

“So what Chris discovered is this second layer of specificity,” notes Wall. “The first layer is, ‘Do you have a compatible TraA receptor?’ If you do, you exchange components. Then the next layer is, ‘Do you have immunity to the collection of toxins I’m going to give you using this exchange process?’”

Not all Death and Destruction

The one-celled creatures are remarkably adept. The bacterial decimation, where kin kill non-kin as packs of cells converge, results in a kill zone (see photo page 3). However, not all exchanges result in death and destruction. Vassallo found in previous research that healthy bacteria repair damaged kin. He designed an experiment where cells had defective membranes and left on their own would die. But if mixed with healthy kin, the clonemates would give them healthy material, and the sick cells became rejuvenated.

Wall’s work, part of a $1.2 million grant from the National Institutes of Health, is basic research, an emphasis on better understanding of and expanding knowledge about the world, and in his case how cells recognize and interact with one another.

His work addresses molecular biology mysteries. One element is the greenbeard gene. Scientists once thought the concept was hypothetical, Walls says. Such a gene could identify others with the same gene and confer something beneficial, increasing the recipient’s fitness for survival.

Scientists have been putting forth various greenbeard candidates.

Wall offers the TraA receptor. It allows a cell to confer beneficial behavior by exchanging cellular components with another cell with the exact same gene.

“We think we have the best example of a greenbeard,” he notes.

The other research area addresses how multicellular animals and plants came into existence.

The evolutionary transition from single cell to multi-cell life is apparently very difficult, he says. The event is thought to have occurred only once for animals and perhaps twice for plants.

“In the microscopic world, it might have happened separately a dozen times,” Wall says. “In the case of myxobacteria, they appear to have made this transition to multi-cellularity, a fairly primitive transition that’s based on an aggregation strategy.”

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