BioEd Online

Some soil microbes will generously give a helping hand to disabled colleagues, researchers have found. The bacteria donate proteins to neighbouring immobile bugs, giving them the ability to move again.

The microbes, called myxobacteria, are common bacteria that live in soils world-wide. They survive by decomposing and eating organic material, and they have adapted a clever way of swarming together to track down these nutrients and digest them. "They feed as gangs," says biologist Dale Kaiser at California's Stanford University.

By lumping together, the microbes stand a better chance of producing enough enzymes to digest their food. And when the food runs out, these colourful collectives may also help the bugs survive. Some speculate that their colour attracts passing animals, allowing the microbes to hitch a ride with curious birds that mistake the lumps for food, says Martin Dworkin, a microbiologist at the University of Minnesota in Minneapolis.

The myxobacteria use antenna-like extensions known as pili to gather together. A bacterium will stick the end of its pilus onto the cells in front of it, and then reel itself in to join the gathering crowd.

But a tiny proportion of the bacteria are mutants that lack the right proteins to control their pili, leaving them stranded and immobile.

Caring and sharing

Kaiser and his colleagues had observed that mutants somehow regain the ability to move when placed in close proximity to normal myxobacteria. But no one knew how this happened.

The team guessed that the mobile bacteria might be passing on the needed proteins, called Tgl proteins, to their disabled mates. To check, they first designed immobile bacteria that express the Tgl protein but lack pili. They then placed these microbes alongside natural mutant myxobacteria, which had pili but lacked the Tgl proteins. The mutants gained the ability to travel and moved to the outer edge of the dish, allowing the researchers to easily separate out the newly mobile mutants from the original cells without pili.

The team then analysed the bacteria for Tgl protein to see what had happened. As expected, the mobile mutants now had Tgl proteins, while the others had lost a significant part of their complement of these proteins.

"They're really a beautiful example of cooperation," says Kaiser. Its unclear why the cells cooperate this way, as the vast majority of cells could gather into a ball and simply leave their disabled colleagues behind. But the team wonders if the cells are exchanging other proteins or information in the same way, which might be advantageous to the colony as a whole.