Bacteria thrive by freeloading

They say that cheaters never win, but some bacteria appear to do quite well by adopting this strategy.

The guilty party is a mutant form of the bacterium Pseudomonas aeruginosa, which can infect people with weakened immune systems and is often the cause of death among people with cystic fibrosis. Because this species is effective only in large numbers, the bacteria wait until they sense chemical messages from many nearby individuals before they begin producing the toxins that cause their virulence—a process called quorum sensing.

Scientists had been puzzled that some bacteria in samples taken from infected people had mutations that caused them to ignore quorum sensing. Losing the ability to detect when the bacterial population reaches critical mass seemed to be a detrimental trait that natural selection would weed out.

Now, Martin Schuster and his colleagues at Oregon State University in Corvallis have shown that these mutants can grow faster than their peers by freeloading on the hard work of their neighbors.

“Some bacteria are taking advantage of others, letting them do the work necessary for survival,” Schuster says.

Once quorum has been reached, the normal, cooperating bacteria start to churn out compounds that convert nutrients into forms that the bacteria can use. By not producing those compounds, the cheating bacteria save energy that they can instead devote to growth, Schuster’s team reports in the Oct. 2 Proceedings of the National Academy of Sciences.

If the cheaters outgrow their peers too much, nutrient supplies for the whole population will suffer. So the mutants also undergo genetic changes that turn their quorum-sensing abilities back on if they become too numerous.