Divide and conquer: coaxing cheaters to battle bacteria

Bacteria are responsible for many human diseases, but rather than striving to kill bacteria which infect us, we could try a more subtle strategy: setting them against each other and reaping the benefits. For such a strategy to work, we have to figure out how to cause trouble among the bacteria — how to interfere with their social lives. A paper by a team of researchers from Denmark and the UK points the way, identifying fracture lines in a community of infectious bacteria.

The team studied Pseudomonas aeruginosa, a bacterium which causes lung infections in cystic fibrosis patients. P. aeruginosa needs to harvest iron from its human host. To release iron from hemoglobin and other molecules that have it bound, the bacteria produce and secrete a molecule called pyoverdine. Pyoverdine extracts iron and binds to it; the iron-pyoverdine complex is then picked up by a receptor on the bacteria.

Enterprising bacteria can take advantage of this system by freeloading on the community's effort. Since P. aeruginosa secretes pyoverdine into its surroundings, an individual could make the receptor but not produce any pyoverdine. Instead, it would pick up pyoverdine (bound to iron) that other individuals had released into the surroundings. The researchers found some P. aeruginosa that weren't producing pyoverdine in two collections from a total of 60 Danish patients, and they showed that the proportion of non-producers grew over time.

The non-producing bacteria may have been cheaters, taking advantage of the pyoverdine made by others, or they may have stopped making pyoverdine because it was no longer useful. To distinguish between these possibilities, the team looked at whether the non-producers had kept the pyoverdine receptor. They found that these bacteria kept the receptor, even though it is costly to produce and maintain. If pyoverdine was no longer useful, the receptor would have disappeared; instead, the bacteria kept it so they could cheat.

Understanding the social dynamics of infectious bacteria offers an avenue towards dealing with them in a different way. Rather than a brute force campaign of extermination, we can try to find a more elegant approach, using the tools of ecology and evolution to weaken them from within. 'Divide and conquer' has long been an favoured military and political tactic; perhaps it can find new value in health and medicine.

Ref
Andersen, S.B. et al. Long-term social dynamics drive loss of function in pathogenic bacteria. PNAS Early Edition (2015). doi: 10.1073/pnas.1508324112

Image credit
The image is by Janice Haney Carr of the CDC and is in the public domain. It shows Staphylococcus aureus which are resistant to methicillin. The evolution of antibiotic resistance by bacteria is one of the pitfalls of existing brute-force treatment strategies.