I’ll admit that when I hear the word, I immediately think of sex. That’s probably because the first pheromone ever discovered, in 1959, was the chemical that female silkworm moths use to attract a mate. Since then, sex pheromones have been identified in many species, from insects to fungi to birds.
But according to Wikipedia, a pheromone is just a chemical that is “capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual”. And it’s not just about attracting mates. Pheromones can also be used to signal others of danger, to lay a trail for others to follow, or even to mark out territory.
In the bacterium Enterococcus faecalis, pheromones have just been discovered that allow harmless commensal strains to ‘kill’ their more harmful antibiotic-resistant forms. E. faecalis is a common commensal bacterium of the human gut. But it can also cause nasty infections in hospital patients, from blood poisoning to meningitis. People who have been on antibiotics, which kill off their normal gut bacteria, often become colonised with these more harmful E. faecalis strains which are also usually also antibiotic-resistant.
It’s long been known that commensal strains of E. faecalis have smaller genomes than the harmful superbug strains. To find out whether the larger genome of the superbugs gave them a growth advantage, Michael Gilmore and colleagues tried out an interesting experiment. They grew commensal or superbug strains of E. faecalis in the presence of stool samples from healthy volunteers. Interestingly, the superbug strain didn’t survive very well, whereas the commensal strain was just fine.
It turned out the healthy stool samples contained another commensal strain of E. faecalis, which the researchers isolated and called Pan7, and which could also kill the superbug strain. E. faecalis is known to make pheromones, so the researchers decided to see if they could be responsible for the killing effect. A quick search of the E. faecalis genome identified 81 potential pheromones so the researchers made synthetic versions of them all and tested them for their ability to kill the superbug version of the bacterium. Three of these synthetic pheromones worked, one of them, called cOB1, especially well. Knocking out the gene that commensal E. faecalis uses to make cOB1 meant it lost its ability to kill it’s superbug ‘sibling’.
Gilmore and colleagues are still trying to figure out exactly how cOB1 kills superbug strains of E. faecalis, but it’s interesting to speculate whether such a pheromone could one day be used to treat people infected with antibiotic-resistant E. faecalis. What I do know for sure, though, is that I’ll never think about pheromones in quite the same way again!
Gilmore et al (2015). Pheromone killing of multidrug-resistant Enterococcus faecalis V583 by native commensal strains. Proceedings of the National Academy of Sciences. doi: 10.1073/pnas.1500553112
If you are interested in hearing about another bacterial pheromone, here’s an animation I made with graphic artist Luke Harris about how Vibrio fischeri uses a pheromone to decide when it’s best to start glowing in the dark.