It doesn't take much to get the fungus that causes thrush and other infections in the mood. New research suggests that in addition to chemical signals from its own species, the yeast, called Candida albicans, also gets turned on by the so-called pheromones sent out by other species.

And when turned on, this yeast isn't selective. If cells of the opposite sex aren't around, then it mates with same-sex partners, according to Richard Bennett, one of the study researchers and an assistant professor at Brown University in Rhode Island.

This type of fungus is a natural inhabitant of our bodies, particularly our guts, but, given the opportunity, the yeast can also cause harmful infections, ranging from a superficial thrush infection in the mouth to potentially lethal blood infections among those with weakened immune systems.

C. albicans cells come in two forms: white and opaque, names derived from the appearance of their colonies. Opaque cells are the reproductive ones. They produce a pheromone that prompts other opaque cells to turn on genes associated with mating. In the presence of this pheromone, the opaque cells also put out long projections that search for another cell with which to fuse (the yeast equivalent of sex), according to Bennett.

The white cells do not reproduce, but they also respond to the pheromone, which activates an entirely different set of genes. They become sticky, and start to glom together and to surfaces, such as a catheter, forming what is known as a biofilm. This is a common route to harmful infections.

The researchers synthesized a variety of pheromones produced by this species and a variety of other fungal species, and found that the white and opaque cells were not picky about a trigger for their responses. The normal opaque cell pheromone is a string of 13 amino acids, which are organic compounds.

“In some of the pheromones eight of 13 residues were different,” Bennett said. “That’s why we were so surprised with these pheromones. We didn’t expect them to work because they look so different.”

It is not unusual for one species to respond to pheromones from another, however, it is unusual for that response to lead to productive mating, he said.  

C. albicans' lax standards may mean that it could respond to other signals from its environment, including signals directly from the host. The next step, Bennett said, is to figure out how these findings fit in with disease.

The research was published online today (Jan. 24) by the journal Proceedings of the National Academy of Sciences.