Pathogen Avoidance by Caenorhabditis Elegans Is a Pheromone-Mediated Collective Behavior

Abstract

A model organism Caenorhabditis elegans is a free-living nematode that feeds on bacteria in the wild. To survive, it has to be able to distinguish between benign and pathogenic microorganisms. Thanks to its simple neural system it is capable of determining whether the source of food is dangerous and physically avoid it.

In this work we present evidence that this avoidance behavior depends on the nematode population density. The worms exposed to a pathogen secrete an alarm pheromone, which drives the population away from the danger. Worms carrying a mutation in the acox-1 gene (involved in the pheromone biosynthesis) are unable to produce the alarm signal and odr-7 mutants fail to react to it suggesting that AWA neurons are involved in the pheromone detection. We developed a simple phenomenological model that quantifies the effects of the secreted pheromone and was able to reproduce major features of the nematodes' behavior at the population level.

We show that the collective avoidance is not restricted to C. elegans as other Rhabditida nematodes also exhibit this behavior. Furthermore, the alarm signal left by C. elegans in the bacterial lawn is detected by most of the other species tested but some of them were attracted to it rather than repulsed which suggests that the chemical signal is not conserved between them.