The bacteria that cause disease may be microscopic, but animals can use senses other than sight to protect themselves from infection. Some bacteria produce harmful toxins, which animals can instinctively recognize as being dangerous using their sense of smell or taste. This is called chemosensation, an innate ability that allows animals to react to chemical stimuli.
But animals can also ‘learn’ to avoid harmful bacteria, though it remains unclear how they do so. Now, Singh and Aballay have used roundworms as a model organism to study this phenomenon.
Roundworms feed on bacteria, so they need to be able to distinguish between disease-causing strains and harmless ones. However, they only start avoiding harmful bacteria after several hours of exposure, which would not necessarily be expected if they were using chemosensation. This prompted Singh and Aballay to investigate whether another mechanism could be teaching the roundworms to avoid disease-causing bacteria, by comparing roundworms that had been exposed to harmful or benign strains.
As observed previously, the roundworms learned to avoid the harmful bacterium Pseudomonas aeruginosa. However, exposing the worms to certain chemicals produced by P. aeruginosa was not enough to teach them to avoid the bacterium. Instead, the experiments showed that when roundworms ingested disease-causing bacteria, the infection caused intestinal bloating. The more toxic the bacteria, the more the intestine swelled, triggering a neural pathway associated with a preference for oxygen. In a few hours, the worms learned to avoid the low oxygen environment associated with P. aeruginosa and developed a preference for high oxygen conditions surrounding harmless bacteria such as Escherichia coli.
These results show how an intestinal infection can send signals to the nervous system to modulate animal behavior. Moreover, Singh and Aballay have identified a neural pathway that stimulates a behavioral host response to defend against infection.