Not to be underestimated: nematodes learn to avoid smelly bacteria

Posted on November 17, 2014   by Jon Fuhrmann

There are many examples of remarkable intelligence from the animal kingdom: chimpanzees can learn sign language and use tools to gather food; elephants have been known to perform basic maths; and a Border Collie named Chaser knows over 1,000 different toys by name.

These remarkable feats are generally associated with mammals; more primitive organisms are rarely considered to be particularly intelligent.

However, this is unjustified – take Caenorhabditis elegans, for example. This nematode (or roundworm) consists of only about 1,000 cells and has no brain, although nearly a third of the cells in its body are neurons responsible for transmitting information around the body. Despite its very simple anatomy, researchers at Rockefeller University in New York found that C. elegans is capable of “learning” which chemical signals are associated with harmful bacteria. They believe that the nematodes’ ability to learn is associated with their olfactory neurons – that is, their sense of smell.

C. elegans usually lives in soil, an environment abundant in bacteria; some, such as Escherichia coli, are a food source for the nematodes, while others are harmful to them. To study the nematodes’ ability to learn from experiences, the scientists looked at how ‘naive’ nematodes – raised without exposure to harmful bacteria – responded to a choice between moving towards edible E. coli or towards pathogens. Having not had the opportunity to learn that pathogens are harmful and not a source of food, roughly equal numbers of these nematodes moved towards the pathogens and towards the E. coli. Repeating the experiment with nematodes that had previously been exposed to pathogens, the researchers found that almost all of them avoided the pathogens and moved towards the safe food source. These results suggest that odour recognition is not instinctive, but can be learned through exposure to harmful bacteria.

The learning ability of C. elegans is now being used to evaluate the pathogenicity of different bacterial strains. Euan Scott, a PhD student at the University of Southampton, is using the nematodes to model how likely the various bacteria are to cause infection and disease in humans. He is presenting his work at the Society for General Microbiology’s Focused Meeting in London today.

Scott uses laboratory-grown, naive nematodes and places them near E. coli patches laced with harmful bacteria. Rather than having to choose between pathogens and safe food, the nematodes now have to learn that their food contains harmful substances. The theory underlying his experiments is that more dangerous bacteria elicit stronger aversive reactions in C. elegans, meaning that more of them will learn to abandon the food patch in search of alternative food sources. Scott is also investigating how the nematodes’ fertility changes when they spend time near pathogens. If the number of eggs were to be directly related to exposure to harmful bacteria, this would be useful for determining the pathogenicity of bacteria.

Knowing how harmful different strains of bacteria are – and which genes cause their pathogenicity – will hopefully translate into human medicine and inform future antibacterial drugs and treatment methods.

The other important aspect of Scott’s research is that while we know which neurons are activated in C. elegans in response to odours, it is not clearly understood how exactly the learning process takes place. He thinks that decoding the learning process of nematodes could mark an important step in understanding how more complex organisms such as humans form memories.

Euan Scott’s work on nematode-based pathogenicity models is just one of many cutting-edge research projects showcased at the Society for General Microbiology’s Focused Meeting in London. To stay up to date on our future events, visit our website.