My extremely versatile little friend
Caenorhabditis elegans is one of the most studied organisms in the world. Whether researchers are looking to find a cure for cancer, discover the mechanisms underlying ageing or study innate immunity, this little fellow can help them. Even on this website you will find more than a hundred articles mentioning studies conducted in C. elegans. I myself devoted three years staring down a microscope at this wriggly little worm.
This is not the place to repeat everything about my little friend that you can look up on Wikipedia or any of the other blog posts, but I will just summarise the most important points for those unacquainted with C. elegans and short of time. If that is not you, just skip the next paragraph.
several C. elegans feeding on a bacterial lawn
Caenorhabditis elegans is a 1 mm long nematode worm. That means you can just about see it with the naked eye, but as soon as you want to say anything about gender, health and age, you are better off looking through a microscope. The nematode can be found in your garden, especially in compost heaps, fresh soil and rotting fruits. In these environments it finds lots of its favourite food, bacteria, though other microbes are also on its menu. This worm is hermaphrodite, which means that ‘females’ can self fertilise. While males do exist, they are not necessary for the survival of the species. This is great news for scientists, because it means that a homozygous (all genes on all chromosomes identical) individual will produce loads of (about 300) identical copies of itself. Every individual is the same, with nothing left to chance, making this animal not only a good candidate for genetic studies, but also ideal for high levels of repetition in experiments. Our worm consists of less than 1000 cells, of which an overwhelming 302 are part of the nervous system (compare that to less than 1/10 for humans). With this nervous system, our friend can exhibit some extraordinary behaviours.
What C. elegans does
My own time with C. elegans was not spent looking at its genes, immune system or anatomy; instead, I was interested in its behaviour. Have you ever looked at some food and decided to stay clear of it? Has your decision to try something new ever been swayed by a trusted friend? Would you believe that our tiny worm can make exactly the same decisions?
No one likes being on the menu and bacteria have come up with different strategies to avoid being eaten. This can include being an awkward shape or size for their main predator [1] or producing a range of substances to make themselves unpalatable or toxic. The bacteria I investigated, plant pathogenic Pseudomonads, fall into this latter category. By genetically modifying them, one can subtly change the substances they produce, and thus how toxic they are. This system of largely identical bacterial strains is ideal to investigate the effect on the nematode of subtle differences in their food. By looking at how the food agrees with them, we can estimate how the toxicity of the bacteria is changed, while experiments where we offer the nematode two different types of bacterial food can reveal altered behaviour. Some substances are volatile, and when sensed through the olfactory system can influence predator behaviour over long distances. Some substances, on the other hand, appear to have an effect only after the bacteria have been ingested, so that nematodes will only differentiate between two different types of bacteria after 2 hours, when they start to feel their upset tummy. Thus over the next few hours more and more of the worms will choose the more edible food.
No matter how few nematodes you start the experiment with, you cannot avoid ending up with hundreds of them, as they are very prolific (about 300 offspring per hermaphrodite). This also allows us to investigate the behaviour of groups of hundreds and thousands of C. elegans. In these cases we often find that they team up in very large clusters and eventually completely destroy the bacterial population. A little bit like finding all your relatives coming round for dinner and completely raiding your fridge and pantry.
Researchers love to talk about their own work. So please do ask questions in the comments below, and I shall answer them in a follow up post.
References
image of C. elegans: By ZEISS Microscopy from Germany – C. elegans, model organism in life sciences, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=52989519
[1] http://aem.asm.org/content/72/1/78.full.pdf+html