It's sometimes said that food is the way to a man's heart and C. elegans are no different; they love tucking into a plate of E. coli. But C. elegans rarely experience E. coli in the wild. Their natural foods of choice must be some of the hundreds of bacterial species cultivated in soil. Intrigued by the nematode's natural dining habits,Boris Shtonda and Leon Avery from the University of Texas Southwestern Medical Center decided to find out whether the worms fared better on some bacteria than others. Offering the nematodes a selection of 40 bacterial varieties gathered from local Dallas soils, the team soon realised that worms thrived on some species while barely growing on others. Recognising that all bacteria are not equal from a nematode's perspective, Shtonda and Avery decided to investigate whether the tiny worms were capable of making choices between nutritious and poor bacteria, and whether past experiences influenced their dining choice (p. 89).
Once Shtonda knew which bacteria the worms thrived on and which they grew less well on, he began offering the tiny worms a simple choice between two bacterial colonies of different nutritional qualities placed close together. At first, the worms distributed themselves evenly between the two dining options, but within several hours, Shtonda began noticing that the worms were expressing a preference. If offered a choice between a nourishing species and a less nutritious bug, most of the worms eventually ended up in the more nutritious of the two. The worms were making a choice between a good and bad diet.
Amazed that such a tiny creature could make the choice between fine dining and junk food, Shtonda decided to test whether the worms actively chose to eat a more nutritious diet, or were they deciding to abandon the poorer meal in search of something better. The team began filming the worms as they ate either nourishing or poor bacteria to see if they could identify particular behaviour patterns. Sure enough, Shtonda quickly realised that worms offered a poor diet had a much higher probability of leaving the dining spot than worms offered a better quality meal. C. elegans was making the decision by leaving behind poor food in the hope of finding something better.
Curious to know how sophisticated the worm's decision making process was,Shtonda decided to investigate whether earlier dining experiences had any affect on the nematode's later decisions. Having allowed the worms to feed on one of the bacterial strains for 2 hours, Shtonda then offered them a choice between good and poor bacteria to see if past experiences influenced their choice. As Shtonda suspected, worms that experienced a good diet at some time in their lives were more prepared to search for good meals than worms that had been treated to a mediocre diet; `a good food experience makes them more motivated to seek better food later' says Shtonda.
Puzzled by what controls the tiny animal's ability to make a choice,Shtonda began screening a wide range of mutant worms to see which ones had the biggest problem making a decision and was surprised when a worm that had lost its ability to migrate towards a warm spot also proved incapable of choosing a good diet. Knowing which neuron was impaired in these mutant worms, Shtonda inactivated the neuron in wild-type worms, and they became less able to make a choice. Having found part of the circuit that controls the nematode's food seeking behaviour, Shtonda is keen to know how it regulates the worm's choice but suspects that it will be a long time before he knows the way to a worm's heart.