To find out whether worms learn from experience to avoid or seek out certain types of bacteria, the researchers compared the behavior of three sets of worms. The first group had been exposed only to a harmless strain of E. coli for their entire lives. The other two groups had been raised since hatching in the presence of both E. coli and a second, pathogenic type of bacteria -- Pseudomonas aeruginosa for one group and Serratia marcescens for the other. These bacteria are not only toxic to C. elegans, but also common causes of hospital-acquired infections in humans.
When the worms that had been grown only on E. coli were later introduced to pathogenic bacteria (either P. aeruginosa or S. marcescens), they were just as likely to select the new, toxic food source as they were to stick with the safer choice. In fact, when the choice was between E. coli and S. marcescens, they migrated towards the latter, despite the likelihood that this would result in a toxic infection. The worms that grew up exposed to a toxic variety of bacteria, however, showed a clear preference for E. coli over the pathogen they were familiar with.
The researchers went on to show that it did not take a lifetime of exposure to a toxic food source for the worms to learn to avoid it. In fact, just four hours in the presence of P. aeruginosa was enough for the worms to choose E. coli instead.
From these experiments, it was clear that the worms modified their olfactory preferences to avoid toxic bacteria ?but it was not yet clear whether they were actively avoiding food that might lead to ill effects, or, conversely, developing a stronger attraction toward food they had found to be safe. With the traditional experimental set-up, where worms migrate toward either of two types of bacteria growing on opposite sides of a lab dish, this was impossible to discern, Bargmann said.
Source:Howard Hughes Medical Institute