Because the frequency of omega turns is negatively correlated with the preference for an olfactory stimulus, a positive choice index indicates Nintedanib ic50 an olfactory preference for pathogen PA14 and a negative choice index indicates a preference for OP50. We defined the learning index as the difference in choice indexes exhibited by naive and trained animals (Figure 1C). We found that naive animals turned much less frequently when exposed to the smell of PA14 and generated a positive choice index, indicating that the naive olfactory preference is to PA14. However, after training, animals

generated similar numbers of turns when exposed either to the smell of PA14 or to the smell of OP50 and produced a choice index close to zero. Therefore, a positive learning index indicates that trained animals learn to decrease their attraction to the smell of PA14 after exposure to PA14 (Figure 1C). C. elegans exhibits spontaneous omega turns even when subjected to clean air that carries no olfactory stimulus ( Figure 1D). Thus, olfactory stimuli do not directly generate omega turns, but instead modulate the frequency of spontaneous turns. When exposed to the alternating air streams odorized with buffer and bacteria culture of either OP50 or PA14, both naive and trained animals exhibited a lower turning rate to the smell of either bacterial culture than to

buffer, indicating that they are attracted to the smell of food (see Figure S1A Selleckchem Cisplatin available online). Thus, learning specifically modulates the olfactory preference between

benign and pathogenic bacteria either without abolishing the general attraction of food smell. This form of aversive olfactory learning occurs and reverses rapidly. Adult animals trained on PA14 exhibited a significant amount of learning with two hours of training and became fully trained after 4 hr. When fully-trained adult animals were transferred back to a bacterial lawn of OP50, the learned olfactory preference gradually diminished over 2 hr (Figures S1B and S1C), indicating that the training process does not generate permanent alteration to the nervous system. Thus, the fully-developed nervous system of adult C. elegans is capable of experience-dependent modulations to avoid the smell of pathogenic bacteria. Although the aversive olfactory learning analyzed by the microdroplet assay reverses rapidly, three observations suggest that it is distinct from adaptation. First, no alteration in olfactory preference was detected in this assay when adult animals were trained with a series of nonpathogenic bacterial strains (Figure S1D), suggesting that the aversive learning is contingent on the pathogenesis of the training pathogen. Second, two adaptation mutants, egl-4 and adp-1, both displayed normal learning ability in the microdroplet assay ( Figure S1E).