Hugh Pickens writes "While it may seem unlikely that single-celled organisms could be trained to salivate like Pavlov's dog at the sound of a bell, researchers say that bacteria can 'learn' to associate one stimulus with another by employing molecular circuits. This raises the possibility that bioengineers could teach bacteria to act as sentinels for the human body, ready to spot and respond to signs of danger. As with Pavlov's dog, the bacteria in the model learn to build stronger associations between the two stimuli the more they occur together. Now called Hebbian learning, it's often expressed as a situation in which 'neurons that fire together wire together.'" (More below.)"Bacteria, of course, don't have synapses or nerve cells, but Eva Jablonka, who just published a paper on conditioning in single-celled organisms (PDF), says it seems 'quite possible at the theoretical level, and I don't see great obvious hurdles for the construction of the suggested vectors.' The trick will be to train bacteria to recognize chemical processes in the body that are associated with danger like an adverse and dangerous reaction to a drug, or to the presence of tumor cells."