The eye does not see what the mind does not know: the bacterium in the worm.

Symbiotic relationships underlie the evolutionary success of many different life forms. The filarial worms are long, slender nematode parasites that cause considerable pathology in large segments of the world's population. About 25 years ago, investigators first reported the presence of bacterial organisms living inside these parasitic worms. Recent molecular biological studies have indicated that these bacteria belong to the genus Wolbachia, members of which have been known to be associated with numerous species of insects. Elimination of the Wolbachia from the nematodes (by, for instance, treatment with broad spectrum antibiotics) results in profound disturbances in the physiology of the latter, including a complete block in reproduction. This observation, taken together with the fact that every individual worm examined to date contains Wolbachia, validates the classification of the latter as "endosymbionts." Many studies indicate that the Wolbachia may also play an important role in the pathology caused by the nematode worms, and that it might be possible to target therapy against the Wolbachia to treat the filarial disease. Intriguingly, the intense recent interest in Wolbachia is in complete contrast with the virtually complete indifference evoked by the original discovery of these organisms.