Obligate parasites of nematodes: viruses and bacteria in the genus Pasteuria .

This chapter covers a group of organisms that are considered 'obligate' parasites because they are nutritionally dependent on nematodes and cannot lead an independent existence outside their host. Viruses are considered obligate parasites in this paper because the resources they use for reproduction are derived from living cells. However, the virus section is relatively brief, as viruses have largely been overlooked as agents capable of causing disease in nematodes. The main focus is on the genus Pasteuria , a widely distributed bacterium that is known to attack many different nematodes, including all economically important plant parasites.