hosphorylcholine is increasingly becoming recognized as a structural component of a wide variety of prokaryotic and eukaryotic pathogens that also possesses immunomodulatory properties. It was first detected in 1967 in the Gram-positive bacterium Streptococcus pneumoniae, where it was found to be associated with a polysaccharide component of the cell wall 1 , subsequently shown to be teichoic acid 2 . In addition, it was later found in cell membrane lipoteichoic acid 3 . It is known that in both structures, PC is attached directly to sugar residues, generally considered to be N-acetylgalactosamine 4 . Since its discovery in S. pneumoniae, PC has been detected in a range of Gram-positive bacteria including other Streptococcus spp. 5 , Clostridium spp. 6 , Lactococcus spp. 7 , Bacillus spp. 7 and also the Gram-negative bacterium Haemophilus influenzae (Ref. 8). Eukaryotic organisms in which PC has been detected include many important disease-causing agents such as the protozoa Leishmania major and Trypanosoma cruzi (Ref. 9), a wide range of fungi 10 , the trematode Schistosoma mansoni (Ref. 9), the tapeworm Diphyllobothrium latum (Ref. 11), several gastrointestinal nematodes 12, and all species of filarial nematode examined for its presence (reviewed in Ref. 14). As with prokaryotes, PC attachment in eukaryotic organisms appears to be via carbohydrate entities although sugars other than N-acetylgalactosamine can be employed for linkage 15, .
The function of PC for this wide range of organisms is generally unknown but roles have been assigned to it in Gram-positive bacteria. For example, depletion experiments have indicated that PC found in Streptococcus oralis appears to play a crucial role in the maintenance of normal cell shape, size and physiology . With respect to S. pneumoniae, anchoring of the pneumococcal surface protein A (PspA) has been shown to be dependent upon interaction with the choline component of PC in membrane-associated lipoteichoic acid 18 . The S. pneumoniae main cell wall degradative enzyme autolysin can also interact with choline residues of lipoteichoic acid and this is thought to have a role in regulating its activity 19 . In addition, this type of interaction is important in autolysin recognition of the cell wall teichoic acid 20 and is a prerequisite for its subsequent hydrolysis 21 . PC is thus necessary for normal growth and cell division in S. pneumoniae. However, it appears to have a number of properties unrelated to these physiological functions, which may or may not be of benefit to the host.