The lipopolysaccharide of Legionella pneumophila serogroup 1 (strain Philadelphia 1): chemical structure and biological significance.

The lipopolysaccharide (LPS, endotoxin) of Legionella pneumophila serogroup 1 (Philadelphia 1) exhibits peculiar chemical features which may account for its importance as a bacterial virulence factor. The O-chain of this LPS constitutes a homopolymer of an unusual sugar, 5-acetamidino-7-acetamido-8-O-acetyl-3,5,7, 9-tetradeoxy-D-glycero-L-galacto-nonulosonic acid (legionaminic acid) of which about 10-75 residues are present. Due to the lack of free hydroxyl groups, this homopolymer renders the cell surface highly hydrophobic and, therefore, supports adherence to the membrane of target cells including alveolar macrophages. Investigations of the serological specificity of the serogroup 1 LPS revealed a monoclonal antibody (mAb 3/1) which recognizes an epitope located in the environment of the 8-O-acetyl group of legionaminic acid. According to epidemiological studies, this determinant appears to be associated with L. pneumophila virulence. The outer core oligosaccharide of L. pneumophila LPS exhibits also hydrophobic properties due to the presence of N- and O-acetyl groups as well as 6-deoxy sugars. The inner core expresses both similarities and differences as compared to enterobacterial core oligosaccharides in containing Kdo but lacking heptose and phosphate groups. Lipid A possesses some unique structural features since its backbone consists of a bisphosphorylated beta-GlcpN3N-(1-->6)-GlcpN3N disaccharide with only amide-linked acyl groups having 14-22 carbon atoms. Long chain fatty acids [28:0(27-oxo) and 27:0-dioic] possessing the double length as enterobacterial acyl groups, may be responsible for the low endotoxicity of L. pneumophila lipid A.