Sialylation of lipooligosaccharide cores affects immunogenicity and serum resistance of Campylobacter jejuni

Three genes involved in biosynthesis of the lipooligosaccharide (LOS) core of Campylobacter jejuni MSC57360, the type strain of the HS:1 seretype, who se structure mimics GM(2) ganglioside, have been cloned and characterized. Mutation of genes encoding proteins with homology to a sialyl transferase ( cstII) and a putative N-acetylmannosamine synthetase (neuC1), part of the b iosynthetic pathway of N-acetylneuraminic acid (NeuNAc), have identical phe notypes. The LOS cores of these mutants display identical changes in electr ophoretic mobility, loss of reactivity with cholera toxin (CT), and enhance d immunoreactivity with a hyperimmune polyclonal antiserum generated agains t whole cells of C. jejuni MSC57360. Loss of sialic acid in the core of the neuC1 mutant was confirmed by fast atom bombardment mass spectrometry. Mut ation of a gene encoding a putative beta -1,4-N-acetylgalactosaminyltransfe rase (Cgt) resulted in LOS cores intermediate in electrophoretic mobility b etween that of wild type and the mutants lacking NeuNAc, loss of reactivity with CT, and a reduced immunoreactivity with hyperimmune antiserum, Chemic al analyses confirmed the loss of N-acetylgalactosamine (GalNAc) and the pr esence of NeuNAc in the cgt mutant. These data suggest that the Cgt enzyme is capable of transferring GalNAc to an acceptor with or without NeuNAc and that the Cst enzyme is capable of transferring NeuNAc to an acceptor with or without GalNAc, A mutant with a nonsialylated LOS core is more sensitive to the bactericidal effects of human sera than the wild type or the mutant lacking GalNAc.