What Makes a Pathogen? Genetic and Structural Heterogeneity of Neisserial Lipooligosaccharide

Abstract

The genus Neisseria consists of both pathogenic and commensal species that colonize mucosal niches. Specific structures of neisserial lipooligosaccharide (LOS) expressed by the gonococcus and meningococcus have been shown to play a role in pathogenesis. Commensal Neisseria also produce LOS, but can express additional structures not found in the pathogenic strains. N. sicca 4320 differs from other commensal Neisseria because it caused disease in a seemingly healthy individual. Therefore, this strain was used to research the extent to which a non-pathogen differs from a pathogen. A bioinformatic examination of the nucleotide sequence of the 4320 chromosome revealed strong homologies with N. meningitidis MC58. These findings suggest that 4320 is a member of the Neisseria and that commensal and pathogenic Neisseria have similar genomic content. Through MALDI-TOF, exoglycosidase digestion, and MSn analyses, 4320 was shown to express both LOS and LPS. The 4320 LOS resembles that of Haemophilus sp. in that it contained 3 heptoses. The LPS consists of a N-acetylglucosamine and rhamnose disaccharide repeat that is not attached to a lipid A, a novel molecule.

PEA transfer onto pathogenic Neisseria LOS was examined, as strains expressing PEA at 3-HepII as opposed to 6-HepII are more serum resistant. Southern hybridization was used to identify the presence of phosphoethanolamine (PEA) transfer genes in N. sicca strains. 4320 showed differences in the presence of Lpt3 transfer genes compared to other strains within the species. Lpt3 was isolated and biochemically shown to mediate the addition of PEA to 3-HepII of LOS that was constructed to lack the 3-HepII decoration. lpt3 nucleotide sequence analysis was done on a diverse group of gonococcal isolates, and phylogenetic analysis showed a high degree of sequence divergence. These data support the idea that the presence of PEA at 3-HepII of LOS increases the pathogenic ability of the Neisseria, as lpt3 was only found in N. sicca strains isolated from fatal endocarditis cases.