Immunochemical studies and genetic background of two Neisseria meningitidis isolates expressing unusual capsule polysaccharide antigens with specificities of both serogroup Y and W135.

We described 2 unusual Neisseria meningitidis strains isolated from epidemiologically unrelated invasive meningococcal disease cases in Ontario, Canada. Both isolates have features typical of serogroup Y N. meningitidis: are of serotype 2c, are of the multi-locus sequence types typical of the serogroup Y strains in Canada, and are genotyped as serogroup Y based on a previously described PCR-ELISA method that detects the serogroup-Y-specific siaD gene. However, both strains were poly-agglutinable in both anti-Y and anti-W135 antisera. Further studies on 1 of these 2 isolates showed the presence of glucose and galactose as well as sialic acids in its purified capsular polysaccharide, suggesting the presence of both serogroup Y and serogroup W135 polysaccharides. Rabbit antisera produced to this strain contained antibodies to both purified serogroup Y and serogroup W135 capsular polysaccharides. Absorption experiments with either serogroup Y or serogroup W135 bacteria confirmed the presence of antibodies to these 2 different polysaccharides. DNA sequencing of the cps operon from both isolates revealed a siaD gene with 99.7% homology to the published siaD sequence from a serogroup Y strain but with 3 point mutations that all resulted in amino acid changes. How these strains may affect results of routine surveillance, PCR diagnosis, and immuno-protection by vaccination are discussed.