is a Gram-negative bacterium that has the potential to cause sudden and acute diseases, including meningitis and septicemia (25
). N. meningitidis
is protected from host-mediated, complement-dependent bacteriolysis and phagocytosis by a polysaccharide capsule (25
) and is classified into 12 serogroups determined by the chemical composition of the capsule. Distinct serogroup capsule compositions differ by either their particular sugar makeup or the linkages between sugars (7
). Meningococci frequently take up and incorporate genomic DNA from the environment via homologous recombination (12
). This may produce a change in capsule genes, resulting in a change in the composition/serogroup of the capsule sometimes referred to as capsule switching (29
). The most typically disease-associated serogroups are A, B, C, W-135, and Y (35
). Effective vaccines are available for disease-associated serogroups A, C, W-135, and Y (3
), whereas there is currently no protective vaccine specifically against the serogroup B capsule (11
). Therefore, capsule switching may produce escape mutants such as C to B. Accordingly, rapid identification of N. meningitidis
capsule serogroups is a critical component of public health response to meningococcal disease.
The chemical composition of the N. meningitidis
polysaccharide capsule is classically inferred by using the slide agglutination serogrouping (SASG) assay (23
). The SASG assay reveals the capsule type with serogroup-specific antibodies that yield positive or negative reactions. Ideally, exactly one reactivity will be positive per isolate but there are occasionally multiple or no positive reactions. In these cases, the isolate is characterized as “nongroupable” (NG). Isolates that have multiple positive SASG assay results are referred to as NG-polyagglutinating. Mothershed et al. introduced the serogroup-specific real-time PCR (SGS-PCR) assays to help resolve NG SASG assay results (22
). The SGS-PCR assay is a real-time PCR test that uses primers and probes specific to each serogroup. The target of these assays is typically the gene that encodes the capsule polymerase specific to each serogroup. The CDC Meningitis Laboratory and other reference labs routinely perform both SASG and SGS-PCR assays to group isolates quickly and unambiguously.
Despite the availability of these two assays for the characterization of N. meningitidis
capsule serogroups, there remain instances of NG isolates and at times the SASG and SGS-PCR assay results do not agree. Examination of these NG isolates reveals three distinct classes: autoagglutinating, nonaggultinating, and polyagglutinating. Autoagglutinating isolates agglutinate in the saline control for the SASG assay via a mechanism unrelated to serogroup specificity. Nonagglutinating isolates show no evidence of capsule expression, and a number of such isolates have been genetically characterized. This work showed that nonaggultination can result from a loss of capsule expression through point mutations and/or phase variation, as well as partial or complete deletions of capsule operon sequences (5
). Polyagglutinating isolates are defined as showing cross-reactivity to two or more serogroup-specific antisera. At this time, the genetic basis of polyagglutination is less well understood. Polyagglutination has been hypothesized to result from the expression of multiple capsule types but has more often been attributed simply to the quality of the antiserum reagents used in the SASG assay.
In this study, we sought to characterize the genomic basis of an NG polyagglutination result for a cerebrospinal fluid (CSF) isolate (M16917) from a patient with acute meningococcal disease. To this end, we characterized and analyzed its complete genome sequence. The M16917 genome sequence was analyzed to determine its likely genomic origin, and the capsule-encoding locus was compared with capsule loci from genomes with known serogroups in order to determine its specific structure. The M16917 capsule locus was found to be chimeric, with sequences derived from both B and C serogroup-encoding regions.
Nucleotide sequence accession number.
The genome sequence data obtained in this study have been submitted to the NCBI sequence read archive and assigned accession number SRX111309.