Neisseria meningitidis is the causal agent of epidemic bacterial meningitis and sepsis. Comparison of isolates from the human nasopharynx without symptoms of disease and those from invasive sepsis reveal that these isolates form two genetically distinct populations. The isolates from asymptomatic carriage most often lacked the capsule biosynthesis (cps) locus which is almost always associated with invasive disease isolates. To determine whether there were other genomic islands associated with asymptomatic carriage, this study compared asymptomatic carriage isolates with pathogenic meningococci.
Twenty four asymptomatic carriage meningococcal isolates were collected during the Kalgoorlie Otitis media study. These strains were whole genome sequenced using Illumina. The genomes were assembled into contigs using SPAdes genome assembler and compared using Genome Comparator on the Bacterial Isolate Genome Sequence Database (BIGSdb) platform.
Six isolates (25%) belonged to clonal complex (cc)53, which is a genetic lineage that is not associated with invasive disease. The remaining strains belonged to eight different clonal complexes, with the exception of 5 isolates which were found to belong to novel clonal complexes. The core genome of these 24 isolates consisted of 1724 genes while the accessory genome consisted of 605 genes. Of the 605 genes, strains within the hype-virulent lineage cc32 possessed the most (300 genes) while isolates within cc53 possessed the least (159 genes). The accessory genome was compared between isolates to identify genes and genomic islands that were unique to each strain. A phage that has previously been found exclusively in N. gonorrhoeae was identified in a single isolate that belonged to a novel clonal complex. This is the first study to identify this island in N.meningitidis. The gonococcal genetic island (GGI) was identified in three isolates. The GGI lacked the genes necessary for expression of a Type IV secretion system, but possessed 13 genes for which the function is currently unknown.