Neisseria meningitidis is the causal agent of epidemic bacterial meningitis and sepsis. Comparison of isolates from the human nasopharynx and those from invasive sepsis reveal that they form two genetically distinct populations. The strains from asymptomatic carriage most often lacked the capsule biosynthesis (cps) locus. The pathogenic isolates have obtained the cps locus via horizontal gene transfer (HGT) and this feature is essential for the establishment of bacteremia. This is a comprehensive study examining the prevalence of HGT events in asymptomatic and disease-causing lineages of meningococci.
Twenty four asymptomatic carriage meningococcal isolates were collected from the Kalgoorlie Otitis media study. The strains were whole genome sequenced using Illumina. The genomes were compared using Genome Comparator on the Bacterial Isolate Genome Sequence Database (BIGSdb) platform. The output showed the presence or absence of specific loci. These 24 genomes were compared with the reference N. meningitidis strains FAM18, MC58 and alpha14.
A genetic lineage was assigned to all strains. Five strains belonged to clonal complex (cc)53 which is a genetic lineage not associated with disease. The remaining 19 strains belonged to known pathogenic lineages. Whole genome comparison revealed 73 genetic islands that were associated with different genetic lineages. These prevalence of HGT events within five lineages was analysed using a total of 4850 whole genome sequenced strains. Four common hyper-virulent lineages were used; cc11 (n=1459), cc41/44 (n=833), cc32 (261) and cc269 (n=601). Six lineages less often associated with invasive disease were used, being cc213 (n=275), cc22 (n=141), cc23 (n=720), cc461 (n=101), cc5 (n=142), cc60 (n=103) and cc865 (n=162). The genetic lineage cc53 was used to represent the asymptomatic carriage isolates (n=52). Overall, cc53 isolates lacked 37 genetic loci that were present in the genetic lineages associated with invasive disease. Additionally, cc53 possessed 10 genetic regions that were not present in genetic lineages associated with invasive disease.