Characterisation of genomic islands in Neisseria meningitidis

Neisseria meningitidis asymptomatically colonises the nasopharynx of about 10 % of the human population. This bacterium is an accidental pathogen, with N. meningitidis serogroup B being the main cause of meningitis and septicaemia in developed countries. Strain MC58 has a genome size of 2,272,360 bp and contains 2160 ORFs, more than half of which have already been assigned a function. Nine conserved genomic islands that are absent from the closely related commensal species N. lactamica were identified and comprise 38 genes. Of these, 14 still encode proteins of unknown function. Two of these islands (pathogenic islands 4 and 8) were investigated, and the genes crucial for two further islands (pathogenic islands 3 and 5) involved in polyamine biosynthesis were successfully knocked out in this work. Genomic island 4 contains six genes, which encode proteins that are involved in the 2-methylcitrate pathway; these genes are clustered together to form the prp operon. Several genes belonging to this pathway (prpC, NMB0432 and ackA-1) were knocked out, and the resulting mutants were unable to utilise propionic acid, which is the substrate for the pathway being investigated. Saliva from over 300 healthy students was analysed for propionic acid content, and the data were compared to the meningococcal carriage status. No significant correlation, however, was found between the concentration of this fatty acid and the carriage status. Genomic island 8 contains coding sequences for the two hypothetical proteins NMB1048 and NMB1049. NMB1049 has been shown to regulate the expression of the divergently transcribed NMB1048 but not prpC. NMB1049 is, in fact, a putative LysR-Type transcriptional regulator. Both genomic islands were also investigated for their involvement in N. meningitidis carriage or infection. Human blood samples from several healthy individuals were inoculated with N. meningitidis MC58, and results showed that only half of the blood samples had bactericidal effects. These results were independent of the strain used, as they were similar for the wild-type and the mutants in the prpC and NMB1049 genes. The catabolism of propionic acid seems to give an advantage to N. meningitidis in colonising the adult nasopharynx.