Association between asymptomatic carriage and sporadic (endemic) meningococcal disease in an open community

We analysed a strain collection representative of the overall Neisseria men ingitidis population circulating in an open community (46000 inhabitants, S pain) during an endemic period (30 isolates from patients and 191 from thro at cultures of healthy individuals) by both phenotypic and molecular techni ques. Almost all patient isolates were assigned to three hyper-virulent lin eages (ET-5 complex, ET-37 complex and cluster A4) by both multilocus enzym e electrophoresis (MEE) and pulsed-field gel electrophoresis (PFGE). In con trast, MEE and PFGE assigned 20% and 15% respectively of carrier isolates t o the hyper-virulent clones (4% for both methods together). There was also a higher correlation between PFGE and phenotypes associated with virulent c lones. These notable differences between the two molecular methods were fur ther observed in more than half the carrier isolates, suggesting that the a ssociations between these strains were distorted by recombination events. H owever, almost one-third of total endemic strains from symptom-free carrier s and almost all patient strains belonged to clones defined by MEE and PFGE , with no known epidemiological connection. These data indicate low transmi ssion and a weak clonal structure for N. meningitidis.