This continual within and among-host evolution is likely to occasionally generate variants that are more likely to cause disease; however, mostly these are maladaptive and will not spread beyond the host in which they arise. One body of theory suggests that it is only mildly pathogenic variants that spread to cause large outbreaks, as they incur only a small cost for their pathogenicity [21]. In any case, it is likely: (i) that particular cell components have ambiguous rolls, promoting asymptomatic

transmission but also increasing the likelihood RAD001 concentration of causing disease; and, (ii) that different circulating genotypes are a consequence of evolutionary forces that act to balance transmission efficiency against their likelihood

to cause invasive disease [22]. In common with the great majority of bacteria that inhabit Osimertinib in vivo the nasopharynx, most meningococci present no risk to human health – a substantial proportion of meningococci possess no capsular locus [23], and only six of the 12 capsular serogroups are associated with disease, with five of these, serogroups A, B C, W and Y, responsible for most cases of invasive disease [24]. Multiple distinct genotypes exist which can be identified by multilocus sequence typing (MLST) as sequence types, which can be grouped into clonal complexes [25]. These are stable over decades and during global spread, but only a small number of them – the so-called ‘hyperinvasive lineages’ – cause most invasive disease [16]. The genetic factors responsible for the hyperinvasive phenotype are incompletely understood: although virtually all invasive meningococcal isolates have a polysaccharide capsule, and a number of other genes and gene products have been implicated in invasion [15]. The role of most of these is much more ambiguous, as none are found in all invasive meningococci, and many are shared with less invasive meningococci and other members of the genus Neisseria that do not cause invasive infections [26], [27] and [28].

The meningococcus thus represents a common member of the Ketanserin microbiota of the human nasopharynx which rarely causes disease. Even in the case of the hyperinvasive meningococci, most episodes of carriage are asymptomatic [29]. It is likely that carriage of these organisms has some benefit to the host, even if this is only preventing other more pathogenic bacteria occupying the same niche. Carriage of the close relative of the meningococcus, the acapsulate Neisseria lactamica, for example, is very common in infants but invasive disease cause by this bacterium is extraordinarily rare [30]. Almost certainly the carriage of these organisms results in the development of an immune response and, as individuals age, they acquire immunity against invasion from carriage [31].