In addition to determination of the meningococcal serogroup by the immunological reactivity of the capsular polysaccharide, more detailed characterization can be achieved by detection of antigenic variation of ''subcapsular'' components of the meningococcal outer membrane (Fig. 2). Meningococci produce two porin proteins, PorA and PorB, which form trimeric pores in the meningococcal outer membrane. The antigenic variation in the PorB and PorA monomers is used to define the meningococcal serotype and serosubtype, respectively. The meningococcal surface is also coated in lipooligo-saccharide (LPS), an amphipathic, phosphorylated gly-colipid. This is composed of a membrane-anchoring lipid, a backbone to which core saccharides are attached, which are in turn linked to structurally variable oligosaccharide moieties. Variation in the oligosaccharide moiety of the surface LPS is used to define the meningococ-cal immunotype.
An inherent limitation of immunological characterization is the antigenically variable nature of the targeted surface components.1-14-1 Such variation means that new reagents are continually required to identify previously uncharacterized variants. A further problem is the dynamic nature within individual bacteria of expression of the structures to which serological typing reagents are targeted. Switching of antigenic types by horizontal genetic exchange of the genes encoding them, or involved in their synthesis, also poses problems for the reliability of immunological characterization techniques. Some carried meningococci even lack the genes necessary for capsular synthesis and transport, thus precluding sero-group determination.1-17-1
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