Sammendrag
S. agalactiae isolates produce a polysaccharide capsule, although occasionally synthesis of the capsular polysaccharide (CPS) may not be detectable. Nine capsular serotypes have been recognized, Ia, Ib, II, III, IV, V, VI, VII and VIII. Serotype Ia, III and V strains most often cause invasive disease in newborns and adults.
GBS expresses a variety of surface-localised proteins. Some of the best characterised of these proteins are the c proteins alpha and beta and the R proteins (R1-R5).
With the exception of c protein beta, probably all these proteins belong to the family of alpha-like proteins, named after the alpha c protein. These proteins are resistant (R) to digestion with trypsin, are organized with a N-terminus, a repeat region composed of a variable number of identical large and tandemly arranged repeat units, each with ~80 aa, and a C-terminus. Homology to variable extent exists between corresponding regions, up to 100%, possibly due to inter-strain exchange of genetic elements followed by recombinational events, resulting in mosaic structures. The structural matching explains why some of the proteins show serological cross-reactivity. The cross-reactivity has hampered the reliability of antibody-based protein detection.
Traditionally, typing of GBS has been based on testing of isolates using rabbit antibodies specific for CPS of each of the CPS types. The discriminatory power of this typing can be augmented if combined with sub typing, i.e. by also testing of expression of the surface-localised proteins. The final GBS categories constitute the serovariants. For instance, type III GBS will be subdivided into the serovariants III/R1 and III/R4, type Ia into the variants Ia/cα and Ia/cαβ. For serotyping and serosubtyping, at least 15 different specific antisera will be required for the time being. This lays burdens on resources, and few centres in the world, if any, has all the antibody-reagents required for complete serovariant determination. Also, occasional isolates may not express the protein of interest in spite of possession of the encoding gene. Recently, molecular methods have emerged as an alternative to antibody-based methods for both CPS type determination and detection of the genes encoding the strain-variable surface, including that the majority of these genes can be detected in a mixed PCR. Testing along these lines probably will replace the antibody-based testing in identification of epidemiological GBS markers.
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