Background: Group B Streptococcus (Streptococcus agalactiae, GBS) is a major cause of neonatal morbidity and mortality and is increasingly recognised as a pathogen in elderly and/or immunocompromised patients. GBS is currently grouped into 10 serotypes, and in our laboratory the referred isolates are characterised by slide agglutination (SA) with sera commercially available from the Statens Serum Institut (SSI). Agglutination visualisation can be problematic, leading to variation in result interpretation, which could hinder outbreak detection, and discrepancies may only be recognised during internal and external quality assessments. H&S risks when using SA assay were identified, such as repetitive strain injury, aerosol/droplet generation, and cuts from glass slides.
Methods: A microtitre assay method (MTP) was devised using the 10 sera from SSI, and 523 GBS suspensions, including 10 type strains, were heat inactivated (96°C + 2°C), cooled and incubated with the latex reagents in a U-shaped well microtitre plate. A total of 8 samples can be tested in a single assay against 10 sera, in addition to prolex grouping reagents.
Results: 91% agreement was initially obtained with 47 strains tested. Further 92% concordance was observed for 476 strains during parallel testing of MTP and SA assays. The discordant results were mainly due to non-typeability (84%, 32/38 samples). The MTP assay has shown high concordance with the SA assay; discrepancies were generally associated with strains agglutinating with more than one sera. The MTP method can be used in routine GBS serotyping, and provide the following advantages compared to SA: clearer agglutination visualisation, discrepancy resolution and H&S risks reduction.