A comparison of four methods for the serotyping of group B streptococci

Group B streptococci are implicated in a wide range of clinical conditions in human adults and neonates. The Group is subdivided into five serotypes Ia, Ib, Ic, II and III, which are differentiated on the basis of capsular polysaccharides. In the interests of epidemiology and efficiency a cheap, rapid method which is easily interpreted would be advantageous. In this study four methods of serotyping, namely, counter-immunoelectrophoresis (CIEP), microimmunodiffusion (MID), coagglutination (COA), and the Lancefield capillary precipitin (CP) test were compared in terms of ease of operation and interpretation, accuracy and rapidity. Todd Hewitt Broth (THB) cultures and acid extracts of the group B streptococcal strains were used as antigens for these methods. It was concluded that COA using THB cultures allows cheap and rapid screening for presumptive serotyping, having a 93-96% correlation with the CP test. MID gives an accurate (100% correlation with the CP test) and unambiguous confirmatory diagnosis of serotype.