Comparison of three different methods in monoclonal antibody-based detection of Streptococcus agalactiae protein serotype markers

Surface‐exposed proteins are important serotype markers in Streptococcus agalactiae (group B streptococci; GBS). The proteins include the c proteins cα and cβ, the R4 protein and a protein provisionally called P. For all of these markers, protein‐specific monoclonal antibodies (MAbs) have been generated. We have compared whole‐cell‐based fluorescent antibody testing (FAT), ELISA, and dot blotting for MAb‐based detection of these proteins by testing a panel of 52 GBS isolates of different capsular antigen types. Of a total of 208 observations with each of the tests, positive signalling in the dot assay was observed in 32.2%, with ELISA in 27.8%, and with FAT in 26.4% of the recordings. Discordant results were noted most frequently with the c13 and cα MAbs. In the case of cα the reason for the discordant test results was further examined and it appeared that this could be attributed to low level expression of the cα protein, although structural variations of cα proteins cannot be excluded. Our findings favour dot blotting as the method of choice although we consider all three methods acceptable for serotyping of GBS.