Capsular typing of Streptococcus agalactiae (Lancefield group B streptococci) from fish using multiplex PCR and serotyping

Streptococcus spp. including Streptococcus agalactiae (Lancefield group B streptococci) are considered emerging pathogens responsible for approximately $1 billion USD in annual losses to the global tilapia (Oreochromis sp.) aquaculture industry. This study evaluated a published multiplex PCR for capsular (CPS) typing and commercially available antiserum to assign capsular serotype to a total of 40 S. agalactiae isolates including American Type Culture Collection (ATCC) isolates as controls. The multiplex PCR was modified from the original description to include primers specific for CPS types Ia, lb, II and III, that are the CPS types reported from aquatic animals. Phenotypic variation was noted among the S. agalactiae isolates in this study based on API (R) 20 Strep strip results. The modified PCR assay that includes a 688 base pair (bp) amplicon based on the cpsL gene diagnostic for S. agalactiae effectively typed piscine S. agalactiae isolates. Most isolates (28 of 30) from North, Central and South America were assigned to CPS type lb based on the presence of three amplicons of 688, 621 and 272 bp. A commercial serotyping system mostly confirmed the molecular CPS types. The modified multiplex PCR assay can be used to determine the CPS types of S. agalactiae present on a farm and/or in a region, and an understanding of this will assist research and disease management strategies (i.e. vaccination and selective breeding).