Background: Group B streptococcus (GBS) is a leading cause of invasive bacterial disease in neonates. The possibility of maternal immunization with GBS-vaccines is being explored. Vaccine candidates include serotype-specific polysaccharide-protein conjugates and GBS surface proteins, including pilus island proteins. In this project, we aimed to undertake capsular serotype identification, pilus island identification and genotypic characterization of GBS isolates associated with colonization in mothernewborn dyads and invasive disease in infants. Methods: Colonizing GBS isolates were identified by vaginal swabbing of mothers (n=541) during active labor and from skin of their newborns post-delivery (n=395). Invasive GBS isolates from infants (n=284) were identified through laboratory-based surveillance. GBS serotyping was done by latex agglutination. Serologically nontypeable isolates were typed by a serotype-specific PCR method. The pilus islands from 541 colonizing isolates and 284 invasive isolates were characterized by real-time PCR targeting the ancillary protein 1 and 2. We undertook sequence typing based on the three most heterogeneous genes (adhP, atr and glnA) of multilocus sequence typing (MLST) on GBS isolates identified in young-infants with invasive disease (n=283) and those associated with maternal (n=525) and newborn colonization at birth (n=369). A total of 121 colonizing and 131 invasive disease GBS isolates that were representative of 55 and 35 clusters respectively were analyzed by the remaining four MLST genes. The gbs2018 locus was characterized by DNA sequencing.