Many microbes exhibit dual roles within hosts existing both as colonizing normal flora and invasive pathogens. One such organism, Streptococcus agalactiae (Group B Streptococcus, GBS) is frequently isolated from the gastrointestinal and vaginal tract of 20-30% of healthy adults but is also the leading cause of infectious neonatal morbidly and mortality due to its virulence capacity to penetrate barriers such as the blood-brain barrier. Currently, the molecular and genetic mechanisms controlling the switch from a colonizing to an invasive pathogen are not well understood. Thus far, the genomes of only three GBS strains isolated from humans and seven strains isolated from other animals have been sequenced and completely assembled. We hypothesize that increasing sequence data of GBS isolated from human hosts will reveal genetic changes caused by specific host tissue interactions and help more fully characterize GBS biology. To study this transition, we have obtained of 45 vaginal colonizing isolates collected from pregnant women and five invasive strains isolated from the cerebral spinal fluid (CSF) of newborns. Following isolation of genomic DNA we have conducted whole genome sequencing using the illumina Miseq platform. Sequence data demonstrated high levels of coverage for most isolates and genome assembly and annotation have been recently completed. Initial genome mapping against a reference strain has revealed non-homologous regions that code for many proteins including those with phage, enzymatic, and hypothetical functions. In addition to genomic comparisons, we have conducted an in vitro assays examining bacterial adherence to human cells. By employing bioinformatic tools, we have studied sequential and functional homology between strains, phylogenetic relationships, and more specifically, genetic differences between colonizing and invasive isolates.