Streptococcus agalactiae, commonly known as Group B Streptococcus (GBS), is a member of the vaginal and gut microbiome of up to 30% of healthy individuals. GBS can cause pneumonia, meningitis, and sepsis in neonates when transmitted from mother to child. In addition, GBS can cause a range of diseases in immunocompromised adults and the elderly, including urinary tract infections, pneumonia, and meningitis. GBS has to compete with other microbiome members in different niches of the human host. Other Firmicutes, Staphylococcus aureus, Streptococcus intermedius, and Enterococcus faecalis, employ the Type VII Secretion Systems (T7SS) for intrabacterial or interbacterial antagonism. Analyses of GBS genomes revealed the presence of putative T7SS loci. These gene clusters encode the core components of T7SS machinery as well as putative secreted effectors/toxins. The goals of this application are to define the function of GBS T7SS in vaginal colonization and to characterize the role of GBS T7SS in interbacterial antagonism. We propose to assess the contribution of the GBS T7SS to GBS adherence, invasion, and host responses of the vaginal epithelium using in vitro human vaginal cell lines and mouse models of GBS vaginal colonization. We will also study the contribution of the GBS T7SS to GBS competition with other microbes in vitro and in humanized microbiota mouse models. The results of our study will significantly advance the understanding of the contribution of GBS T7SS to interbacterial interactions that shape the outcome of the host infection.