We propose to leverage existing longitudinal rectal samples from the TRUST/RV368 study, a cohort of 2737 Nigerian MSM followed every 3 months for up to 18 months with rectal NG/CT testing, exams and behavioral questionnaires. We will assess the rectal microbiota at the visit prior to and at time of rectal STI diagnosis in 300 incident cases (150 CT, 150 NG) against samples from 200 controls who acquired no rectal STIs during follow-up. We will assess the impact of behavioral factors on the rectal microbiota, and compare the rectal microbiota before and at the time of rectal NG or CT infection. Finally, we will identify rectal microbiota signatures that associate with risk for incident rectal NG and CT infection, controlling for confounders such as receptive anal intercourse. In the long term this project will provide foundational data to inform future, larger studies, and may lead to novel interventions to treat and prevent rectal STIs in MSM.