Preparing for a clinical trial of interventions to maintain normal vaginal microbiota for preventing adverse reproductive health outcomes in Africa Human body surfaces are covered by bacteria called the human microbiota. This microbial community is believed to contain at least 10 times the number of cells and 100 times the number of genes in the human body. Considerable progress has been made in characterising the human microbiota in recent years. Researchers have discovered that these microbial communities are needed for proper functioning of the human body, for example to digest complex carbohydrates in the gut and to prevent pathogen invasion. At the same time, imbalances of the normal microbiota have been shown to be associated with a number of diseases.   The human vagina also contains bacterial communities of mostly lactobacilli that are thought to protect women and their foetuses from pathogen invasion. When these bacterial communities are imbalanced, pathogens may enter the uterus, placenta, membranes surrounding the foetus, or abdominal cavity, causing pelvic inflammatory disease, miscarriages, pre-term births, or maternal or neonatal sepsis. These imbalances have also been associated with increased transmission of HIV and other sexually transmitted infections (STIs) between sexual partners and mother-child pairs. Some of the imbalances have been diagnosed in the past (using symptoms reported by the patient and microscopy of vaginal fluid) as bacterial vaginosis (BV). Unfortunately, BV has been notoriously difficult to treat, and even when treated successfully, recurrence rates are high. Because researchers have not been able to prevent BV recurrences during pregnancy, they have also not been able to prevent subsequent poor pregnancy outcomes.  In this development grant, we will develop interventions to maintain normal vaginal microbiota after initially having cured BV. The study participants will be Rwandan women who are at high risk of HIV and other STIs but who are not pregnant. We will ask one group of 15 women to use a vaginal gel (0.75% metronidazole gel) twice per week for 2 months, a second group of 15 women to use a vaginal probiotic tablet called Gynoflor thrice weekly for 2 months, and a third group of 15 women to use a vaginal probiotic capsule called Ecologic Femi thrice weekly for 2 months. Both probiotics contain 'healthy' bacteria called lactobacilli, and Gyneflor also contains a very low dose of oestriol. A further group of 15 women will form the control group. All women in all groups will receive safer sex and family planning counselling as well as individualised counselling on how to integrate the study interventions into their usual vaginal hygiene practices. At the end of the project, we will evaluate whether we were able to maintain healthy vaginal microbiota during the 2-month intervention period as well as the 4 months after cessation of the interventions. We will also evaluate the acceptability and feasibility of these interventions.  A novel aspect of this trial is that we will not only use standard methods to evaluate the vaginal microbiota (i.e. reporting of symptoms and microscopy of vaginal fluid) but also new molecular methods based on the sequencing of bacterial genes. This allows for a holistic and in-depth characterisation of microbial communities.  The results of this trial will be used to plan a larger randomized controlled clinical trial of the most promising intervention(s). The Global Challenges Research Fund (GCRF) supports cutting-edge research to address challenges faced by developing countries. The fund addresses the UN sustainable development goals. It aims to maximise the impact of research and innovation to improve lives and opportunity in the developing world.