Engineered bacteria are increasingly being explored as living medicines to prevent, sense, or treat disease directly in the body. These approaches have shown promise across a wide range of applications, including infections, inflammatory disorders, and cancer, but most efforts have focused on model organisms that are not naturally adapted for in situ stable colonization. In contrast, using host-associated, niche-adapted bacteria as therapeutic chassis may offer important advantages in colonization, persistence, and local efficacy. In the female genital tract (FGT), the vaginal microbiota is a major determinant of susceptibility to HIV, bacterial vaginosis (BV), and other reproductive tract disorders. Among vaginal microbes, Lactobacillus crispatus is strongly associated with mucosal health, reduced inflammation, improved epithelial barrier function, and protection against pathogen acquisition. This makes it a particularly attractive candidate for the development of vaginal live biotherapeutics designed to act directly at the mucosal surface. However, L. crispatus has long remained underexploited as a therapeutic platform because of major technical barriers, including low transformation efficiency and the lack of robust, standardized genetic tools for clinical isolates. To address this gap, we are developing a synthetic biology framework to engineer vaginal L. crispatus for microbiome-based intervention. We established optimized transformation methods and modular expression tools, enabling stable engineering of multiple clinical L. crispatus strains. Using this platform, we also achieved secretion and membrane display of heterologous proteins in this historically intractable non-model species, expanding the range of therapeutic strategies that can be implemented in vaginal lactobacilli. As a proof of concept, we engineered L. crispatus to express anti-HIV nanobodies for localized protection at the vaginal mucosa. More broadly, this platform opens the way for the development of bacterial therapies aimed not only at reducing HIV susceptibility, but also at preventing BV and restoring a protective vaginal microbiota. Beyond therapeutic delivery, these tools also provide a foundation to functionally interrogate vaginal lactobacilli and better understand the mechanisms through which the microbiota shapes women’s reproductive health.