Abstract 4893: Profiling the ovarian cancer-specific intratumoral microbiome

Abstract Interactions between tumors and their associated microbiota have recently emerged as a novel dimension of cancer biology. These microbes can influence tumor initiation, immune evasion, and therapeutic response. Although several cancers, such as colorectal and esophageal cancer, harbor distinct microbial communities that shape their tumor microenvironment, the presence and functional relevance of intratumoral bacteria in ovarian cancer, the most lethal gynecologic malignancy, remain largely unexplored. Using pan-bacterial 16S rRNA fluorescence in situ hybridization (FISH), we detected bacterial signals in approximately 60% of ovarian cancer specimens, whereas benign ovarian tissues were nearly negative. The signals were predominantly localized (80%) within the tumor stroma. To further characterize the bacterial composition, 16S rRNA sequencing identified a unique intratumoral microbiota dominated by Phyllobacterium, Rhodococcus and Fusobacterium. To assess the biological impact of these bacteria, we established an in vivo mouse model by introducing the ovarian cancer-resident bacteria via intraperitoneal injection, followed by transplantation of mouse ovarian cancer cells. Tumor growth was approximately 3-fold higher in bacteria-exposed mice than in controls. These findings demonstrate that bacterial exposure can enhance tumor progression, potentially through remodeling of the tumor microenvironment. Collectively, our results reveal that ovarian cancer harbors a distinct, stromal-localized intratumoral microbiota that contributes to tumor progression. We propose that bacterial colonization of the tumor stroma may act as a chronic inflammatory niche, promoting fibroblast activation, angiogenesis, and immune modulation. Citation Format: Ryosuke Sonobe, Miho Suzuki, Keiko Shinjo, Yutaka Kondo. Profiling the ovarian cancer-specific intratumoral microbiome abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4893.