Abstract Abstract Chemoresistance remains a significant obstacle in breast cancer therapy, while the role of intratumoral microbiota in this process has yet to be fully elucidated. Here, we identify Lactobacillus iners (L. iners) as a critical microbial factor contributing to resistance against standard anthracycline-taxane-based neoadjuvant chemotherapy. Through integrated multi-omics analyses of longitudinal patient cohorts and 4T1 murine models, we demonstrate that L. iners promotes chemoresistance by reprogramming tumor GSH metabolism and inhibiting ferroptosis. Mechanistically, L. iners-derived L-lactate induces lysine lactylation of the RNA methyltransferase NSUN2, thereby enhancing m5C modification and post-transcriptional stabilization of glutathione synthetase (GSS) mRNA. This subsequently upregulates GSS, augments GSH biosynthesis, and reinforces redox homeostasis, ultimately protecting tumor cells from ferroptotic cell death. Clinically, high intratumoral abundance of L. iners correlates with diminished treatment response and shorter recurrence-free survival. Notably, selective depletion of L. iners using a tumor-targeted antibiotic nanoplatform restores chemosensitivity, suppresses NSUN2 lactylation, and reactivates ferroptosis without disrupting systemic microbiota homeostasis. Collectively, these findings uncover a microbiota-NSUN2 lactylation-GSS regulatory axis underlying chemotherapy resistance and provide a translational strategy for microbial-targeted therapy in breast cancer. Citation Format: Y. Lu, S. Pilei. Intratumoral Lactobacillus iners Drives Chemoresistance in Breast Cancer: A Ferroptosis-Evasion Mechanism via NSUN2 Lactylation abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-11-19.
Lu et al. (Tue,) studied this question.