Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 17 5 2025 10 14 783 790 Farzaneh Zarei Mahmoodabadi Department of Biology, ET.C., Islamic Azad University, Tehran, Iran Akram Tabatabaee Bafroee Department of Biology, ET.C., Islamic Azad University, Tehran, Iran Arfa Moshiri Departments of Mycobacteriology and Pulmonary Research, Pasteur Institute of Iran, Tehran, Iran; Laboratory of Molecular Medicine, IRCCS Instituto Giannina Gaslini, Genova, Italy Seyed Davar Siadat Departments of Mycobacteriology and Pulmonary Research, Pasteur Institute of Iran, Tehran, Iran; Department of Biochemistry, Vanderbilt University Class of 2025, Microbiology Research Center (MRC), Pasteur Institute of Iran, Tehran, Iran Nayereh Ebrahimzadeh Departments of Mycobacteriology and Pulmonary Research, Pasteur Institute of Iran, Tehran, Iran 2025 04 28 2025 08 07 Background and Objectives: Breast tissue microbiota differs between healthy and cancerous tissues, with some bacteria influencing tumor progression. Staphylococcus epidermidis, a common skin commensal found in breast tumors, may play a role in epithelial-mesenchymal transition (EMT), a key step in metastasis. This study evaluated the effects of S. epidermidis culture and cell-free supernatant (CFS) on MDA-MB-231 breast cancer cell survival and expression of EMT-related genes Snail1, fibronectin 1 (FN1), and N-cadherin (CDH2). Materials and Methods: Different concentrations of S. epidermidis cultures and their CFS were applied to MDA-MB-231 cells. Cytotoxic effects were assessed by MTT assay at 2, 4, and 24 hours post-treatment. Real-time PCR analyzed gene expression after 24 hours of exposure to non-toxic concentrations (MOI 50 and 100 for cultures; 14% for CFS). Results: Low concentrations did not affect viability, while higher doses (MOI 100 and 14% CFS) reduced viability by up to 60% and 90%, respectively, at 24 hours. MOI 50 did not significantly alter gene expression. At MOI 100, Snail1 and FN1 were significantly upregulated, but CDH2 was unchanged. Treatment with 5% and 7% CFS significantly increased all three EMT gene expressions, indicating EMT induction. Conclusion: S. epidermidis affects EMT gene expression and cell viability, indicating potential involvement in breast cancer progression. https://ijm.tums.ac.ir/index.php/ijm/article/view/5468 https://ijm.tums.ac.ir/index.php/ijm/article/download/5468/1824