Breast-Cancer-Derived Secretomes from MCF-7 Cells Modulate Bacterial Pathogenic Traits

Breast cancer is the most frequently diagnosed malignancy among women worldwide, with the luminal A subtype being the most prevalent. Several studies have reported a complex interplay between breast cancer cells and the local microbiome; however, the mechanisms by which tumor cell-secreted factors influence bacterial biological properties remain insufficiently explored. In this study, we established an in vitro model that partially recapitulates the luminal A breast cancer microenvironment by exposing three breast-associated bacterial species, <i>Pseudomonas aeruginosa</i>, <i>Enterococcus faecalis</i>, and <i>Escherichia coli</i>, to conditioned media (CM) derived from MCF-7 (tumor) or MCF-10A (non-tumor control) cell lines. A combination of complementary approaches, including ultrastructural morphological assessment, biofilm formation assays, antimicrobial susceptibility testing, and virulence gene abundance profiling by genomic qPCR, was employed to reveal distinct tumor-microbiota interactions. Exposure to MCF-7 CM induced dose-dependent structural alterations in <i>P. aeruginosa</i> and <i>E. faecalis</i>, with pronounced membrane blebbing and structural disruption in <i>E. faecalis</i>. Biofilm formation was differentially modulated in a species- and concentration-dependent manner, with a persistent increase observed in <i>E. coli</i>. Antibiotic susceptibility profiles were selectively altered in <i>E. faecalis</i>, which displayed increased sensitivity to vancomycin, penicillin, and imipenem, along with decreased sensitivity to chloramphenicol. <i>P. aeruginosa</i> exhibited increased sensitivity to imipenem along with reduced sensitivity to meropenem and gentamicin, whereas no significant changes were observed in <i>E. coli</i>. qPCR analyses demonstrated that MCF-7 CM was associated with enrichment of multiple virulence-associated genes (e.g., <i>lasB</i>, <i>exoS</i>, <i>pilB</i>, <i>plcH</i>, <i>fsrC</i>, <i>esp</i>, <i>fimH</i>, and <i>papG</i>), reflecting enhanced pathogenic and adhesive potential. Collectively, these findings suggest that luminal A breast cancer-derived factors can reprogram microbial phenotypes in a species-specific manner, providing mechanistic insight into breast tumor-microbiome crosstalk and a platform to explore microbiome-targeted interventions.