Mammary tissue microbiome analysis in PyMT mice reveals Methylobacteria as a commensal organism with potential therapeutic applications

Studies have reported differences in specific bacteria comparing the tissue microbiome in human breast cancer versus normal breast tissue, prompting hypotheses for potential therapies or theragnosis. To test these hypotheses using controlled experiments animal models are needed. Therefore, we investigated the microbiome in the gut and mammary tissue in a mouse model of breast cancer. C57BL/6 mice expressing the polyoma middle T antigen in the mammary gland (PyMT) develop spontaneous multifocal breast tumors. Microbiota in the gut and mammary tissue were studied prior to and after development of mammary gland tumors by amplicon and shotgun DNA sequencing. In parallel, RNA sequencing was performed on tumor and normal tissue to measure differences in gene expression associated with breast cancer. Bacteria identified in these studies were administered to mice to test their effects on cancer progression. Bacterial community composition in the gut of healthy or tumor-bearing mice showed wide fluctuation over time and did not organize into discrete clusters. In tumor versus healthy mammary gland tissue, relative abundances of six bacteria were significantly different: Ralstonia, Methylobacterium, Pelomonas, Staphylococcus and Tepidimonas. Methlyobacterium sequences were significantly higher (PERMANOVA, P = 0.013) in healthy tissue when compared to tumor, leading to a hypothesis that Methylobacterium may promote health. When co-transplanted with breast tumor cells, Methylobacterium reduced growth in immune competent mice. Here we describe the gut and mammary tissue microbial composition of healthy and breast tumor-bearing animals, identifying Methylobacterium sp as a commensal bacteria that might have therapeutic potential to reduce breast cancer progression.