The Gut Microbiome as a Potential Predictive Biomarker for Breast Cancer: Emerging Associations and Geographic Differences

Byeongsang Oh^1*Gillian Lamoury¹Susan Carroll¹Marita Morgia¹Frances Boyle²Nick Pavlakis¹Stephen Clarke¹Alexander Guminski¹Alexander Menzies¹Connie Diakos¹Katrina Moor¹Thomas Eade¹Mark P Molloy³Michael Back¹

• ¹Northern Clinical School, Faculty of Medicine and Health, The University of Sydney, Sydney, Australia
• ²The Mater Hospital, North Sydney, Sydney, Australia
• ³Bowel Cancer and Biomarker Laboratory, Kolling Institue, St Leonards, Sydney, Australia

Background: The gut microbiome may influence breast cancer (BC) development by modulating estrogen metabolism, immune responses, and microbial metabolites. Altered microbial patterns have been reported in BC, but their value as predictive biomarkers remains uncertain. Methods: We reviewed 13 case–control studies that compared gut microbiome composition in women with and without BC, focusing on diversity, compositional shifts, and geographic variation. Results: Reduced microbial richness (alpha diversity, the number and balance of bacterial species) was observed in more than half of the studies, although findings were not uniform. Differences in community composition (beta diversity) were common. Across studies, BC was consistently associated with elevated Bacteroides and reduced Faecalibacterium, a genus linked to anti-inflammatory effects. Other recurrent findings included enrichment of Eggerthella and Blautia in BC, though results for several taxa were inconsistent. Geographic variation was evident: Eggerthella was enriched in U.S. cohorts, Blautia in European cohorts, and in Chinese cohorts, Prevotella was elevated while Akkermansia was reduced. Conclusions: Despite heterogeneity, converging evidence supports reduced diversity and shifts in select taxa, particularly enrichment of Bacteroides and depletion of Faecalibacterium, as emerging features of the BC microbiome. Geographic differences underscore the influence of host and environmental factors. These findings suggest biomarker potential but highlight the need for larger, longitudinal, and standardized studies to establish causality and clinical utility.