Preferential localization of intratumoural bacteria in breast cancer revealed by fluorescence in situ hybridization.

Intratumoural bacteria have emerged as potential components of the breast tumour microenvironment, yet their spatial organization remains poorly understood. This study aimed to characterize the spatial distribution of three frequently identified bacterial genera (Bacillus, Pseudomonas, and Staphylococcus) in breast tumour tissues using fluorescence in situ hybridization (FISH). For this purpose, thirty formalin-fixed, paraffin-embedded breast tumour specimens from Moroccan patients were analyzed using genus-specific probes along with a universal bacterial 16 S rRNA probe. FISH analysis revealed a non-random spatial distribution of all three bacterial genera, characterized by preferential localization within tumour regions. Bacterial signals were consistently observed in close proximity to tumour cell nuclei, suggesting a recurrent perinuclear organization. Conversely, signals were absent or rarely detected in normal epithelial cells, even though they were present in the lumen of breast ducts. These findings demonstrate a distinct spatial organization of intratumoural bacteria in breast cancer tissues and support a preferential association with tumour cells. Although causality cannot be inferred, this work provides morphological evidence that establishes a framework for future studies investigating bacteria-tumour cell interactions and their contribution to the breast cancer microenvironment.