The gut microbiota’s function in regulating health has seen it linked to disease progression in several cancers. However, there is limited research detailing its influence in breast cancer (BrCa). This study found that antibiotic-induced perturbation of the gut microbiota significantly increases tumor progression in multiple BrCa mouse models. Metagenomics highlights the common loss of several bacterial species following antibiotic administration. One such bacteria, Faecalibaculum rodentium, rescued this increased tumor growth. Single-cell transcriptomics identified an increased number of cells with a stromal signature in tumors, and subsequent histology revealed an increased abundance of mast cells in the tumor stromal regions. We show that administration of a mast cell stabilizer, cromolyn, rescues increased tumor growth in antibiotic treated animals but has no influence on tumors from control cohorts. These findings highlight that BrCa-microbiota interactions are different from other cancers studied to date and suggest new research avenues for therapy development.

Clostridium perfringensis an anaerobic toxin-producing bacterium associated with intestinal diseases, particularly in neonatal humans and animals. Infant gut microbiome studies have recently indicated a link betweenC. perfringensand the preterm infant disease necrotizing enterocolitis (NEC), with specific NEC cases associated with overabundantC. perfringenstermedC. perfringens-associated NEC (CPA-NEC). In the present study, we carried out whole-genome sequencing of 272C. perfringensisolates from 70 infants across 5 hospitals in the United Kingdom. In this retrospective analysis, we performed in-depth genomic analyses (virulence profiling, strain tracking and plasmid analysis) and experimentally characterized pathogenic traits of 31 strains, including 4 from CPA-NEC patients. We found that the gene encoding toxin perfringolysin O,pfoA, was largely deficient in a human-derived hypovirulent lineage, as well as certain colonization factors, in contrast to typicalpfoA-encoding virulent lineages. We determined that infant-associatedpfoA⁺strains caused significantly more cellular damage thanpfoA⁻strains in vitro, and further confirmed this virulence trait in vivo using an oral-challenge C57BL/6 murine model. These findings suggest both the importance ofpfoA⁺C. perfringensas a gut pathogen in preterm infants and areas for further investigation, including potential intervention and therapeutic strategies.

Establishing commensal dysbiosis, defined as an inflammatory gut microbiome with low biodiversity, before breast tumor initiation, enhances early dissemination of hormone receptor–positive (HR+) mammary tumor cells. Here, we sought to determine whether cellular changes occurring in normal mammary tissues, before tumor initiation and in response to dysbiosis, enhanced dissemination of HR+ tumors. Commensal dysbiosis increased both the frequency and profibrogenicity of mast cells in normal, non–tumor-bearing mammary tissues, a phenotypic change that persisted after tumor implantation. Pharmacological and adoptive transfer approaches demonstrated that profibrogenic mammary tissue mast cells from dysbiotic animals were sufficient to enhance dissemination of HR+ tumor cells. Using archival HR+ patient samples, we determined that enhanced collagen levels in tumor-adjacent mammary tissue positively correlated with mast cell abundance and HR+ breast cancer recurrence. Together, these data demonstrate that mast cells programmed by commensal dysbiosis activate mammary tissue fibroblasts and orchestrate early dissemination of HR+ breast tumors.

The diverse community of commensal microbes that comprise the gut microbiota is known to play an integral role in human health, not least through its ability to regulate host immune responses and metabolic pathways. Alterations to the homeostasis of this community, including through the use of broad-spectrum antibiotics, have already been associated with the progression of several cancers, namely melanoma and liver. The aggressive nature of breast cancer (BrCa), largely due to its ability to metastasize early, has ranked the disease with the second highest mortality rate of all cancers globally. Yet the body of research into the complex relationship between the microbiota and BrCa is still limited. This study found that a depletion of the microbiota, through the administration of antibiotics, significantly increased the rate of primary tumour progression in mouse BrCa models. We show that antibiotic-induced microbiota disturbances lead to changes in behaviour of a relatively obscure tumour-immune cell population: mast cells. We observed increases in tumour stroma-associated mast cells in antibiotic treated animals. Moreover, inhibition of mast cell degranulation, via cromolyn, slowed tumour progression in antibiotic treated animals but not in control animals. Thus, it appears that a perturbed microbiota drives stroma-associated mast cell recruitment and activation, which in turn promotes primary tumour growth through an as yet unknown mechanism. One Sentence Summary: We show that breast cancer progression is accelerated through a unique/novel immune response involving mast cells as a result of an antibiotic induced perturbation of the gut microbiota in a mouse model.

Early-life vascular development is a tightly regulated process that influences organ maturation and long-term cardiometabolic and neurovascular health. While the maternal microbiome is increasingly recognised as a determinant of neonatal immune, metabolic, and neurodevelopmental programming, its role in developmental angiogenesis remains poorly defined. Here, we investigated whether maternal gut microbiota regulates neonatal retinal vascular development through maternally derived metabolic signals. Using antibiotic-induced dysbiosis with a broad-spectrum antibiotic cocktail consisting of vancomycin, neomycin, metronidazole, amphotericin B, and ampicillin (VNMAA), together with germ-free animals, we assessed retinal angiogenesis at postnatal day 6 (P6) and P12. Maternal microbiome depletion significantly delayed retinal vascular extension at P6 and produced persistent abnormalities in P12 retinal vascular architecture, including reduced deep plexus vascular density, altered descending branch density, and reduced branching complexity after normalisation to vessel density. Whole-genome shotgun sequencing demonstrated pronounced remodelling of the maternal caecal microbiome following antibiotic treatment and reciprocal faecal microbiota transplantation (FMT), whereas P12 pup caecal communities were comparatively limited and did not mirror the dramatic restructuring observed in dams. Restoration of the maternal microbiome by FMT rescued neonatal vascular defects, supporting a causal role for maternal microbial state. Untargeted LC–MS metabolomics of maternal milk identified glycerophospholipid metabolism as a prominent microbiome-sensitive pathway. Choline and related lipid metabolites were prioritised as candidate mediators because of their position within this pathway and their biological relevance to membrane metabolism and endothelial function. In vitro endothelial assays demonstrated that choline modulates endothelial viability and promotes tubule formation. Together, these findings identify a maternal microbiota–milk metabolite–vascular axis regulating neonatal retinal angiogenesis and suggest that lactational metabolic signalling may represent an important mechanism through which maternal microbial ecology shapes offspring vascular development.

Establishing commensal dysbiosis, defined as an inflammatory gut microbiome with low biodiversity, before breast tumor initiation, enhances early dissemination of hormone receptor–positive (HR+) mammary tumor cells. Here, we sought to determine whether cellular changes occurring in normal mammary tissues, before tumor initiation and in response to dysbiosis, enhanced dissemination of HR+ tumors. Commensal dysbiosis increased both the frequency and profibrogenicity of mast cells in normal, non–tumor-bearing mammary tissues, a phenotypic change that persisted after tumor implantation. Pharmacological and adoptive transfer approaches demonstrated that profibrogenic mammary tissue mast cells from dysbiotic animals were sufficient to enhance dissemination of HR+ tumor cells. Using archival HR+ patient samples, we determined that enhanced collagen levels in tumor-adjacent mammary tissue positively correlated with mast cell abundance and HR+ breast cancer recurrence. Together, these data demonstrate that mast cells programmed by commensal dysbiosis activate mammary tissue fibroblasts and orchestrate early dissemination of HR+ breast tumors.