Comparative Analysis of Tumor Microbiota Identifies a Metastatic-Specific Bacterial Signature, Highlighting Streptococcus spp. As the Predominant Hub across Cancers

Background: Cancer remains one of the leading global health challenges, with lung cancer (LC), breast cancer (BC), and colorectal cancer (CRC) among the most prevalent and deadly malignancies. The intratumoral microbiota (IM), a distinct microbial ecosystem within tumor tissues, has recently emerged as a potential modulator of carcinogenesis, immune responses, and metastatic progression. However, comparative cross-cancer analyses remain limited. Therefore, this study aimed to compare the IM across these cancer types, with particular emphasis on distinguishing metastatic from non-metastatic malignancies, to identify tumor-specific microbial signatures with potential relevance for biomarker discovery, patient stratification, and microbiota-informed therapeutic strategies. Methods: We performed 16S rRNA gene sequencing to profile the IM in formalin-fixed, paraffin-embedded (FFPE) samples from 20 BC patients, 20 CRC patients and 15 non-small cell lung cancer (NSCLC) patients. Results: spp. were central in non-metastatic networks. Conclusions: Overall, our findings highlight cancer-type and metastasis-specific microbial signatures, supporting a potential role for the IM in tumor progression and offering novel avenues for biomarker discovery and therapeutic targeting.