Sixteen percent of cancers worldwide are attributable, at least in part, to infection, showing the link between microbial agents and tumorigenesis . Studies to further investigate the role of the microbiome in cancer have accumulated in recent years, with a particular interest in intratumoral bacteria . Intratumoral bacteria with specific pathophysiological roles have been detected in several types of cancers [3,4]. However, identification of tumour-associated bacteria is challenging due to potential external contamination and the low bacterial content in most tumours, resulting in a low ratio of bacterial to host DNA . The improved sensitivity of next-generation analytical tools over the past decade helped to overcome these technical issues.
In 2020, Nejman et al.  performed a comprehensive analysis of the tumour microbiome using 16S rRNA gene sequencing in seven cancer types including breast, ovary, pancreas, bone, melanoma, lung, and brain tumours. By analysing 1526 tumours with a single platform, they showed that each tumour type had a distinct microbiome composition with specific metabolic functions and that intratumor bacteria are predominantly intracellular in both cancer and immune cells.
In another study published in 2017, Bullman et al.  studied the role of the Fusobacterium nucleatum in metastatic colorectal cancer. They showed that this bacterium was detected in 50% of patient samples and that colonisation of human colorectal cancers with Fusobacterium and its associated microbiome -including Bacteroides, Selenomonas, and Prevotella species – was maintained in distant metastases, demonstrating the stability of the microbiome between matched primary and metastatic tumours. These findings suggested that bacteria from the primary tumour can migrate to metastatic sites.
In the context of these two major studies, we aimed at testing the hypothesis that the intratumoral microbiota of metastases differs depending on the primary location. To this end, we performed microbiome analysis on biopsy samples from different sites (lymph node, lung, liver) of patients with different primary tumours (breast, lung, colorectal cancers) previously included in the SHIVA01 trial . This randomised phase 2 trial evaluated the efficacy of several molecularly targeted agents in patients with different types of advanced cancer. In addition, we analysed the association of microbiome features with clinico-pathological parameters common to all tumour types and patient outcomes.