Deciphering the intratumor microbiota in malignant gastrointestinal tumors: multifaceted interplay and clinical implications

With the development of next-generation sequencing methods, the presence of flora has been detected in otherwise sterile tumour tissues, and intratumoural flora has a role in tumour diagnosis suggesting prognosis due to its heterogeneous nature. Studies have shown that intratumoural flora may contribute to cancer development and progression by inducing DNA damage, affecting immune system homeostasis, influencing signalling pathways, affecting barrier effects, and the effects of intratumoural flora metabolites on the organism. Moreover, intratumoural bacteria have been combined with various therapies to play a role in the clinical management of upper gastrointestinal tumours. In addition, immune cells act as either promoters or suppressors of tumours, and there are various crosstalks between intratumoral bacteria and immune cells; intratumoral bacteria enhance anti-tumour immunity through mechanisms such as T and NK cell activation and intratumoral microbiota-derived antigen presentation, and also reduce anti-tumour immune responses and promote cancer progression through the promotion of anti-inflammatory environments, T-cell inactivation and immunosuppression. In recent studies, intratumoral bacteria have been combined with nano-engineering to target not only the tumour tissue but also the tumour microenvironment. In this review, we summarise the roles and mechanisms played by intratumoural flora in cancer development, metastasis and therapy, and hope to provide ideas for follow-up studies.