Analysis of intratumoural bacteria: Opportunities and challenges for cancer therapy

Intratumoural bacteria have been shown to play conflicting roles within tumours, especially in the modulation of immune responses. Moreover, bacteria have been linked to a variety of cancer types and have different prognostic values depending on the cancer. This heterogeneity is not only shaped by tumour molecular subtypes, spatial location, cancer progression stage, genetic alterations, and the presence of multiple subclones, but is also influenced by variations in angiogenesis, oxygen levels, microbial sources, endocytosis and micropinocytosis. In this review, we describe the diversity of intratumoural bacteria across 25 cancer types covering an extensive spectrum of analyzed intratumoural bacteria. Furthermore, the dual roles and mechanisms of their involvement in tumour progression and anticancer activity are summarized. Additionally, interventions and applications of engineered bacteria in cancer therapy, especially strategies used in clinical trials, are illustrated. This work describes the roles of intratumoural bacteria and their metabolic byproducts as regulators within the tumour microenvironment and highlights the potential translation of bacteria for cancer therapy.