For millions of years, human beings have coexisted and coevolved with a vast number of bacteria, fungi and viruses. Now, the epidemiological relationships between several specific bacteria and cancer are explored at the molecular level. With the increasing maturity of next-generation sequencing technology, it will definitely promote the in-depth study of gut bacteria and thus help understand the taxonomic and metabolomic associations between bacteria and cancer. This review summarizes the tumorigenesis mechanisms of bacteria.