Bacteria hiding out in tumours can shield them from attack by the immune system.

The oral bacterium Fusobacterium nucleatum has been linked to premature birth, rheumatoid arthritis and colon cancer. Gilad Bachrach and Ofer Mandelboim at the Hebrew University of Jerusalem and their colleagues studied the impact of the bacterium on cancer cells. They found that F. nucleatum sticks to tumour cells grown in culture and inhibits immune cells by activating an immune-cell receptor called TIGIT. Many immune-cell types found in human colon cancer and melanoma samples also expressed TIGIT, and were inhibited by F. nucleatum.

The results could explain why certain tumours, especially intestinal ones, seem to have high levels of bacteria.

Immunity 42, 344–355 (2015)