Bacteria to blame? Science Photo Library

Most colon cancers may be caused by infections with bacteria that are normally found in cows. For decades we have known that Streptococcus gallolyticus gallolyticus (SGG) is sometimes found in colon tumours, but now the microbes have been found to directly cause tumour growth in mice.

Cancer arises when cells have mutations in their DNA that allow them to start quickly proliferating. Sometimes a particular cause is strongly suspected – such as cigarette smoke in the case of lung cancer, or human papillomavirus for cervical cancer. But often there is no clear cause, and tumour-causing mutations can happen randomly.

Colon cancer, which is the third most common cancer in the UK, has previously been linked with dietary risk factors, such as eating a lot of red or processed meat, or not enough fibre. But researchers have wondered if SGG, which can be found in some colon tumour samples, might also play a role.


To investigate, Yi Xu of Texas A&M Health Science Center and her team fed the bacteria to mice that were pre-disposed to develop colon tumours. Those who were fed SGG developed around twice the volume of tumours as those given a different microbe for comparison.

The team also found that human colon cancer cells in a dish multiplied faster when SGG bacteria were present too. “Somehow it can stimulate pathways that are important in cell proliferation,” says Xu.

When the team tested tumour samples from people with colon cancer, they found 74 per cent had SGG bacteria in them, and 26 per cent had very high levels of it. This suggests the bacteria play a role in up to 74 per cent of colon cancers, says Xu.

Heart infections

It’s still unknown if SGG is behind the initial cancer-causing mutations. Instead, the bacteria may accelerate the growth of existing small tumours, says Xu. That’s what her team plans to investigate next, by giving the bacteria to healthy mice that aren’t pre-disposed to colon cancer. If the bacteria’s role in cancer is confirmed, colon cancers may one day be treated with antibiotics or even prevented with a vaccine, she says.

It’s thought that around 2 to 8 per cent of healthy people have SGG in their colon tissue. It isn’t known for sure how people catch it, but the bacteria are also known to infect cows and previous work has shown infections of people are six times more common in a rural farming area of Spain than in the city of Barcelona.

SGG sometimes causes heart valve infections, and doctors usually recommend anyone with this infection should also be checked for colon cancer. A previous study found that about half of people with an SGG heart valve infection also had an undiagnosed colon cancer.

Anyone with an SGG heart infection that doesn’t have a tumour should have repeat check-ups, says Juan Corredoira of the Lucus Augusti University Hospital in Lugo, Spain.

Journal reference: PLOS Pathogens, DOI: 10.1371/JOURNAL.PPAT.1006440

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