Scientists have uncovered a chain reaction which could link a type of bacterium living in our intestines to the development of colon cancer.
Enterococcus faecalis is harmless in the vast majority of people, but US scientists have found that it can produce harmful chemicals.
The Journal of Medical Microbiology study found these can damage DNA, and prompt gene activity linked to cancer.
A UK expert said it was plausible that bacteria could cause colon cancer.
However, he stressed that E.faecalis was very unlikely to be the only bacterium which had such an effect.
Our guts provide a home to dozens of different types of bacteria, many of which actually provide a useful service, helping break down indigestible sugars in food by fermentation, or even "training" the body's immune system.
However, in recent years, scientists have suggested that in certain, susceptible individuals, these bacteria can actually do harm.
E.faecalis, sometimes also known as Group D Streptococcus, is one of those under suspicion, and the research by the Department of Veterans Affairs Medical Center in Oklahoma City strengthens the link.
The researchers investigated how colon cells in the laboratory reacted to the presence of the bacterium, when it is in a "fermentation" state.
In this state, it produces a kind of oxygen molecule called "superoxide", and it is this which can damage DNA in surrounding cells.
Professor Mark Huycke, who led the research, found that the apparent effects were not limited to this.
"We found that superoxide led to strong signalling in immune cells called macrophages - it also altered the way some cells in the gut grew and divided and even increased the productivity of genes which are associated with cancer."
In total, the expression of 42 genes linked to vital processes in human cells was altered by the presence of E. faecalis in this state.
"This research puts into perspective the complexity of the effects normal gut bacteria can have on the health of the individual."
Dr Barry Campbell, a gut microbiology researcher from the University of Liverpool, agreed that E.faecalis was a candidate for cancerous changes.
However, he said that other bowel bacteria could also be behind the cell changes which eventually lead to tumours.
He said: "There is not going to be only one culprit. Our own team is interested in a particular type of E.coli with this in mind.
"There are also many other factors which are involved, such as genetics and environment."
Professor Ian Rowland, a specialist in gut bacteria from Reading University, said: "This shows how it could happen, although whether this actually does happen in a human is another matter.
"There is a lot of circumstantial evidence that gut bacteria are important in colorectal cancer, although we don't fully understand why.
"In the case of Enterococcus faecalis, we know that most people have this in their gut, but most people don't get colon cancer, so there must be other factors involved."