The study, published in the Journal of Translational Medicine, also suggests the bacteria may play a role in causing the cancer, called oral squamous cell carcinoma, the researchers said on Thursday.
"Finding bacteria associated with [oral squamous cell carcinoma] encourages us to hope that we have discovered an early diagnostic marker for the disease," said Donna Mager of the Forsyth Institute in Boston, who led the study.
"If future studies bear this out, it may be possible to save lives by conducting large-scale screenings using saliva samples."
The American Cancer Society estimates about 29,370 people will be newly diagnosed with oral cavity and oropharyngeal cancer in the US in 2005 and 7320 people will die.
A test is important because the five-year relative survival rate for all the cancers is 59%, mostly because they are not detected until they have spread to other parts of the body.
"We cannot rule out the possibility that the bacteria themselves may be causally involved in the development of the disease"
Mager's team compared bacteria samples from the saliva of 229 healthy people to samples from 45 oral cancer patients. The team found unusually high levels of three bacterial species - C gingivalis, P melaninogenica and S mitis - in the oral cancer patients.
It could be that cancer changes mouth chemistry, allowing the bacteria to flourish, the researchers said.
"We cannot rule out the possibility that the bacteria themselves may be causally involved in the development of the disease," said Max Goodson, Director of Clinical Research at Forsyth.
Bacteria and viruses are known to cause cancer. Helicobacter pylori bacteria are the main cause of stomach cancer, for example, while human wart virus is the only known cause of cervical cancer.