A whole unknown community of bacteria makes its home in the human stomach, it has been discovered.
The bacteria H. pylori causes stomach ulcers
Scientists had believed that the acid in stomachs made them inhospitable.
But the discovery of the ulcer-causing bacteria H. pylori led Stanford University scientists to explore this undiscovered world.
The Proceedings of the National Academy of Sciences study even found a bacterium related to one previously detected in radioactive waste sites.
Helicobacter pylori, or H. pylori, was discovered in stomachs 20 years ago and has been the subject of a large number of scientific studies.
But it has not been clear how wide a range of micro-organisms live in the stomach.
The Stanford researchers carried out molecular analyses of gastric samples from 23 volunteers and found at least 128 different types of bacteria.
Some were types which had been identified in the mouth or oesophagus.
But 10% of the types were genetically distinct from any previously reported bacteria.
One was related to a group that includes Deinococcus radiodurans, a bacterium found in radioactive waste sites, hot springs, and animal faecal samples.
'No queue for causes'
The researchers say this is the first time a similar bacterium has been identified in a human.
In addition, H. pylori was found in samples from 19 people studied - although conventional tests had only detected its presence in 12 of them.
The researchers, led by Dr Elisabeth Bik, writing in PNAS, said larger studies analysing gastric micro-organisms, focusing on whether there were significant differences between genders or in people from different ethnic and racial groups were needed.
They added: "A better understanding of the resident microbial communities at healthy and diseased sites should shed light on the pathogenesis [development of disease], diagnosis and treatment of gastrointestinal illnesses."
Professor Charles Penn, a microbiologist at the University of Birmingham, told the BBC News website: "It's quite surprising that they found these bacteria. But they do say finding a trace of DNA doesn't necessarily mean the bacteria are there."
And he said there were unlikely to be discoveries along the lines of H. pylori.
"We're not queuing up to find causes for other diseases that may be tracked down in this bacteria."