The mouth is home to over 600 different species of bacteria, some of which can cause disease.
Helicobacter pylori has recently been shown to cause stomach ulcers and is behind a large proportion of gastric cancers.
"Recently, scientists discovered that H. pylori can live in the mouth. We wanted to determine whether the bacteria can cause bad breath, so we tested patients complaining of halitosis for the presence of H. pylori," said Dr Nao Suzuki from Fukuoka Dental College in Fukuoka, Japan.
The researchers found that the bacteria in the mouths of 21 out of 326 Japanese people with halitosis.
In these people, the concentration of a bad breath gas and the level of oral disease were significantly higher.
In patients with periodontal (gum) disease, 16 of 102 people had H. pylori in their mouths.
"Halitosis is a common problem in humans, and bad breath is largely caused by periodonitis, tongue debris, poor oral hygiene and badly fitted fillings," said Dr. Suzuki.
"Bacteria produce volatile compounds that smell unpleasant, including hydrogen sulphide, methyl mercaptan and dimethyl sulphide. Doctors often measure the levels of these compounds to diagnose the problem. Gastrointestinal diseases are also generally believed to cause halitosis," Dr. Suzuki added.
Patients who were carrying H. pylori had more blood in their saliva and were also carrying Prevotella intermedia, which is one of the major periodontal bacteria.
"Although the presence of H. pylori in the mouth does not directly cause bad breath, it is associated with periodontal disease, which does cause bad breath," said Dr. Suzuki.
"We now need to look into the relationship between H. pylori in the mouth and in the stomach. We hope to discover the role of the mouth in transmitting H. pylori stomach infections in the near future," Dr. Suzuki added.
The study is published in the December issue of the Journal of Medical Microbiology.