Microbe hunt: novel bacterium infects immune-deficient people.

A newfound bacterium can cause illness in people, its discoverers have concluded. However, it may infect only people with a rare, inherited form of immune deficiency.

All three patients known to carry the microbe had a preexisting immune deficiency, chronic granulomatous disease (CGD).

In people with CGD, the bacterium may account for a significant number of infections, says Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID) in Bethesda, Md. The finding, he says, "clearly will have very important implications for CGD patients:'

In the approximately 1,000 people in the United States who have CGD, immune cells called phagocytes have trouble digesting certain pathogens. People with CGD develop frequent, persistent infections, some of which are caused by known bacteria and fungi that normally succumb to phagocytes.

Other illnesses among people with CGD result from unidentified infections. "The discovery of a brand-new pathogen [puts] a little bit of a dent in that black box," says Fanci.

The novel microbe's discoverers, several of whom work at NIAID, have dubbed it Granulobacter bethesdensis. They first identified it in 2003 in swollen lymph nodes removed from a 39-year-old man. He had unexplained fever and chills and had lost 4.5 kilograms of body weight during the previous 3 months.

To prove that the newfound bacterium could cause illness, David E. Greenberg of NIAID and his colleagues put the microbe into lab mice.

Animals that had been bred to have CGD-like defects developed abnormalities in their lymph nodes similar to those seen in the patient. Mice with normal immune systems had at most mild symptoms, the researchers report in the April PLoS Pathogens.

Tests of blood from some people with healthy immune systems indicate past exposure to G. bethesdensis, but there's no evidence that they got sick, Greenberg says.

The opportunistic bacterium might have lived in the original patient for several years before he became ill, says medical microbiologist David A. Relman of the VA Palo Alto (Calif.) Health Care System. "The evidence that this organism caused this patient's disease is suggestive but not conclusive; he says.

The patient was still infected with G. bethesdensis as of late last year. The bacterium has since been found in two other people ill with CGD.

Where G. bethesdensis usually lives is a mystery. Genetic tests show that its closest relatives include benign soil and plant bacteria, some of which are used to manufacture vinegar.