Legionella kin found, linked to pneumonia.

Packed inside a tiny amoeba with hundreds of its ilk, this minuscule bacterium soon bursts the amoebic bubble to seek out a less crowded home. This quest bodes illness if it occurs in the lungs of an unwary human host.

The unnamed bacterium is one of a group called Legionella-like amoebic pathogens, or LLAPs. These pathogens are close cousins of Legionella pneumophila-the microbe thought to cause Legionnaires' disease-report Barry Fields of the Centers for Disease Control and Prevention in Atlanta and his colleagues in the July-September Emerging Infectious Diseases, a CDC online journal.

The LLAPs-so named because, like Legionella, they flourish in amoeba-are genetically similar to Legionella as well. A strand-to-strand comparison of their ribosomal DNA turned up a mere 2 percent difference. This analysis prompted the investigators to propose that the 12 LLAPs they studied represent at least "five new species of Legionella," although they have not been formally named.

Overall, epidemiologists had calculated that up to 15,000 of the 250,000 pneumonia cases of known origin that occur in the United States each year are caused by Legionella. The new study suggests that LLAPs may cause some of these 15,000 cases, as well as some of the 250,000 pneumonia cases of unknown origin that crop up in the United States annually.

"This indicates that there's a lot of Legionnaires' disease we don't know about," says Fields.

Indeed, the 12 examples of the newly described species were found using experimental methods after standard tests had failed to detect them. They were spotted either in people with Legionnaires' disease or in the sources of Legionnaires' disease outbreaks, including a whirlpool bath and a hospital shower.

The first LLAP recognized by scientists turned up more than 40 years ago in soil. This early finding and recent unpublished work by biologists at Middle Tennessee State University in Murfreesboro, who found the microbes in amoeba collected from moist soil under an air conditioner, indicate that LLAP bacteria may be abundant in nature.

"In all likelihood, these bacteria are widespread in the environment," concludes microbiologist Anthony Newsome of Middle Tennessee State.

"They're out there, everywhere," Fields agrees.

The study by Fields and his colleagues may explain, in part, why Legionnaires' disease is difficult to diagnose. Standard tests, which rely on antibodies to the 39 previously recognized Legionella strains, fail to detect many cases. In addition, the finicky bacteria are notoriously difficult to grow in the laboratory, says Paul H. Edelstein of the University of Pennsylvania in Philadelphia.

In some instances, the new study suggests, researchers may have been looking for the wrong species.