Pathogen preference: infected amoebas flourish in cooling towers.

Cooling towers on buildings harbor amoebas infected with both known and unknown types of bacteria. New research raises the possibility that such towers are more effective than natural waters at fostering novel bacterial species that cause illnesses in people.

Scientists suspect that many microbial species responsible for human illnesses first preyed upon amoebas, unicellular organisms found in aquatic environments. Though microscopic, amoebas normally are shaped like "a piece of chewing gum that's been chewed," says Sharon G. Berk of Tennessee Technological University in Cookeville. But after they've been infected by bacteria, they become spherical. Finally, they burst and release the bacteria, she says.

Well known among those pathogens are Legionella pneumophila bacteria, which cause Legionnaires' disease. Cooling towers--structures that cool water streams for air conditioning, power generation, or other purposes--are a source of Legionella-infected amoebas. These towers cool water by exposing it to outside air and then blowing out warm air. That airflow carries fine water droplets, which can release bacteria.

Berk and her colleagues wondered whether infected amoebas would be more prevalent in cooling towers than in natural water environments. They took samples from cooling towers on hospitals and industrial buildings and also from rivers and lakes.

The researchers found infected amoebas in 22 of 40 cooling tower samples but in only 3 of 40 natural samples. A statistical analysis revealed that cooling towers were 16 times as likely to contain infected amoebas as were natural aquatic environments, the researchers report in an upcoming Environmental Science & Technology.

The scientists then attempted to grow the amoeba-infecting bacteria on laboratory plates for identification but succeeded for only a small number of the bacteria. Of the strains identified from the cooling tower samples, three were L. pneumophila, three more were new strains related to known bacteria, and one was novel, notes Berk.

Berk says that she expects that many more unidentified pathogens were living in the amoebas. The researchers don't know why amoeba-infecting bacteria flourish in the cooling tower environment.

The study "adds some insight relating to both the environmental complexity and the potential for emergence of new disease from cooling towers," says microbiologist Richard Bentham of Flinders University in Adelaide, Australia. "The implication of cooling towers as 'pathogen accelerators' makes them an intriguing research focus."

James M. Barbaree, a microbiologist at Auburn University in Alabama, agrees that conditions in cooling towers appear conducive to the growth of infected amoebas. But considering how many different water environments there are, he adds, "I don't think you would say that it explicitly is going to occur more in all cooling towers than [in] all natural samples."