In studies sponsored by the Environmental Protection Agency, Tennessee Tech microbiologist Sharon Berk has hypothesized how Legionella resists our best efforts to destroy it, and she has shown that the bacteria might, in fact, be concentrated into a form that's more threatening than previously thought. Her research could well lead to the prevention of Legionnaire's disease.
Berk, professor of biology in Tennessee Tech's Center for the Management, Utilization and Protection of Water Resources, earned the university's 1996-97 Caplenor Faculty Research Award.
Every year, Legionnaire's disease claims the lives of between five and 15 percent of the estimated 10,000 to 15,000 people infected by Legionella bacteria, which live in the stagnant water of hot water tanks, whirlpool spas and air-conditioning cooling towers.
For some time, scientists have known how Legionella bacteria infect humans. What they haven't known was the exact form in which the bacteria spread.
Plumbing systems hosting Legionella also contain amoebas -- single-celled organisms that live in water and feed on bacteria. Once ingested, the bacteria multiply in the amoebas' digestion vacuoles. At temperatures close to that of the human body, amoebas literally explode, freeing the bacteria, and the amoebas die.
But in lab studies, Berk has found that at the lower temperatures of cooling towers, the amoebas expel protective sacs called "vesicles" full of the bacteria. The amoebas survive and continue feeding on the bacteria, thus continuing the cycle. And the bacteria in vesicles survive, too, despite the presence of biocides.
It's the vesicles that appear to be the "missing link" in our understanding of how Legionella spreads and also how current monitoring techniques may be inadequate.
Once expelled, the bacteria stuffed in vesicles resist biocides for at least 24 hours -- plenty of time for them to escape and infect anyone near enough to inhale aerosol droplets released by the plumbing systems.
"The vesicles just happen to be the perfect size to fit in the lower spaces of the lungs, which is where Legionnaire's disease starts," says Berk. "About 90 percent of vesicles fall into this size category."
Berk hypothesizes that the vesicles are actually protecting the bacteria. Moreover, she and her team found that each vesicle may contain several hundred bacteria -- possibly an infectious dose. And each amoeba is releasing about 25 vesicles a day.
"We also looked at other single-celled organisms from cooling towers, and we were astounded to find that some were releasing as many as several hundred vesicles a day," says Berk.
That's a lot of bacteria -- and a lot of potentially infectious doses. In water where amoebas are releasing vesicles, current monitoring techniques could be underestimating the number of bacteria present. Berk's goal is to identify ways to stop the production of Legionella. Developing techniques for the more accurate detection of vesicles should lead to the reduction of Legionnaire's disease.
"Dr. Berk has contributed more to our understanding of Legionella-amoeba interactions over the last few years than any other investigator, and I expect she will continue to advance in this field," says Robert M. Wadowsky, chief of microbiology at Children's Hospital of Pittsburgh.
Says Dennis George, director of the Water Center, "If true, this work will reshape our thinking on disinfection and the role amoebas and protozoa play in the environment."
Legionella causes two distinct illnesses: the more severe Legionnaire's disease and the milder, non-fatal Pontiac fever. Legionnaire's disease, a form of pneumonia, tends to strike heavy smokers and the middle-aged and older population, as well as those with suppressed immune systems, such as people with AIDS.
From 1993 to 1995, Berk served as a Foundation Lecturer for the American Society for Microbiology -- an honor granted only a handful of the society's 43,000 members. Her research team -- which includes Water Center associate professor Martha Wells, lab associate Rebecca Ting, several Tennessee Tech students and Middle Tennessee State University faculty member Anthony Newsome -- is currently supported by a $359,300 EPA grant. Nationally, only 12 of the 150 proposals for EPA funding in environmental biology were granted last year, including Berk's.
The Caplenor Award is the university's premier research award and is named in honor of Donald Caplenor, former associate vice president for research and dean of instructional development.