They found: when the bubble bursts, the bacteria disperse, and lifeguards get pneumonia-like symptoms.
Lars Angenent, Ph.D., Washington University assistant professor of chemical engineering, and collaborators from San Diego State University and the University of Colorado, took what is known as a molecular survey of a common gene found in all life forms, 16 S ribosomal RNA (rRNA) gene, by cloning the different forms, sequencing them, and making evolutionary-distance trees, or phylogenetic trees. They then were able to match the genetic sequence of the bacterium Mycobacterium avium to the same bacterium found in the lungs of nine lifeguards who had become ill with hypersensitivity pneumonitis, a lung condition that mimics pneumonia symptoms.
The therapeutic pool was unlike regular swimming pools - but similar to hot tubs - in that the water temperature is about 92-94 degrees F. The pool water was treated with hydrogen peroxide, rather than chlorine, as a disinfectant. Patients who take hydrotherapy can be in the pool for up to four hours and the skin is sensitive to chlorine when exposed to it that long.
Lifeguards became ill because they were exposed to the bacteria in aerosols eight hours daily, five days a week. No patients became ill at the hospital, which must remain anonymous.
"The presence of Mycobacterium avium would be a concern for the very young, or old or immunocompromised," Angenent said. "This isn't a concern for a regular swimming pool, so there shouldn't be any reason to panic."
Angenent said that typically bioaerosol researchers examining a problem like this would sample the air, capture the cells and grow them on an agar plate and count colonies of species, but this approach misses too many airborne bacteria, which are difficult to grow in a laboratory environment. The approach he and his collaborators took enabled them to survey greater than 1,300 rRNA genes from the different bacteria and fungi found in the air and pool water, giving them a total of 628 unique sequences, the most common being Mycobacterium avium, which was found in the ill people's lungs.
"Our results show that molecular surveys are much better tools to gain knowledge of pathogens in the environment compared with conventional approaches," Angenent said. "If you use conventional tools, you might think there is no problem when there really is one."
The study was published in the March 14, 2005 issue of the Proceedings of the National Academy of Sciences.
The preferred site for these bacteria in the watery environment is a water bubble because the species is hydrophobic. They prefer to cluster together near the air and avoid the water. The pool had many bubbles in it because of the addition of hydrogen peroxide.
Whenever the bubble bursts, bacteria can become airborne. Mycobacterium avium is a gram-positive bacterium, known to be resistant to disinfection in large part because of a strong, waxy cell wall. In this particular hospital environment, the bacteria had even more freedom to grow because the hydrogen peroxide killed off nearly all its competitors.
"We did an exhaustive survey of everything we could find in the water and air, both at the pool, and outside the pool environment," Angenent said. 'We wanted to be sure the troublesome aerosol came from the pool and not from outside. We did find this mycobacterium at incredibly high levels in air, but we also found fungi spores and plant materials."
While no immunocompromised patients using the hospital pool became ill, there are cases in the literature of the immunocompromised becoming ill from Mycobacterium avium, Angenent said.
"The pool was closed down for a short while, then re-opened," he said. "There were no reported cases of anyone else becoming ill from exposure, and the pool would have been safe for healthy people, as long as they were not in the environment for long stretches of time."
Angenent said he has results from a related study that shows air filters he and his coworkers emplaced by the same pool captured and removed a large percentage of bacteria from air. The pool has now reopened and no new cases of illness have been seen.