In the summer of 1976, 4,000 American Legionnaires descended upon the Bellevue-Stratford Hotel in Philadelphia, Pennsylvania, for a four-day convention. Several days later, many of the attendees experienced symptoms of severe pneumonia. By the beginning of August, 22 people had died. The Centers for Disease Control and Prevention (CDC) estimate that about 180 people were sickened and 29 people died before this mysterious outbreak burnt out.
In the months that followed, the CDC went into full epidemic investigation mode, and by January 1977, an organism had been isolated from lung tissue of one of the fatal cases of the mysterious Philadelphia respiratory disease. The bacteria would later be named Legionella pneumophila, the infectious agent that causes what we call Legionnaires' disease.
The bacteria was breeding in the cooling tower of the hotel's air conditioning system, and had been distributed throughout the hotel by the air handling system where it was inhaled by members of the American Legion.
We've learned a lot about the microbe in the last 40 years, and guidelines have been established to reduce its growth in water systems—an identified source of the infection-causing L. pneumophila. The bacteria likes to grow in water systems, particularly those containing warm tap water with lots of dissolved organic material for them to feed on.
Yet L. pneumophila still causes more than 10,000 cases of Legionnaires' disease worldwide every year—many of them in the US. In 2015, five Legionnaires' disease outbreaks occurred in the States—three of which occurred in the Bronx, New York, sickening 140 and killing 13.
To drill down on how this microbe continues to thrive in some water systems, a group of researchers from the KWR Watercycle Research Institute in the Netherlands honed in on the role of biofilms, a physical structure created by the deposit of bacteria such as Legionella. Their study was recently accepted for publication in Applied and Environmental Microbiology.
A biofilm is a deposit of bacteria, other microorganisms, and organic and inorganic materials that stick together and accumulate within a matrix called a slime layer. They can form on solid or liquid surfaces when nutrients and water are present.
To study how biofilms factored into Legionella growth in water systems, microbiologist Dick van der Kooij and colleagues created a model system that contained organic matter needed for biofilm development. Then they exposed their model system to warm drinking water that did not contain disinfectant. This system provided the organic matter and conditions where the Legionella bacteria would grow.
Results of the study showed that organic compounds in the warm drinking water induced the formation of a biofilm and Legionella grew there. Higher concentrations of organic matter induced higher biofilm concentrations. Heating the water increased the organic matter, and that, in turn, enhanced biofilm formation.
While the biofilms were developing and accumulating a high concentration of different types of bacteria, the researchers found the bacteria were joined by amoebae—single-celled animals that live in damp environments. The amoebae became a source of nutrition for Legionella and they thrived as part of the biofilm.
It isn't exactly clear how the Legionella growing in a biofilm leads to human infections, but cells, including bacteria, constantly break off from biofilms and could go on to infect someone.
Scientists, engineers, and water quality experts recognize that biofilms are more than a collection of bacteria, organic material, and slime. They represent a community of organisms that feed off each other, provide structure to the biofilm, and are held together by chemical bonds. Preventing or removing biofilms is not as easy as disinfecting water—and this could be a big reason that Legionnaires' disease keeps popping up, even after we know what causes it and how it spreads. These biofilms could be helping Legionella resist disinfection.
The Occupational Safety and Health Association lists chemical agents to be used for disinfecting, water temperatures recommended to limit Legionella growth, frequency of cleaning, and other considerations for decreasing Legionella growth in water cooling towers. Constant maintenance and monitoring for Legionella appear to be important factors in the success of a Legionella management program.
"Nine out of 10 times, the disinfection will be effective," Tim Keane, a consulting engineer at Legionella Risk Management Inc., told The New York Times. "But if the treatment program and risk management program isn't in place after the disinfection, nine out of 10 times the bacteria will regrow again if it was there before."
For now, scientists have figured out some of the complexities and nuances of the relationship between Legionella pneumophila growth and biofilms. The next step may be to figure out how to effectively disrupt or prevent these relationships from forming.