Researchers Have Discovered Best Conditions for Legionella Growth

Researchers have identified the optimal conditions for the growth of Legionella which causes a very serious, often deadly, type of pneumonia, Legionnaires’ disease. The best conditions were ones that encouraged biofilm growth: “warm tap water installations with ample dissolved organic matter.”

Since first being identified in an outbreak that occurred back in 1976 at an American Legion convention in Philadelphia, at least 20 outbreaks are reported annually; 10,000 cases of Legionnaires’ worldwide, each year, are caused by Legionella pneumophila. In fact, L. pneumophila accounts for 90% of infections, and those who acquire the disease are known to suffer, “long-term impaired health-related quality of life.”

Inspired by a number of large Legionnaires’ outbreaks, Dutch scientists sought to figure out what conditions incited bacteria growth on surfaces that had been exposed to drinking water. The research, published in Applied and Environmental Microbiology, was conducted at KWR Watercycle Research Institute in Nieuwegein, Netherlands.

According to a recent press release, in controlled hydraulic conditions, the researchers developed a “model system” that allowed them to measure biofilm formation as well as Legionella bacteria growth that had been “exposed to drinking water without disinfectant.” This system allowed them to compare two different water supply systems; one with low concentrations of dissolved organic matter, and one with high concentrations.

According to first author Dick van der Kooij, PhD, recently retired principal microbiologist at KWR Watercycle Research Institute, “Drinking water prepared from aerobic groundwater with a low concentration of dissolved natural organic matter induced a very low biofilm concentration that did not support growth of L. pneumophila. Drinking water from two other sources with higher concentrations of organic matter induced higher biofilm concentrations that supported Legionella growth.” This means that the higher the biofilm concentration, the higher the likelihood of Legionella growth.

When speaking of the implications of their findings, Dr. van der Kooij explained, “Our research demonstrated that microgram-per-liter concentrations of biodegradable compounds in warm drinking water can induce sufficient bacterial growth on surfaces for proliferation of the amoebae that support growth of Legionella. Heating the water increases the concentration of biodegradable compounds, thereby promoting biofilm formation.”

Amoebae are essentially hosts for Legionella bacteria—especially because they contain amino acids that Legionella needs to grow—and biofilms, “can support a high concentration of bacterial species that serve as prey for amoebae.”

Currently, there are no vaccines available to prevent legionellosis. Because of this, the Centers for Disease Control and Prevention (CDC) stress that the best form of prevention to reduce risk of Legionella growth, is to ensure that water systems within buildings are consistently maintained. Hot water tanks and heaters, larger plumbing systems, hot tubs, and cooling towers are all water systems that are capable of spreading Legionella bacteria. They remind the public that any kind of water system that uses hot water is an optimal environment for Legionella to grow and it may be difficult to keep the disinfectants that are typically used to kill these kinds of bacteria at levels that are needed to kill said bacteria. However, the CDC provides steps for testing water as well as questions to ask hot tub operators in order to increase protection against the potentially deadly bacteria on their website.