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Reducing Biofilms in Building Water Systems to Abate Risk of Legionella

July 17, 2018
Improperly maintained water supply systems and other factors can lead to an environment that allows Legionella survival and growth.

While biofilms occur naturally on many surfaces, including bodies of water and living tissues, they can become a concern when they invade a building water system, potentially damaging water quality and threatening public health. An example of harmful biofilm formations can be found in those that capture and harbor opportunistic pathogens, such as Legionella pneumophila, the bacteria responsible for causing Legionnaires’ disease.

The Centers for Disease Control (CDC) has recognized that opportunistic pathogens, or “bad bugs” like Legionella, are the primary cause of waterborne disease in the U.S. Legionella can multiply to dangerous levels under certain conditions and potentially cause Legionnaires’ disease, a serious lung infection caused by inhaling small droplets of contaminated water through aerosolized mist or vapor.

Improperly maintained water supply systems and other factors, including complex system designs, high water age and breaks and leaks, can lead to an environment that allows Legionella survival and growth within the water system. The new Plumbing Manufacturers International (PMI) document, “Just the Facts: Legionella and Water Supply Systems,” describes several key elements that can promote the growth of Legionella bacteria in a water supply system, including biofilm, a sticky substance created by bacteria that forms on the inside wall of water supply piping and protects Legionella from heat and disinfection.

“In my experience, biofilm will be present in all building plumbing systems. Anywhere there is water and a surface, biofilm can form,” said Frank Sidari, technical director at Special Pathogens Laboratory, a firm internationally recognized for clinical and environmental expertise in Legionnaires’ disease prevention with solutions for detecting, controlling and remediating Legionella and other waterborne pathogens. “Legionella and other microbes occur naturally in our water supplies. Certain conditions, many of which are still being clarified, tip the balance in favor of Legionella in some building water systems. Simple maintenance practices have not been successful in eliminating Legionella from these water systems”

Since biofilms are an inherent part of a building’s water system and can potentially protect Legionella bacteria, proactive management of building water systems to minimize risk is important.

“The design and complexity of current building plumbing systems, the continued exposure to water containing nutrients and microorganisms, and the need to maintain disinfectant levels within drinking water standards provide the opportunity for Legionella to exist in potable water systems,” Sidari said. Studies have shown that Legionella can be found in up to 70 percent of complex building water systems including large buildings and hospitals compared to 10 percent to 40 percent of simpler plumbing systems in residential houses, he added.

Stages of Biofilm Formation

Biofilm forms anywhere non-sterile water comes in contact with a surface, providing an opportunity for Legionella and other opportunistic premise plumbing pathogens (OPPPs) to grow and avoid disinfection. Biofilm formation and growth involves three stages: attachment, growth and detachment. Free-floating (or planktonic) bacteria that come into contact with a surface may attach to the surface via cell surface appendages, referred to as adhesions. Following attachment, cells grow and divide, with new cells generally sticking to the biofilm and increasing its coverage and thickness.

It’s important to note that older water infrastructure can be more vulnerable to contamination through leaks and breaks, according to PMI’s Legionella document.

Eventually, if the biofilm is not subject to disinfection or some control means, it may grow to a thickness of hundreds of microns. This condition inhibits the efficacy of disinfection efforts and provides a variety of environmental niches in which different types of bacteria can survive, such as both aerobic and anaerobic organisms. A biofilm may reach a quasi-steady state condition, referred to as maturity, in which the thickness stabilizes, but growth continues. In this condition, chunks of the biofilm containing hundreds or thousands of cells may detach and float into the bulk water phase. These cell clusters are also resistant to disinfection because of their size, and they may harbor pathogens, such as Legionella. 

Although Legionella generally do not grow within the biofilm in potable water lines, the presence of a thick biofilm increases the chances of Legionella survival and growth. The diversity of environmental niches in a thick biofilm may lead to the entrainment of larger organisms, such as protozoa, which may act as a host for Legionella, leading to growth and replication within the biofilm. Thus, cell clusters which detach from a thick biofilm could harbor Legionella. If these cell clusters are aerosolized or aspirated, human infection could occur.

It’s important to note that older water infrastructure can be more vulnerable to contamination through leaks and breaks, according to PMI’s Legionella document. A leak or break increases the possibility of OPPPs, like Legionella. entering the infrastructure, becoming entrained in biofilms, and then being released into the water supply.

In addition, high water age has possible implications for OPPPs, including Legionella, which are more likely to grow when water lies stagnant in pipes leading to the tap. Recent research by Dr. Marc Edwards indicates potential consequences to public health with the increasing number of buildings being built or retrofitted with new types of plumbing systems that keep water in pipes longer. His research also showed that the significant disinfectant loss in buildings with stagnant water needs to be studied further.

The Case for Antimicrobials

Bacteria like Legionella that survive inside biofilm in a water system can be challenging to eradicate. Many established treatment methods, such as chemical and ultraviolet light disinfection, can kill Legionella; however, there can be challenges with achieving the necessary widespread distribution within a water piping system to fully destroy the bacteria and biofilms that harbor them. In addition, some disinfectants, like monochloramine and copper-silver ionization, can be successful in controlling Legionella, but do not remove biofilm in a plumbing system, according to Sidari.

The Center for Biofilm Engineering at Montana State University is currently working on developing a repeatable laboratory model to assess the efficacy of antimicrobials against Legionella. This model will help determine the necessary chemistries, concentrations and contact times to control biofilm containing Legionella.

Better detection and eradication tools and methods must be developed to minimize the opportunity for Legionella to grow and spread within building water systems. Laboratory models, such as the one being created at Montana State, will offer manufacturers and regulators a way to compare products and treatment regimens for better Legionella control.

To view PMI’s “Just the Facts: Legionella and Water Supply Systems,” visit the PMI website at

Paul Sturman is research professor and industrial coordinator specializing in biofilms in waste remediation and industrial systems with the Center for Biofilm Engineering at Montana State University, Bozeman.