You might not be familiar with the term biofilm, but you sure would have encountered biofilms regularly. The plaque that forms on your teeth and causes tooth decay is one type of bacterial biofilm. The “gunk” that clogs your household drains is also a biofilm. If you have ever walked in a stream or river, you may have slipped on rocks that were slimy with biofilm. And so it goes: biofilms—they’re where you want to be.
What is Biofilm?
A common misconception of microbial living is that bacteria exist as individual organisms in a ‘planktonic state’. Rather, microorganisms have been shown to naturally accumulate on a wide variety of surfaces; where they form communities.
Those surfaces include household and industrial pipes, biomaterials such as contact lenses, medical devices including implants and urinary catheters, as well as plant and animal tissues.
These accumulations of microorganisms of mono- or poly-microbial aggregates are commonly referred to as biofilm and can consist of diverse communities of bacteria and fungi.
How and Why Biofilms form?
Biofilms form when microorganisms such as bacteria come in contact with a surface that allows them to grow and multiply. The microorganisms form an attachment to the surface of the object by secreting a slimy, glue-like substance. The sticky slime allows other microorganisms to attach to it, creating layers and eventually forming a complex three-dimensional structure.
Biofilms can form on just about any imaginable surface: metals, plastics, natural materials (such as rocks), kitchen counters, the walls of a hot tub or swimming pool. Indeed, wherever the combination of moisture, nutrients, and a surface exists, biofilms will likely be found as well.
A biofilm community can be formed by a single kind of microorganism, but in nature biofilms almost always consist of mixtures of many species of bacteria, as well as fungi, algae, yeasts, protozoa, and other microorganisms, along with non-living debris and corrosion products.
Favourable Conditions for Biofilm Formation
After the maturation of biofilm, it starts releasing microorganisms into the water which continuously contaminates the water system. Although biofilms start out microscopic in size, they can grow into visible biofouling deposits in just a matter of days.
Biofilm formation is encouraged where water temperatures are warm and sufficient nutrients are present to establish the colony.
Biofilms only grow in wet areas. The faster the moving water is, the thinner the biofilm. Conversely, the slower the moving water, the thicker the biofilm.
Biofilms can be quickly formed if water remains stagnant in the water distribution system. The stagnant water in dead legs provides ideal conditions for biofilms to form. Without flow, the water in a dead leg does not receive disinfectant or biocide treatment. This allows bacteria to attach to system surfaces to start the biofilm formation process.
The presence of scale and corrosion in a system will increase the available surface area and allow the formation of microniches that are protected from circulating disinfectants. Scale and corrosion also increase the concentration of nutrients and growth factors, such as iron, in the water system.
How Biofilms impact Man-Made Water Systems?
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.
Within water systems, dense biofilm formations can break apart and pieces can be carried off to other areas of the system, getting clogged in dead legs and filters and compromising further sections of the system.
In poorly managed heating, cooling and domestic water systems, biofilms can contribute to operational inefficiencies including poor heat exchange and poor circulation and may also give rise to corrosion or the presence of pathogenic organisms.
In cooling tower systems, biofilm control is especially challenging since systems are continuously inoculated with dust, nutrients, bacteria, and other contaminants during their normal course of operation.
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.
Biofilms Provide Safe Haven for E.Coli & Legionella
For microorganisms such as amoebas, protozoans, algae and bacteria, living as part of a biofilm has its advantages.
The biofilm facilitates nutrient and gaseous exchange and protects microorganisms not only from biocides but also from periodic increases in temperature and attempts at physical removal, especially in areas where surfaces are scaled or corroded.
Bacteria living as a group are more resilient to stressors, such as dehydration, ultraviolet light, high or low pH or to the presence of bacterial toxic substances, such as antibiotics, antimicrobials, disinfectants or heavy metals.
This is why once established, biofilms and associated microorganisms provide a protective environment for any Legionella entering a building with the source water to multiply to high levels.
Uncontrolled biofilms can occlude pipework, resulting in areas of poor flow and stagnation which further contributes to Legionella growth.
Why Are Biofilms Difficult to Treat?
Established biofilms are difficult to treat, even when subjected to high levels of chlorine or other disinfectants. The lack of flow and access for cleaning makes biofilms especially difficult to remove from surfaces inside plumbing of water distribution systems.
Biofilm-forming pathogens are very challenging to treat with conventional antibiotics because of their greater resistance behaviour.
Chlorine or bromine (generally used as primary biocides) oxidising biocide residuals are quickly consumed by reaction with the protective biofilm matrix and can only penetrate the outer layers.
If the concentration and contact time is long enough, non-oxidising biocides (generally used as secondary biocides) can diffuse into biofilms, but will only kill certain microorganisms and are prone to deactivation.
Microbial populations can also shift over time to favour microorganisms that are not susceptible to the specific non-oxidising biocide used.
Any disturbances in a water distribution system, such as water main construction or maintenance, a burst pipe or an explosion, can shake the biofilm loose. The freed biofilm and/ or plume of contaminated water travels in the water column downstream to seek out a new home. When another surface is found, such as new plumbing systems or newly installed evaporative cooling towers, the biofilm re-establishes, thus “seeding” the new plumbing system.
Reducing legionella risks by controlling biofilm in water systems
Preventing the formation of biofilm has been one of the public health and water treatment strategies for reducing water system contamination and therefore reducing the number of people who contract Legionnaire’s and other water-borne diseases.
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.
- Limiting the amount and type of nutrients (particularly organic nutrients) that are available to the bacteria in the water system is an important control measure. Nutrient levels can be controlled by:
- Selecting materials that will not serve as substrates or provide nutrients for biofilm development
- Ensuring that chemical additives used to control scaling, corrosion and microorganisms are applied at appropriate and effective concentrations and are chemically compatible (i.e. non-reactive) with one another and with the system
- Considering the properties of materials used in the water system (e.g. insulating properties, the potential for corrosion, interaction with chemical disinfection processes)
- Ensuring that system design is appropriate and will prevent the accumulation of biofilms, sediments and deposits (e.g. the design should eliminate dead ends and stagnation, and allow access to all parts of the water system for maintenance and cleaning)
- Use filtration in systems where water quality is poor and water is recirculated. Filtration of water entering a system can remove particulates and organic matter, reducing the rates of biofilm formation and Legionella growth.
Preventing low flow rates and stagnation of water is an essential and important control measure, and the system should be designed to minimize areas of stagnation and low flow.
Care should be taken to ensure that any modifications to the system do not introduce areas of stagnation and low flow. Where such areas are unavoidable, design and operation should aim to at least reduce stagnation and low flow.
Water systems at risk from stagnation should be periodically flushed or disinfected, and temperatures that are optimal for the growth of Legionella should be avoided. However, where flushing is used, the likely exposure of people to aerosols generated during flushing must be considered.
Take a Proactive Approach
The best method to fight the presence of biofilms is to prevent their formation from occurring in the first place. To mitigate the risks associated with legionella outbreak due to biofilms, it is critical to understand the complexity of your facilities water system.
And the most comprehensive way to identify any risks in your water systems is by getting a legionella risk assessment. Working with a legionella risk assessor, your facility manager can mitigate risk and ensure compliance with legislation.
Legionella risk assessments from our expert team help keep you one step ahead in the battle against Legionella.