How to Test for Legionella in Water: Essential Steps and Modern Solutions
How to test for legionella in water involves collecting water samples and sending them to a laboratory for analysis. The most common method is the culture method, where samples are plated on selective agar and incubated for several days to allow Legionella to grow. Alternatively, molecular methods like PCR can provide faster results. Accredited laboratories use these methods to detect and quantify Legionella bacteria, including specific species and serogroups.
How to test for Legionella bacteria
When inspecting water systems, there are several visual warning signs indicating a higher risk of the presence of Legionella bacteria, including:
* Corrosion—Corroded pipes, fittings, or valves provide a breeding ground for biofilm formation, which can harbor and protect Legionella bacteria from disinfection methods. Rust-colored water or the presence of flakes and rust particles are indications of corrosion.
* Scale—The accumulation of scale provides a favorable habitat for bacteria, including Legionella, to colonize and proliferate. Visual signs of scale include white or off-white deposits on faucets, showerheads, or internal plumbing.
* Sediment—The presence of sediment such as dirt, debris, or particulate matter can be a potential source of nutrients and support the growth of Legionella bacteria. Brown or cloudy water, or visible particles in the water, can indicate the presence of sediment
There are also several ways of testing for Legionella bacteria. The technology is rapidly changing, and each method has its own positives and negatives. The merits of each one should be explained in depth by a water treatment professional who can offer advice based on the requirements of your building and the type of water system in it.
The culture method
Traditionally, water samples are collected from water systems using the bottle bulk water sampling method. This allows water to be collected from potable water fixtures or from warm-water-containing mechanical equipment.
The water sample is then sent to a UKAS accredited laboratory, such as WCS Group, which will identify the presence or absence of Legionella in the water sample collected, the bacterial count and Legionella species present. It is essential to collect the water samples at the right points within the water system and with the right methodology for accurate results. It is also important that samples are transported in the correct way and in the correct timescales.
However, these traditional lab-based culture methods are time-consuming, and it can take up to two weeks before the building owner or manager is aware of the result.
The PCR method
The PCR (or polymerase chain reaction) method is a technique that only takes a few hours to complete. The method consists of several steps:
- Viable cells are concentrated through membrane filtration.
- They are then subjected to sonication and heating procedures, which lyse the cells and free the DNA.
- Finally, the DNA is purified for the polymerase chain reaction.
PCR is a fast and accurate way of detecting Legionella bacteria, often succeeding in detecting Legionella types when traditional methods fail; however, this method is traditionally more expensive, requires specialist equipment and results can be difficult to interpret. PCR’s are very useful in managing outbreak situations.
The rapid swab method
Rapid swab-testing kits that enable the detection of Legionella in just 25 minutes are now widely available. They are incredibly accurate at identifying the fatal Sero group 1 strain of Legionella.
Detecting Legionella so quickly allows rapid remedial action, which can significantly reduce the possibility of human infection. However, while being a helpful self-test tool in the toolkit, the rapid-swab method is not a substitute for a robust water testing programme and UKAS laboratory certification and can only currently detect Sero Group 1.
From where should you take Legionella samples?
Legionella samples should be taken from the locations identified in the risk assessment or from areas where there is a potential concern. It is often not suitable just to take one test as this will not provide the full picture as to the system’s health.
As well as the water that regularly flows through the system, special consideration should be given to high-risk areas:
* Where water temperatures drop-out of specification
* Where stagnant water may be sitting in pipes or tanks
Using temperature control to monitor and control Legionella
Another way to detect places that potentially contain Legionella bacteria is to test water temperature. While physically testing is necessary to ensure public safety, as always, prevention is the best cure. The best way to do this is to control water temperature, as the bacteria only grow to levels that can become dangerous between 20 and 45°C.
* Hot water storage cylinders (calorifiers) should store water at 60°C or higher
* Hot water should be distributed at 50°C or higher (thermostatic mixer valves must be fitted as close as possible to outlets, where a scald risk is identified)
* Cold water should be stored and distributed below 20°C
The temperature for water storage cylinders should be checked in accordance with the risk assessment, but this is likely to be every month, and cold water tank temperatures should be checked at least every six months. Also, ‘sentinel’ outlets, which are the closest and furthest from each tank or calorifier, should be checked for their monthly distribution temperatures.
Keeping the system clean
To control the risk, you will need to ensure that hot and cold water storage tanks, distribution pipework, and outlets remain clean.
Stagnant water favors legionella growth. To reduce the risk, you should remove dead legs/dead ends in pipework, flush out infrequently used outlets (including showerheads and taps) at least weekly, and clean and descale showerheads and hoses at least quarterly.
Cold-water storage tanks should be cleaned periodically, and water should be drained from hot water cylinders to check for debris or signs of corrosion.
Design systems
Design systems to minimize Legionella growth by:
* keeping pipe work as short and direct as possible
* adequately insulating pipes and tanks
* using materials that do not encourage the growth of legionella
* preventing contamination, for example by fitting tanks with lids and insect screens.
Conclusion
Testing for Legionella in water involves collecting samples and analyzing them in accredited laboratories. Common methods include the culture method, which takes days for results, and PCR, offering faster detection. Visual signs like corrosion, scale, and sediment indicate higher risks. Rapid swab tests can detect Legionella in minutes but are limited to specific strains. Regular temperature checks and system maintenance are crucial for prevention.
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References
1- When and how to test for Legionella bacteria in water systems
https://www.wcs-group.co.uk/wcs-blog/when-to-test-for-legionella-bacteria-in-water-systems
2- Managing legionella in hot and cold water systems
https://www.hse.gov.uk/healthservices/legionella.htm#
