Legionella sampling; does “Not detected” mean there are no Legionella present?

Have you recently tested your water system, be it cooling water, hot or cold water or process system, and had the laboratory report “Not Detected”? Did you feel pleased that you don’t have Legionella bacteria in your system?

Perhaps you need to reconsider what “Not Detected” actually means…

A laboratory test for Legionella is considered to be the “gold standard” for testing as it can identify what type of Legionella bacteria and how many are present. However you need to be aware of several factors.

Planktonic versus sessile bacteria


When you take a sample of water from your system generally you are taking a sample of the body of the water and that is where free swimming or planktonic bacteria are found. So you sample will only be measuring planktonic bacteria.

Legionella bacteria can settle onto surfaces and become part of the biofilm on surfaces. It is thought that as many as 60% of all Legionella bacteria within a system can be found within the biofilm. This means only 40% of Legionella bacteria are planktonic and available to be picked up in your sample.

The next consideration is that Legionella bacteria are not homogeneously mixed throughout your system, they can travel around in “clumps” within the system. This means that when you go to take your sample it is possible that Legionella are not at your sample point when you take the sample. It also means that in two samples taken soon after each other it is possible that one sample contains Legionella and the other doesn’t!

The next consideration is the “Limit of Detection” that the laboratory test is capable of and this can vary between laboratories with some laboratories offering as low as 20cfu/litre and others 100cfu/litre. This means that for a laboratory offering 100cfu/litre then 90cfu/litre would be classed as “Not Detected” whereas for the laboratory capable of 20cfu/litre the sample would return with a positive result!

Finally is the consideration of how capable the recovery rate of the laboratory methodology. It is often said that a laboratory sample can only recover 40% of bacteria within the sample. This means that a “Not Detected” i.e. below 100cfu/litre could actually be significantly more that the number reported.

When you look at all this you can see that a “Not Detected” result should not make you complacent that your system is free from Legionella. Any bacteria, including Legionella, can grow to large numbers very rapidly as bacteria grow by “asexual reproduction” and can double in number every 20 minutes given ideal conditions. It does not take long therefore for a “not Detected” to become a large number in a short space of time.

Consequently it is very important to always be vigilant in managing Legionella bacteria and not assume that a “not Detected” means that Legionella is not present.

There are other tools in the box that could be used to confirm this is not the case, for example PCR analysis can confirm the presence or absence of Legionella DNA and so a “Not Detected” really would mean Legionella ae not present.

However it is important to understand all the pros and cons for all sampling methodologies and use the tools as appropriate to really understand whether your system is free from Legionella bacteria.

If you want to discuss your sample results with a company that understands then please contact Collaton Consultancy Limited via our website: http://www.collatonconsultancy.com, by email on general@collatonconsultancy.com or phone us on +44 (0)7958 124563.

For further information on the training we can offer why not check out our online training at: http://www.collatonconsultancy.com/online_training.htm

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