Legionella Sampling - Culture v PCR
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The gold standard for Legionella testing has always been the culture method (often referred to as the standard reference method) whereby the bacteria are filtered from a water sample, grown and then the number of colony forming units (cfu) estimated. However, this is a labour intensive process with a 12 day turnaround. Given the penalties for non-compliance and costs associated with taking water services out of use for 12 days, the culture method is rapidly becoming outdated.
A number of recent scientific advances have revolutionised the face of Legionella analysis and now, with a decent courier, samples can be turned around in 24 hours (4 in an emergency). Progress has been made possible due to the use of Polymerase Chain Reaction (PCR) technology. Rather than actually growing the Legionella bacteria, PCR works by isolating fragments of DNA from the water sample and amplifying them to a detectable level. This process is a lot quicker, more efficient and can largely be automated.
PCR has been of inestimable value especially for trend analysis and when investigating outbreaks. It is not without its problems. The initial stumbling block was that of detecting ‘dead’ or ‘non viable’ cells, that would not show up on culture. This has since been largely overcome, but PCR in general terms still returns far more positive results than the culture method. This leads on to the next problem of reporting units and their correlation with action levels.
The current Legionella guidance ACOP L8 contains recommendations for ‘Action in the event of a positive sample’ as below (for domestic water systems);
• Legionella bacteria (cfu/ litre) – More than 100 but less than 1000
ACTION REQUIRED (L8) – Either:
(a) If only one or two samples are positive, system should be re-sampled. If a similar count is found again, a review of the control measures and risk assessment should be carried out to identify any remedial actions.
(b) If the majority of samples are positive, the system may be colonised, albeit at a low level, with legionella. Disinfection of the system should be considered but
an immediate review of control measures and risk assessment should be carried out to identify any other remedial action required.
• Legionella bacteria (cfu/ litre) – More than 1000
ACTION REQUIRED (L8) – The system should be re-sampled and an immediate review of the control measures and risk assessment carried out to identify any remedial actions, including possible disinfection of the system.
The reporting units in this table refer entirely to colony forming units (cfu), whereas PCR sample results are returned in genomic units (GU). Therefore, if we get a negative culture test but a positive PCR test – what action to take? Or similarly, if PCR is used in isolation at what point do you start spending large amounts of money disinfecting water systems and taking services out of action? There is no way of correlating the two units and no set levels for genomic units.
Currently a large number of companies offering PCR, back up the results with a culture test (at extra cost). It is certainly in no doubt that future analysis will move towards PCR (molecular photocopying as it is sometimes known). In the meantime, we will be looking to the HSE for guidance on the newly UKAS accredited PCR testing method.
For more information about Legionella sampling or managing the risks associated with Legionella, please contact Nemco Utilities at www.nemco-utilities.co.uk
About the Author
Charles has been working with the various facets of legionnaire's disease for some time now and has a wide expertize on the same. He shares his insights with the readers through his scholarly articles.
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