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RAPID LEGIONELLA TESTING

FREQUENTLY ASKED QUESTIONS

WHAT IS THE STANDARD CULTURE METHOD?

Based on ISO 11731:1998 – Water Quality – Detection and enumeration of Legionella.  Results are generated as colony forming units per litre for Legionella pneumophila and Legionella spp.  Samples are concentrated, pre-treated and inoculated, before incubation at 35°C.  Samples are examined at three, seven and ten days and any suspect Legionella colonies are removed from the plate for confirmation.  Results are expressed as colony forming units per sample volume.

WHAT IS THE RAPID TESTING METHOD?

Rapid detection by Polymerase Chain Reaction (PCR) indicates the number of viable Legionella bacteria in a sample by quantifying the level of ‘target’ DNA (i.e. regions of DNA that are specific to these organisms).  The method captures the target organism via membrane filtration followed by extraction of DNA from intact cells.  A PCR run is carried out to amplify the target DNA to a threshold level in order for it to be detected by the instrument.  Results are expressed as genomic units per sample volume.

WHAT IS THE TURNAROUND?

Rapid Detection by Polymerase Chain Reaction (PCR) normally gives reported results within 48 hours of sample receipt for Legionella pneumophila and Legionella spp.

WHAT ARE THE BENEFITS?

  • The main advantage is the speed with which you get the result.  Traditional legionella analysis takes around 10-14 days to produce a valid test result.
  • The rapid Legionella test allows the detection of viable non-culturable Legionella bacteria (those which have been stressed sufficiently by disinfection or temperature to be unable to multiply, but are not killed).  When the stress is removed this group will then be able to multiply.  The standard culture method will not detect these organisms.  This is why results are expressed as genomic units per sample volume, not colony forming units as for the standard method.
  • The test can be used for day-to-day monitoring of treatment performance and 'fine-tuning' of chemical applications.
  • Useful for assessment of the cleanliness of a water system, for example, during pre-commissioning investigations or where water systems have previously failed.

The rapid Legionella test should not be viewed as a replacement for existing standard culture method based analysis but rather as a tool to provide an early warning of significant Legionella contamination.