Legionella Sampling – PCR versus Culture Technique
PCR is a molecular biology technique in which the DNA of micro-organisms is extracted and then amplified (multiple copies are made). This enables the laboratory to determine the presence and quantity of that organism’s DNA in a water sample. As it is the DNA that is being assessed, the technique can detect live cells, cells that are alive but cannot be cultured (so-called Viable Non-Culturable) as well as dead cells.
In contrast the traditional culture technique for Legionella relies on the bacteria’s ability to grow and multiply in order to produce visible colonies on an agar plate which can be counted by the laboratory. Any Legionella cells which do not grow and multiply therefore remain undetected.
Legionella bacteria are known to be difficult to recover from samples in a laboratory situation. Therefore a certain proportion in a sample will always remain undetected when using the culture technique.
Advantages of Using the Legionella PCR Method Rather than the Culture Technique
The traditional Legionella culture technique takes 10 days to arrive at a confirmed negative result as Legionella bacteria are slow growing in the laboratory. A further 3 days are required for confirmation in the case of suspect colonies being identified on the agar plates. In contrast, the Legionella PCR technique is able to arrive at a confirmed positive or negative result in 24 hours. This includes being able to quantify Legionella species and Legionella pneumophila serogroup 1. This can be particularly useful when determining how successful a treatment has been in a water system.
Current action and alert levels for L. pneumophila in L8 are given in colony forming units (cfu)/L. For this reason whenever a Legionella test is conducted by PCR, this must be carried out alongside a test by traditional culture technique. Therefore 2 samples must be provided to the laboratory for this purpose.
Interpretation of Results
PCR gives results as genomic copies (GU) per litre of sample, whereas the traditional culture technique gives results in colony forming units (cfu) per litre of sample. As PCR can detect dead and viable non-culturable cells the quantitative results from PCR are expected to higher than those obtained by culture.