LEGIONELLA - World Health Organization

Regular monitoring of Legionella concentrations should be included to build up a picture of
the trend. Legionella tests are not recommended as a guide for control measures, because their
inherent unreliability means that the results cannot be used as a reproducible, sensitive and
timely measure of system control (see Chapter 11). Legionella testing should only be used to
verify and validate a WSP — test results should not be seen as a surrogate for a comprehensive
control strategy (Bentham, 2002).
Conditions and frequency of testing
The heterotrophic plate count (HPC) technique (also known as heterotrophic colony count,
total colony count, total viable count and total heterotrophic count) is useful in assessing the
efficacy of antimicrobial treatments of cooling tower water (WHO, 2004).
Microbial testing should preferably be carried out in a laboratory that is competent and accredited
to do this work. If dipslides are used to test environmental samples (e.g. from cooling tower
waters), an incubator should be used for temperature control, and the slides should be incubated
at 30 ºC for at least 48 hours before interpretation of the result. Dipslides are simple, convenient
and inexpensive, but their accuracy is limited. They are useful in detecting major changes in bacterial
levels and for verifying that a water treatment programme is being implemented. Periodic
counts by the agar plate method are required for a more reliable and reproducible assessment of
HPC. Regular (e.g. monthly) HPC on tower water should be undertaken, to assess the efficacy
of the biocide treatment and general cleanliness of the system. A count of 5 × 10 5 colony forming
units (CFU)/ml in HPC is an acceptable upper limit for treated tower water in a clean system
(HSE, 2004). If this level of HPC is exceeded, the frequency of testing should be increased to
weekly, until control has been re-established.
Culture techniques and detection limits
Deficiencies in culture sensitivity and precision (discussed in Chapter 11) diminish the use
of action levels as a meaningful control measure in cooling water systems. Any Legionella test
result should be considered in the light of the detection limit of the method used, which
should be clearly reported with test results.
Sampling
Chapter 11 discusses requirements for sampling.
Where open basins are involved, water samples should be taken below the surface of the water.
When samples are obtained from taps, it is preferable to select those that are connected directly
to pipes containing the circulating water. Sample taps should be clean and free of leaks and
external fittings, such as hoses. Taps should be run so that the entire length of the fitting is
flushed with water for at least 30 seconds before taking the sample.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS