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| Laboratory - Microbiological Analysis |
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The HSE document L8, recommends that analysis of water samples for legionella bacteria should be carried out by a UKAS accredited laboratory, and the laboratory is required to take part in the PHLS External Quality Assessment Scheme for the isolation of legionella bacteria from water. Qualified and experienced microbiologists should carry out interpretation of the results. Further supplementary guidance is available in British and International Standards on legionella sampling and the detection and enumeration of legionella bacteria by culture. L8 provides action levels for legionellae in hot and cold water systems and cooling water. Effectively, a response is required where legionella bacteria are detected in numbers greater than 100 colony forming units (cfu) per litre of water sample. In cooling systems additional microbiological monitoring is needed, and L8 recommends weekly testing for total bacteria (TVC). Action levels are provided for TVC’s and a review of the biocide treatment programme is required where microbiological counts exceed 10,000 cfu per ml of sampled water. It is important to note that TVC analysis is a general guide to the level of microbiological control being achieved, but it is not a reliable indicator of the number of legionella bacteria present in the cooling system. Low TVC results have been reported in the presence of high legionella counts. Guidance on sampling for the presence of legionella bacteria in water systems is available, and is currently being updated by the Standing Committee of Analysts. The quality of the water sample is particularly important as investigations into the environmental source of legionella infections have shown that there is significant variation in the procedures used for the routine sampling of systems, and this will inevitably introduce considerable variation in the results of analysis. In particular, most cooling systems and many potable water systems are treated with biocides, and sample containers should contain suitable compounds that will neutralise these biocides. If this is not carried out microorganisms in the sample, which are representative of the water system, may killed off while the sample is transported to the laboratory, and therefore a false negative result may be reported. Showers have been implicated as the source in a number of legionella infections, the preferred sample is normally the first catch i.e. the sample of water that first passes from the outlet. As legionella bacteria are obligate aerobes, the highest count of legionellae in potable water systems is often close to the outlet, i.e. where oxygen levels are highest. The sample volume collected from water systems for the detection of legionella bacteria is normally one litre. A plastic bag may be used to collect a sample from the showerhead. During sampling the mixer valve should be adjusted from the cold to the hot setting to collect a representative sample from the shower. A new plastic bag should be used for each shower sampled. The primary reservoir for legionella bacteria in most hot water systems is the base of the calorifier where due to temperature stratification the temperature is often particularly conducive to the growth of legionllae (30-40°C). This water may be sampled if a drain valve is fitted, but the valve should first be disinfected (with chlorine or isopropyl alcohol) and flushed for a few seconds before a sample is collected. Care should always be taken by the person who is collecting water samples, to avoid generating aerosols during the sampling process. Individuals who may be prone to infection should not be involved in collecting samples. Article by:
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Increasingly, showers and spa pools are associated with Legionnaires’ disease. If they are not adequately maintained they can provide conditions that support the growth of legionella bacteria and readily produce fine aerosols which may then be inhaled.  |
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