How to control Legionella - Part 1: the risks and the options…

by Paul Limbrick, on 31-07-2018
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IMG_1519Water undertakers have a responsibility to ensure that ‘wholesome’ water is delivered to the point of entry of buildings, this does not necessarily mean that such waters are free from microorganisms. Moreover, whilst drinking water should be free from parasites (cryptosporidium) and faecal contaminants (coliforms/E. coli), it’s common for wholesome water to contain aquatic bacteria, as such bacteria make-up the ‘normal’ microbiology of drinking water. Therefore, bacteria such as Pseudomonas and Legionella live preferentially in the environment and these soil-derived bacteria frequently reside within water supplied to buildings.

 

Bacteria in their ‘planktonic’ form (free floating within the water system) generally pose little threat due to the documented impact of temperature; bacterial viability (whether they live or die) or a bacterial cell’s propensity to multiply. Moreover, hot water temperatures of 60˚C and greater will cause cell lysis (break-open the bacterial cell wall) typically after 2 minutes of exposure time to this temperature and hot water temperatures in excess of 70˚C will kill ‘planktonic’ bacterial cells upon contact. Conversely cold-water temperatures of less than 20˚C will typically encourage waterborne bacteria to stay dormant (not multiplying/metabolically active), which substantiates the rationale for identifying temperature as the recognised ‘traditional’ method of control.

 

Inadequate water usage within the distribution system may encourage the settling of nutrients within water systems which in-turn will provide a ‘food source’ for waterborne bacteria. Moreover, as nutrients accumulate within a water system - waterborne bacteria will adhere to the inner surface of pipework to utilise these nutrients for growth and eventual proliferation. This is essentially the early stages (conditioning phase) of biofilm formation and therefore underpins the importance of maintaining good system pressure and adequate water usage – to ensure that nutrients don’t settle, and all freely-suspended particles and microorganisms are flushed out of the system, thus mitigating the risk of biofilm formation. From a ‘practical perspective’ this supports the importance of identifying and flushing infrequently used outlets as well as identifying and remediating dead-ends – cut off ends. As well as ensuring adequate circulation of water through the distribution system in order to mitigate the risk of stagnation within a water system.

 

Biofilm bacteria are different to free-floating (planktonic) bacterial cells in that they may gain protection from being within the biofilm. As such, it’s accepted that the use of temperature [heat] is not the most robust treatment strategy as biofilms may increase the temperature tolerance of bacterial cells living within it due to their ‘architecture’ and associated density. In addition, biofilm bacteria may demonstrate phenotypic differences to their previous (planktonic) form and develop antimicrobial resistance characteristics by virtue of propagating (growing) within a biofilm containing bacteria of varying antimicrobial resistance. The ‘pili’ structures on the surface of the (Gram negative) bacterial cell wall have been associated with binding to the inner surface of pipework (mentioned above) but have also been associated with the exchange of resistance characteristics (genes) between bacteria, therefore increasing the ‘virulence’ of waterborne bacteria and making them more difficult to treat.

 

For these reasons careful consideration should be given to the choice of remediation strategy when controlling bacterial contamination within water systems as some will be more effective than others – depending on the condition of the water system being treated and the strategy of use employed. Moreover, key considerations may also include; the age of the water system and associated dilapidation – including the presence of corrosion scale, materials of construction of water systems, level of organic material and particulate that may support the growth of waterborne bacteria, as well as the nature of bacterial contamination when detected (biofilm or planktonic).

 

The Responsible Person [Water] / Authorised Person [Water] with responsibility for buildings, of which the majority will have complex distribution systems [i.e. Hospitals, Universities, Hotels, Care Homes etc.] will need to ensure such conditions are avoided to reduce the risk of waterborne bacteria establishing. A broad summary of the ‘principal conditions’ that support the growth of waterborne bacteria within water distribution systems, which are;

  • water temperature between 20-45˚C;
  • poor circulation and water flow;
  • available nutrients;

Whilst understanding the ecology of waterborne bacteria and the limitations of a given water system will undoubtedly help with choosing an effective treatment regimen. Choosing an appropriate water treatment regimen requires careful consideration and should not be neglected as this may result in an unsatisfactory outcome – either by impacting upon the integrity of the water system being treated and or by demonstrating poor efficacy in terms of biocidal effect (bacterial cell death and or removal from the water system not achieved).

Editors Note: The information provided in this blog is correct at date of original publication - July 2018. 

© Water Hygiene Centre 2019

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About the author

Paul Limbrick

Paul is a Microbiologist who has diversified over his career into supporting clients with understanding of water control strategies, water sampling strategies along with consultancy support of specialist advisory services. These services have included membership of Water Safety Groups, shaping water safety plans [policy and operational control plans]. All of which reflect the offering of our Authorising Engineer (Water).

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