I have a Pseudomonas aeruginosa count – what next?

by Water Hygiene Centre, on 16-01-2019
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To understand the actions that should be taken when a positive Pseudomonas aeruginosa [P. aeruginosa] test results is received, it is prudent to first understand more about this opportunistic pathogen and the associated guidance. This knowledge will help to shape the actions…

What is Pseudomonas aeruginosa?

  • Pseudomonas aeruginosa is a microorganism that is part of the natural flora and fauna of the human body;

  • It is an ‘opportunistic’ pathogen that can contaminate / colonise moist / wetted environment, such as the end of taps [and up to 2-metres of associated pipework];

  • P. aeruginosa is well-known for its antibiotic resistance capabilities and within the healthcare environment the associated risk and cost of infection from this pathogen is significant;

HTM 04-01 - Water Safety Group & Water Safety Plan 

HTM 04-01 Parts B and C prescribe methods for mitigating the risk of contamination/colonisation with P. aeruginosa; including the need for the Water Safety Group [WSG] to commission and agree a Water Safety Plan [WSP] that considers the risk from P. aeruginosa to patients. If there are augmented care areas within the healthcare organisation then these need to be risk-assessed and a sampling plan established. The HTM 04-01 Part B details where samples should be taken, sampling should not be a ‘fishing’ exercise! The Water Safety Group should agree the sample points and ensure that the Water Safety Plan outlines the actions to take.


Pseudomonas aeruginosa positive results?

On confirmation of a positive P. aeruginosa test result, infection prevention and control will decide whether an outlet is to remain in normal use, be fitted with a point-of-use microbiological filter or taken out of service. The HTM 04-01 Part B, Appendix D ‘Action Plan’ can assist this decision-making process by offering clear guidance on recommended remedial and re-sampling actions dependent upon the number of bacterial cells recovered within a given sample.

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Flushing of outlets

A significant proportion of P. aeruginosa positive test results are attributed to infrequent use or poor flushing of the outlet(s). Upon receipt of a pre-flush positive test result, it’s indicated to increase flushing thus increasing the movement of water and retest the outlet in three days. Moreover the bacterial count will often reveal ‘how’ an outlet is used in practice; so whilst it’s recommended to commence flushing of affected outlets (so as to not exacerbate stagnation), the bacterial count will often determine whether or not the outlet is to be kept in service.


Sampling for Pseudomonas aeruginosa

Should routine sampling return a pre-flush water sample test positive for P. aeruginosa with a reported count between 1-10 cfu/100ml, then it’s indicated to re-sample pre and post-flush. If the re-samples return negative results (i.e. ‘Not Detected’) then sampling will revert to once every 6-months.

When P. aeruginosa counts between 1-10 cfu/100ml are initially reported from a previously unaffected outlet it’s not indicated to remove the outlet from service. However, it is prudent to consider that the only safe level of P. aeruginosa is ‘0’ and water sample results are only a ‘snapshot in time’. This often results in the outlet being removed from service or the installation of a point-of-use filter until a subsequent sample tests negative for P. aeruginosa – considering that sampling for this pathogen is only indicated within high-risk patient areas and many of these patients will have suppressed immunity and or underlying health problems which make them more susceptible to infection and subsequent disease.

When test results report bacterial counts of >10 cfu/100ml then it’s indicated to risk assess removing the outlet(s) from service and re-sample the affected outlet(s) after three days – during which time flushing should be increased. Following receipt of a negative test result from the 3-day test, then the outlet can be reinstated to normal service and subject to retesting after an additional 2-weeks, 4-weeks and 6-months provided that outlets continue to test negative. Should outlets test positive following the 3-day test then additional remedial works are typically required:

  • Clean, disinfect or replace affected outlet / components / pipework;

  • Review cleaning processes – avoiding cross contamination;

  • Ensure that hot water system is operating within specification and that hot water circulation is properly balanced;

  • Review the use of the outlet – a wash hand basin is designed for washing hands and not for disposal of body fluids or dirty water!

  • The position of the tap into the wash hand basin – avoid splash and spray including direct fall of the water on to the waste opening at the bottom.


In summary, for as long as water outlets are well maintained, used or flushed and water temperatures remain within specification then problems seldom occur with P. aeruginosa.

When issues occur with positive water sample results, refer to the Water Safety Plan, which will in turn detail the need for reviews to take place involving the three key stakeholders of estates, domestics and healthcare workers. Each to ensure that their individual interactions with the affected outlet are not the root cause.

Avoid water stagnation – slow moving or stagnant water encourages the growth and proliferation of P. aeruginosa [and other waterborne bacteria].

Adequate engineering control – ensure that hot and cold-water systems remain within specification i.e. control water temperature and eliminate components that encourage P. aeruginosa.

Cleanliness – ensure that outlets are cleaned correctly to help mitigate the risk of P. aeruginosa [and other waterborne bacteria] proliferating the ‘local system’ and outlets.

Appropriate use of hand-wash basins – they are designed for washing hands and nothing else e.g. disposal of body fluids!

If you have any questions about Pseudomonas aeruginosa, please don’t hesitate to get in touch with Water Hygiene Centre by clicking here.

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Editors Note: The information provided in this blog is correct at date of original publication - January 2019. 

© Water Hygiene Centre 2019


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Water Hygiene Centre

The Water Hygiene Centre was established in 2009 to address the lack of independent water hygiene consultancy within the industry. From our humble beginnings, we have established ourselves as a market leader, helping clients identify and minimise the risk of waterborne contamination and disease, whilst improving compliance performance.

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