Are Legionella resistant to chlorine?

Legionella bacteria live in both the planktonic phase i.e. in the body of water within a system, and in the sessile phase i.e. within bioflims. It is well understood that chlorine finds it difficult to penetrate biofilms so is not a surprise that Legionella in biofilms may not be affected by chlorine.

A study in Italy found that:

“many microorganisms of aquatic origin, such as Pseudomonas, produce bacteriocins that may actually be inhibitory to Legionella species. Thus, chlorine levels that eliminate these microorganisms could increase the population of indigenous Legionellae. Furthermore, Legionella species are more resistant to chlorine than other bacteria because they can enter a viable but nonculturable state, as observed using PCR methods, can be protected by amoebae, and/or can survive in pipe biofilms.

In an experimental study of microbial changes during and after application of various disinfection treatments, amoebae resisted all the treatments and probably acted as reservoirs for Legionella pneumophila, allowing quick recolonization of the system once the treatments were interrupted. As multiplication in protozoans can differ depending on the Legionella species, it is conceivable that Legionella pneumophila sero-group 1 is more able than other serogroups to survive within protozoans or biofilms under stressful environmental conditions, such as high temperature and high chlorine levels”.

The study went on to say:

” Legionella species and serogroups may have different ecological niches and/or that their ability to survive in man-made water environments varies with the environmental conditions. In particular, Legionella pneumophila serogroup 1 shows a special ability to colonize “cleaner” water systems and to survive under more stressful conditions, such as higher temperature and chlorine levels, which are not consistent with the survival of other serogroups.”

Clearly then chlorine may not be able to remove all Legionella bacteria and may even help Legionella survive under some circumstances.

In a cooling water system this issue can be mitigated using a biodispersant to help remove biofilms from surfaces allowing chlorine to act as intended.

The same is not true for hot and cold water systems where the use of a biocide would not be allowed and so different strategies are required for managing these systems.

For a discussion on how Legionella can be controlled in your system contact Collaton Consultancy Limited via email on

The above scientific report cited is:

Borella et al., ‘Legionella Contamination in Hot Water of Italian Hotels’. 2005

Collaton Consultancy Limited are a specialist consultancy offering technical advice on Legionella, Pseudomonas and water treatment. We also act as Authorising Engineer (Water) in healthcare sites and as Expert Witnesses. We also offer both online and face to face training in a host of subjects.

More information can be found on the following websites:

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.