The recent 9th International Legionella Conference in Rome had some new and interesting information regarding temperature and chemical disinfection of Legionella in building water systems. Some of the information we accept as ‘gospel’ is probably wide of the mark. Disinfection by heat or chemicals doesn’t appear to be what we all thought…


International Legionella Conferences are rare events. Once every four years these days – but originally once every 10 years. The last was in Melbourne in 2013. So once they happen the geeks all get together and it is on for young and old. This conference in Rome, just a quick stroll from the Colosseum, was no exception. Four and a half days of non-stop Legionella frenzy! With a few breaks for refreshment. Of course disinfection is high on the list of topics relating to Legionella and there are plenty of opinions. This year VBNC was one of the hot topics in the disinfection area. So what is VBNC?

Details of the conference content are here: 

Viable Non-Culturable (VBNC)

Before we can get into the ups and downs of disinfection it is important to understand the ‘viable non-culturable’ concept. So here goes! Aside from becoming ‘dormant’ some bacteria including Legionella and Mycobacteria enter another state. In response to stress they shut down things they don’t need and focus on surviving. This may mean that things like reproduction go on hold until the outlook improves. One consequence of entering this state is they will not grow on conventional culture plates. So culture test results will come back negative. This is a very quick description of the VBNC state. Of course it’s a little bit more complex than this. However, the thing to note is that the bacteria are not dormant. They are waiting to pounce! VBNC cells are still just as capable of causing infection.

VBNC and infection

The Rome conference contributors made it very clear that VBNC is not something to forget about. Bacteria in this state are just as infectious as their less stressed counterparts. They will be ‘under the radar’ as far as test results go – but still just as dangerous. A number of different presentations and posters were focusing on this issue and it’s potential impacts. It seems that stressing Legionella without killing them can hamper verification of a Water Safety and Risk Management Plan. Below is a quick run down.



This one is a bit alarming. One presentation, which is due for publication, showed Legionella surviving up to 70°C for 10 minutes! After this it entered the VBNC state. A major worry is that routinely we use heat to ‘pre-treat’ Legionella test samples in laboratories. What does that mean for our test results? Obviously keeping water above 50°C is important to stop Legionella multiplying. Above 50°C Legionella will not multiply which makes it a good control strategy as part of a multi-barrier system. However, it does not kill them quite as effectively as we used to think. This has  obvious implications for pasteurisation as a routine decontamination / disinfection strategy.


There is quite a long history of reports of monochloramine inducing the VBNC state. The conference just added more weight to this evidence base.


Another poster testing different stresses on Legionella in biofilms showed chlorine, salt concentration and monochloramine all produce the VBNC state


Now this is a bit of a shocker. Conventionally we believe that reducing nutrients is a good thing. However, starvation induces the VBNC state in Legionella. So a low nutrient environment around biofilms may ‘hide’ positive Legionella test results. On the other hand low nutrients usually means better disinfection.

In short, it seems that any disinfection strategy, by its’ nature, is likely to stress Legionella. Stress starts up the VBNC reaction and the culture results won’t show what is actually there. More importantly culture results will not show how effective the disinfection has been.



So what now – do we panic? Well, panic is rarely a good idea and usually expensive. Understanding that disinfection is only one part of a multi- barrier system is a good start. Recognising that it is relatively low down on the hierarchy of control is also important. None of the above reports suggest that the disinfection methods do not have an effect. They just show that disinfection is not sterilisation. They also show that disinfection can help Legionella hide from sample test results. It is also important to recognise that sampling is only verification of an effective control strategy and in itself gives you no control at all.

Good system design, good maintenance and good plumbing controls come before disinfection in the hierarchy. If these are in place then disinfection becomes a ‘mop-up’ measure at the end of the process. Sampling gives some reassurance that what you have put in place is working.  Reducing human exposure at the outlet is then the final barrier Legionella must overcome. Cleaning outlets, reducing hose lengths and removing or disinfecting aerators, cleaning shower roses and maintaining sensor taps all make the job of the disinfection process easier.


Sampling has a place in risk management but where it fits needs to be understood before the sample is taken.

Legionella culture results are unreliable.

Disinfection often makes Legionella test results even less reliable.

Good design, management and maintenance are the keys to successful Legionella control.



The 9th International Legionella Conference, Rome 2017

Platform presentations: Friday 9.15-9.30 am, Survival patterns and virulence of Viable but Non-Culturable Legionella induced by starvation. Kirshner, A.

9.45-10.00 am, Temperature induced Viable but Non-Culturable Legionella pneumophila cells are virulent against amoebae and macrophage-like cells. Cervero-arago, S.

18.00-18.15 pm, Maxi impact of temperature, Copper and Silver exposure on the viability and recovery of clinical and environmental strains of Legionella pneumophila. Prevost, M.

Poster Session 2. Poster 37, Adaptation of Legiolert (TM) for amoebal co-culture of VBNC cells. Dey, R, Ashbolt, N.

Poster Session 3. Poster 17,Investigation of the effects of various stress factors on Legionella pneumophila in biofilm layer. Vatansever, C., Turetgen, I.

Poster Session 3. Poster 29, Preliminary data on detection and quantification of viable and VBNC Legionella spp. by culture and PMA-qPCR in water distribution systems treated with monochloramine Ditommasoa, S. et al.