Some like it hotter

Legionellae grow at significantly higher temperatures than previously thought



HZI/ AG Höfle/Elisa AndreozziEpifluorescence micrograph of a biofilm by Legionella pneumophila (green, immunofluorescence staining) with other water bacteria (blue).

Legionella pneumophila, a bacterial pathogen known to dwell in warm water systems proliferates at temperatures between 50 and 60°C. This was shown by scientists of the Helmholtz Centre for Infection Research (HZI) in Braunschweig, Germany, in a study just published in "The ISME Journal". This finding does not indicate an additional risk to humans as far as we know at this time. However, in the opinion of the researchers, the consequences for the management of hot water systems, air conditioning facilities and cooling towers needs to be investigated in further studies.

According to estimates, legionellae cause approximately 100,000 cases of severe pneumonia in Europe each year. Legionellosis, including its most severe form that is also called Legionnaire's disease, often manifests in the form of outbreaks affecting many people at the same time. Unless Infection is detected at an early time, it can quickly be fatal. Although Legionella pneumophila, the most important pathogen amongst the legionellae, has been known and studied intensively since 1976, there is yet no efficient way of preventing outbreaks of legionellosis. The germs proliferate mainly in warm and hot water systems and reach the human lung in water droplets. Showers, cooling towers and air-conditioning facilities are significant sources of infection. Accordingly, German law requires that all relevant hot water systems are regularly tested for the presence of legionellae.

"In order to control the hazard posed by legionellae in the long term, a sound and detailed understanding of the Ecology of these bacteria in our warm water systems is required," says Prof Manfred Höfle, who is the director of the "Microbial Diagnostics" department at the HZI. Höfle coordinates a DFG-funded (German research funding organisation) tri-national project investigating the Ecology of legionellae, in which German, Palestinian and Israeli partners are involved. In addition, his research group is integrated into the EU project "Aquavalens" working on improving the diagnostics of pathogens in water.

Höfle and his colleagues, Dr Ingrid Brettar and René Lesnik, looked at the legionellae population in drinking water along its way from surface reservoirs through water storage tanks and pipes all the way to the water faucet. They use molecular biological methods to find out: There are clearly more legionellae in hot tap water than in cold water. "It was evident that the number of legionellae increases at 50 to 60°C and, specifically, that we must presume Legionella pneumophila to grow well in this temperature range," says Ingrid Brettar. "This result was surprising," the lead author of the publication, René Lesnik, commented. "All previous studies seemed to indicate growth of legionellae at up to 42°C, may be up to 45°C."

BCYE agar plate (the typical charcoal-containing medium for legionellae) showing typical Legionella colonies

Credits: René Lesnik

Even if some caution should always be exercised whenever legionellae are present in the water, these findings are no reason for increased concerns in the opinion of the scientists: "From what we know at this time, the health hazard posed by Legionella has not changed fundamentally," says Höfle. But the present insights might help improve the management of hot water systems and make them safer against the colonisation and growth of legionellae. Höfle and his colleagues hope to be able to address this question and other issues - for example, how the legionellae manage to proliferate at elevated temperatures - in further collaborative studies involving their research partners.


Original publication:

R.Lesnik, I. Brettar & M.G. Höfle 2015: Legionella species diversity and dynamics from surface reservoir to tap water: from cold adaptation to thermophily. The ISME (International Society for Microbial Ecology) Journal (2015), 1–17; doi: 10.1038/ismej.2015.199

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