The strange life of Legionella in biofilms.

The optimum growth temperature of L. pneumophila ranges from 37 to 42oC and a warm temperature is essential for its growth because host amoebae digest the bacterium at a temperature < 20oC. Several studies showed that the colony counts of L. pneumophila in potable water installations correlated with the concentration of organic carbon, iron, manganese and corrosion products in the water.

Water quality also affects biofilm formation, but the relationship between the concentration of the water-induced biofilm and growth of Legionella is still unclear.

A stable or declining Legionella colony count at a biofilm concentration increasing with age shows that the quantitative relationship with the biofilm concentration is complicated.

A constant flow in engineered water systems can decrease the presence of L. pneumophila through the use of Venturi systems by preventing the attachment of the bacteria to surfaces.

Interestingly, although iron is essential for biofilm formation, high iron concentrations can inhibit biofilm formation.

Summer seasons, which coincide with legionellosis outbreaks, favour the proliferation of L. pneumophila in cooling tower microbial populations while other Legionella species decrease in number.

Although there is a body of evidence suggesting that L. pneumophila can coexist in biofilms with P. aeruginosa, these studies were performed with inoculums from natural environmental sources, which may contain several different bacterial species. In contrast to these studies, mono-species biofilms with P. aeruginosa were shown to prevent L. pneumophila colonization.

The presence of amoeba seems to also effect whether P. aeruginosa is antagonistic to L. pneumophila colonization, as biofilms which contain both Acanthamoeba castellanii and P. aeruginosa, increase the uptake of L. pneumophila within A. castellanii, and the colonization of L. pneumophila in biofilms.

It is evident that environmental L. pneumophila found in biofilms are extremely resilient to treatment with biocides L. pneumophila exposed to environmental stresses and/or found within biofilms can enter a viable but non-culturable (VBNC) state, and treatment of water systems with biocides can make L. pneumophila enter the VBNC state.

L. pneumophila bacteria grown on a solid surface are more resistant to killing by iodine than bacteria grown in broth, suggesting that there are metabolic differences between surface associated and planktonic phase bacteria. This is consistent with data suggesting that sessile and planktonic L. pneumophila in biofilms have different gene expression profiles.

An area of increasing interest is the role of other bacterial species in L. pneumophila biofilm production, and the mechanism with which certain species promote L. pneumophila growth while other species inhibit it.

Legionella and biofilms is a complex area and one that needs further study before we can fully eradicate Legionella. One of the key areas should be how do we eradicate biofilm in potable water systems as this is difficult to achieve without chemicals that may not be suitable for drinking water systems.

 

Collaton Consultancy Limited offer training and consultancy services on managing Legionella in potable and process waters, including cooling towers. Should you need our help then contact us on general@collatonconsultancy.com or phone on +44 (0)7958 124563.

 

Bibliography.

Kooij, Dick van der, Geo L. Bakker, Ronald Italiaander, Harm R. Veenendaal, and Bart A. Wullings. “Biofilm Composition and Threshold Concentration for Growth of Legionella Pneumophila on Surfaces Exposed to Flowing Warm Tap Water without Disinfectant.” Edited by Johanna Björkroth. Applied and Environmental Microbiology 83, no. 5 (March 1, 2017): e02737-16. https://doi.org/10.1128/AEM.02737-16.

Abdel-Nour, Mena, Carla Duncan, Donald Low, and Cyril Guyard. “Biofilms: The Stronghold of Legionella Pneumophila.” International Journal of Molecular Sciences 14, no. 11 (October 31, 2013): 21660–75. https://doi.org/10.3390/ijms141121660

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