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Understanding the Complex Relationship Between Legionella Bacteria and Amoebas in Drinking Water Systems

Understanding the Complex Relationship Between Legionella Bacteria and Amoebas in Drinking Water Systems

The Relationship Between Legionella and Amoeba

In the intricate ecosystem of drinking water systems, a noteworthy relationship exists between Legionella bacteria and free-living amoebas (FLA) that can impact public health significantly. This blog post delves into how these two organisms interact, the potential risks to human health, and what measures can be implemented to mitigate these risks.

The Hidden Dangers in Water: Legionella and Free-living Amoebas

Legionella bacteria are often highlighted in public health discussions due to their role in causing Legionnaires' disease, a severe form of pneumonia. These bacteria naturally occur in freshwater environments but become a health concern when they grow and spread in man-made water systems like cooling towers, plumbing systems, and even decorative fountains.

Amoebas, on the other hand, are protozoans that also inhabit water systems. They are generally harmless to humans directly; however, their interaction with Legionella bacteria can create a perilous situation. Amoebas can serve as a reservoir host to Legionella, providing a protective environment where the bacteria can thrive and multiply unchecked.

The Symbolic Relationship

The relationship between Legionella and FLA (e.g. Acanthamoeba spp., Hartmannella spp., and Naegleria spp.) is symbiotic and complex²,³. Amoebas ingest Legionella bacteria through phagocytosis, a process meant to typically destroy the bacteria. However, Legionella has evolved an adaptation to resist this destruction and, instead, uses the amoeba as a host to grow and replicate²,³. This makes Legionella bacteria an intracellular parasite to various types of protozoa such as amoebas³. Inside the amoeba, Legionella is shielded from many water treatment methods that would typically kill free-floating bacteria. This is especially the case among encysted amoeba². 

This relationship is critical because it allows Legionella to increase in number and become more virulent¹. When water containing these amoeba-protected Legionella is aerosolized — as in showers, fountains, or cooling towers — it can be inhaled, leading to infections in humans.

One of the first responders in our immune systems to invaders such as Legionella are cells called macrophages. These cells are actually very similar to environmental amoebas in that they engulf invaders and destroy them. Legionella bacteria can apply the skill sets in dealing with amoeba and in turn also resist destruction by alveolar macrophages²,6. While inside macrophages, Legionella are able to hide and remain undetected from other types of immune cells that would eliminate them by other means. 


Figure 1. Electron micrographs of U937 macrophages (A) and A. polyphaga (B) infected by L. pneumophila (strain AA100) at 24 h. The infection is very similar in both host cells. Legionella is cytoplasmic at 24 h in the two panels. Image taken from Molmeret et al., AEM, 2005.


Public Health Implications

The presence of Legionella in water systems is not just a matter of bacterial contamination but involves its interaction with amoebas, making it a more complex issue for water safety³. When systems are inadequately maintained, the risk of Legionella outbreaks increases, often with severe public health consequences4.

COVID-19 lockdowns in 2020-2021 resulted in many buildings being vacant or underutilized for extended periods of time creating long-term stagnation issues. Waterborne pathogens such as Legionella and others such as FLA capitalized on this opportunity to establish a foothold in the form of biofilm within the plumbing system4.

Strategies for Prevention and Control

Understanding the relationship between amoebas and Legionella has significant implications for managing water systems. Here are some strategies that can help control their presence and reduce the risk of Legionnaires' disease5:

Water System Maintenance1. Regular Water System Maintenance: Ensuring that water systems are regularly checked and maintained can help prevent the conditions that allow amoebas and Legionella to thrive. This includes adjusting chlorine levels, which can kill both amoebas and Legionella when properly managed.

Temperature Control

2. Temperature Control: Legionella and amoebas both prefer warmer water temperatures², growing between 20°C (68°F) and 60°C (140°F). Keeping water systems outside of these parameters can help reduce the risk of growth.

Biofilm Control3. Biofilm Control: Both organism types thrive in biofilms, which are layers of bacteria and other microorganisms that form on wet surfaces. Regular cleaning, flushing, and application of disinfectants can help remove biofilms and reduce the populations of amoebas and Legionella.

System Upgrades4. System Design and Upgrades: Designing water systems that prevent water stagnation and ensure adequate flow can help limit the growth of amoebas and Legionella. Retrofitting old systems with designs that consider these factors is also crucial.

 

Understanding the Legionella-Amoeba Interaction

The interaction between Legionella bacteria and free-living amoebas in water systems is a prime example of how complex microbial interactions can impact public health. By understanding these dynamic symbiotic relationships, we can better design, maintain, and manage our water systems to safeguard against health risks. Public awareness and rigorous adherence to water safety practices remain our best defense against the potentially deadly impact of these microscopic inhabitants.

This relationship underscores the importance of a proactive approach in water system management, ensuring safe water not just in terms of chemical composition but also microbiological integrity.


References

1. Amaro, F., Wang, W., Gilbert, J. et al. Diverse protist grazers select for virulence-related traits in Legionella. ISME J 9, 1607–1618 (2015). https://doi.org/10.1038/ismej.2014.248

2. Boamah DK, Zhou G, Ensminger AW, O'Connor TJ. From Many Hosts, One Accidental Pathogen: The Diverse Protozoan Hosts of Legionella. Front Cell Infect Microbiol. 2017 Nov 30;7:477. doi: 10.3389/fcimb.2017.00477. PMID: 29250488; PMCID: PMC5714891.

3. Taylor M, Ross K, Bentham R. Legionella, protozoa, and biofilms: interactions within complex microbial systems. Microb Ecol. 2009 Oct;58(3):538-47. doi: 10.1007/s00248-009-9514-z. Epub 2009 Apr 14. PMID: 19365668.

4. Xin Li, Juan Xu, Jianfeng Wu, Mark H. Weir, and Chuanwu Xi. Prevalence of Legionella in a Public Building Water Plumbing System During COVID-19 Lockdown. Environment & Health. 2023 1 (5), 352-359 DOI: 10.1021/envhealth.3c00058

5. Centers for Disease Control and Prevention. Reopening Buildings Guidance. https://www.cdc.gov/control-legionella/php/guidance/building-water-system.html. Accessed August 29, 2024.

6. Molmeret M, Horn M, Wagner M, Santic M, Abu Kwaik Y. Amoebae as training grounds for intracellular bacterial pathogens. Appl Environ Microbiol. 2005 Jan;71(1):20-8. doi: 10.1128/AEM.71.1.20-28.2005. PMID: 15640165; PMCID: PMC544274.


 

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