When it comes to household utilities, except for Legionella, domestic hot-water boilers and installations have been overlooked as potential hotspots for microbial life. Yet, these everyday appliances harbour distinct thermophilic bacterial communities that thrive in the hot water, opening up a new field of study in drinking water microbiology. In this insightful interview, Professor emeritus Dr Thomas Egli, a microbiology expert and scientific consultant to bNovate, shares the surprising discovery that led his team to investigate these microbial communities and the implications of their findings.
Below, Prof. Egli sheds light on the significance of this research and its impact on water quality and safety.
1. What initially motivated your team to investigate the microbial communities present in domestic hot water boilers? Was there a specific observation or problem that sparked your interest?
It all started quite by accident. In 2018, Stefan Zimmermann, Technical Sales Advisor at bNovate, brought me a BactoSense instrument to test. As a curious microbiologist/scientist, I enjoy exploring new versions of such devices. It’s also a good sign if I can handle it. While measuring various types of water at home, I noticed something unexpected: the total cell count in hot water from our kitchen tap and shower hose was ten times greater than in the supplied cold drinking water. Also, the fingerprints of cold and hot water looked completely different. Initially, I thought it was a mistake, but repeated tests confirmed the observation. I asked Stefan to check cold and hot water at his home, and his findings were identical. A check of relevant literature reveals that – world-wide – there are no regulations concerning the microbiological quality of hot or warm water. According to Swiss regulations ‘warm water’ is simply ‘drinking water of which the temperature was raised by heat input’. Therefore, we contacted several long-standing microbiologist colleagues from the Vienna and Zurich regions, who all owned a BactoSense instrument, so that we could directly compare the results to explore this in more detail.
The Vienna University of Technology also did similar tests and found similar data. We discovered no existing regulations or literature addressing microbiology in hot water, only in cold drinking water, except for Legionella concerns. This gap in knowledge and regulation drove us to investigate further. After all, you have to observe your environment; if you see something unexpected, you must follow it.
2. Your study found distinct thermophilic bacterial communities thriving in hot water boilers. Can you explain the significance of this finding and its potential implications for water quality, energy efficiency, or other areas?
Hygienically speaking, aside from Legionella, there are no known microbiological or viral diseases explicitly spread by hot water, which might be why this issue hasn’t been a major concern. However, it’s disconcerting that hot water has been overlooked, especially given the wide temperature range over which boilers and other warm water installations are operated in different countries. For instance, most countries recommend running boilers at 50-60°C, with a short raise to 60°C once a week to prevent Legionella growth. However, often they are kept at 40-50°C to save energy. Hence, the implications of these different temperatures on microbial growth in hot-water systems deserve thorough investigation. This is a new field of study that could reveal how hot-water microbial communities interact with pathogens like Legionella, potentially either competing with or supporting them. The findings could lead to a better understanding of microbiology in hot water systems and improved regulations.
3. The study suggests that hot water boilers act as semi-continuous bioreactors. Can you elaborate on this concept and explain how the conditions within boilers facilitate the growth of these thermophilic communities?
Heating water generates nutrients, as demonstrated in our paper. According to our calculations, hydrolysis of 1-2% of the dissolved organic carbon (DOC) present in cold water appears to be the primary source of nutrients; less likely are the decay of cells from the supplied cold drinking water or the release of nutrients from material’s surfaces or biofilms. In growth experiments we demonstrated that bacterial flora from hot-water boilers was able to grow at 50-60°C, whereas cold-water flora was unable to proliferate at such temperatures. Essentially, such a boiler acts like an individual thermophilic bioreactor. E.g., in three neighbouring buildings supplied with drinking water from the same mains each boiler had its own bacterial flora.
4. Your team employed advanced techniques like flow cytometry and 16S rRNA gene amplicon sequencing. Can you briefly explain how these methods contributed to your analysis and what insights they provided?
Flow cytometry was pivotal in observing the phenomenon of increased abundance and possible changes in community composition in hot and cold water. This technique allows for rapid and direct access to the characteristics of various waters. The flow cytometry fingerprints indicated immediately that the bacterial flora in hot water differed vastly from that in cold water (but not how with respect to composition). Gene analysis definitely confirmed that hot-water boilers contained a different bacterial flora consisting of about 35 different thermophilic strains with three to four of them dominating. In contrast, cold-water floras exhibited a ca. 10-times higher diversity, with almost 300 strains unique to this habitat. Interestingly, the regular occurrence of one of our dominating thermophiles was also found in a recent study on household hot-water installations in the US, suggesting a worldwide phenomenon. Ecologically, ecosystems with a broad spectrum of organisms are considered more stable against intruders than those with a narrow spectrum, making the study of hot-water systems intriguing regarding microbial stability and vulnerability to pathogens.
5. The BactoSense flow cytometer played a crucial role in your study. How did this tool specifically aid in rapidly analysing and comparing the bacterial communities in hot- and cold-water samples?
The BactoSense flow cytometer was a major advantage in our study because all collaborating groups (our Austrian colleagues, bNovate and myself) used the same standardised instrument. This ‘inter-instrument’ reproducibility allowed us to accurately compare results from different sources, instruments and individuals. This reliability made our findings robust and comparable, enhancing the credibility of our analysis.
6. While thermophilic bacteria may not directly pose health risks, you mentioned potential interactions with opportunistic pathogens like Legionella. Can you expand on this and discuss any concerns or areas that require further investigation?
Expanding on this is challenging, but during the last 40 years, understanding of Legionella growth in hot- and cold-water systems has seen minimal advances. Our findings, particularly the fact that nutrients are generated during heating, might offer a new perspective on growth and interactions between cold- and hot-water organisms and Legionella. Although this is speculative, it’s something that warrants further investigation.
7. Based on your findings, what recommendations or best practices would you suggest for homeowners or building managers to maintain and monitor their domestic hot water systems?
Flow cytometry, BactoSense!
Using flow cytometry, particularly the portable, easy-to-handle BactoSense instrument, can provide quick ‘on-site’ insights into microbial activity in domestic water installations. Flow cytometry offers a fast, less expensive method to monitor abundance and changes in microbial communities without extensive genetic testing. Additionally, monitoring the ratio of hot- to cold-water organisms might/may give an indication of water treatment effectiveness and drinking water biostability, and a guide for improving treatment.
With respect to temperature, I think that the recommendation to maintain boiler temperatures between 50 and 60°C makes much sense to minimise the risk of opportunistic pathogens.
8. What are the next steps or future research directions you envision in this area? Are there any specific factors or variables you think should be explored further?
Our findings have opened up a largely unexplored area of hot-water microbiology in buildings. Future research should focus on the stability of carbon in treated water to prevent regrowth during stagnation or in hot-water systems. Understanding how to treat water to achieve a (bio)stable is crucial. This will involve collaborations between microbiologists and chemists to delve deeper into the factors affecting water stability and microbial growth.
9. Beyond domestic hot water boilers, are there other household or building systems where similar thermophilic microbial communities might be present and warrant investigation?
Particularly in developing countries or the Global South, rooftop storage tanks often reach temperatures of 35-40°C, which can be ideal for pathogen contamination. This issue needs urgent attention. Additionally, for energy-conservation reasons, many hot-water systems are operated at 35-40°C, which might be critical regarding microbial growth. Also, the fate of disinfectant residuals during heating and their role in oxidative nutrient generation are not well understood. These areas certainly warrant further investigation to ensure water safety and quality.
10. Is there anything else you would like to add or emphasise regarding the significance or implications of your study's findings?
It’s a pity that I am retired and no longer have access to a lab, as there is so much more to explore in this field. However, I hope young researchers will take up this mantle and continue investigating the fascinating world of microbial life in domestic hot-water installations we are exposed to every day.
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