Microbiota in hot water boilers: Surprising discoveries when comparing cold and heated drinking water
The Swiss-Austrian research team with the participation of the Division of Water Quality and Health at Karl Landsteiner University discovers specific microbial communities for each hot water boiler that are independent of the cold drinking water supplied. The work sheds light on an aspect of drinking water microbiology that has received little attention to date.
Hot water taken from the tap in private and public buildings contained more and different bacteria, but fewer different species overall, than the same unheated drinking water. Water samples were taken at the building entrance and after distribution through the building's hot water system. The researchers counted the total number of dead and live cells in the samples using a mobile flow cytometer, identified the species of bacteria present through genetic analysis, and carried out bacterial growth tests. While the bacterial numbers in the cold water samples remained constant, they doubled in the growth test in the hot water samples after just 5-7 hours.
Drinking water hygiene regulations and hot water distribution systems
In the European Union, the distribution of cold drinking water from the building inlet to the consumer is regulated. The heating of drinking water in building internal systems has been little studied, except for the colonisation and spread of pathogenic Legionella. Few studies show differences between cold and hot water, which is the starting point for the present study. Water samples were taken from 19 sites in Zurich and central Switzerland, Lower Austria and Vienna. They came from buildings with boilers of different ages, sizes and surface/volume ratios. One of these locations uses instantaneous water heaters in the building and therefore provided the control samples for the study.
Specific, thermophilic microbiota detected
Four sites received their drinking water from the same supply pipe. Analyses of these four cold water samples, taken directly at the building inlet, showed a comparable and easily reproducible pattern of bacterial species present. However, the characteristics of the hot water samples differed significantly, supporting the hypothesis that the physical-chemical conditions in boilers lead to the development of thermophilic (heat-loving) microbiota. Neither the age, size or surface area/volume ratio of the boilers had any effect on the number or type of bacteria initially present, nor on the growth of bacteria in the growth experiments. The scientists suggest that heating releases organic matter that serves as food for thermophilic bacteria such as Hydrogenophilaceae, Nitrosomonadaceae and Thermaceae, while the strains in the cold water samples are unable to grow at temperatures of 50-60°C.
Causes of microbiota changes still unclear
Growth experiments with the building's cold drinking water, heated to over 55°C after being drawn from the tap, did not give similar results to the same experiments with the same water heated within the building's system. The influence of specific biofilms is possible, but cannot fully explain the observed phenomenon. If boiler-specific biofilms were the cause, there should be an effect of boiler age or the surface/volume ratio.
No known negative effects on consumers
In the future, the results of the study will help to better weigh up energy-saving approaches and measures to prevent pathogenic Legionella in hot water distribution systems. Despite the detection of specific bacterial species in heated drinking water samples, there have been no reports of demonstrable negative effects on consumers, as this issue has not yet been investigated in detail so far. A possible interaction of harmless thermophilic bacterial strains with potential pathogens in hot water requires further investigation. This work is an important reference for the selection and combination of different genetic and molecular biological methods.
The Division of Water Quality and Health at KL is concerned with microbiological and hygienic water quality relevant to health. The focus is on the development and application of advanced methods for the analysis of water quality in natural and technical systems. Together with the Medical University of Vienna and the Technical University of Vienna, the Division forms the Inter-University Cooperation Centre for Water and Health (ICC).
Original paper
The study has been published open access in the Q1-ranked journal "Water Research" in the category "Water Resources".
Egli, T, Campostrini, L, Leifels, M, Füchslin, HP, Kolm, C, Dan, C, Zimmermann, S, Hauss, V, Guiller, A, Grasso, L, Shajkofci, A, Farnleitner, AH & Kirschner, A 2024, 'Domestic hot-water boilers harbour active thermophilic bacterial communities distinctly different from those in the cold-water supply', Water Research, pp. 121109. https://doi.org/10.1016/j.watres.2024.121109
