Current routine analysis of the microbiological quality of drinking water is very often based on plate count methods, where the number of bacterial colonies on a nutrient medium are counted. This method has been applied worldwide for more than 100 years. Despite its ubiquitous use, this method is rather time consuming. It takes approximately 3 days to detect microbiological contamination of drinking water. The detection of additional pathogens, such as Legionella, can easily take several days to weeks. A further disadvantage of culture methods lies in the fact, that only a minor fraction of microorganisms found in environmental samples (0.1-1%) grow in culture media and can therefore be detected.
A much rapid and more complete method for the analysis of the microbiological quality of water resources and drinking water is provided by Flow Cytometry. The potential and the limits of this technique have been investigated in detail in the last few years by the research group of Prof. Thomas Egli in collaboration with Dr. Frederik Hammes (Drinking Water Microbiology and Ecophysiology). Originally used in medical routine analysis, Flow Cytometry is now finding its way into the quality control of drinking water as a promising alternative to existing methods.
Principles of Flow Cytometry and potential applications for the analysis of drinking water
The measurement principle of Flow Cytometry is quite simple: Prior to the actual measurement, the bacteria in the water sample are stained with a fluorescent dye. The water sample is then forced through a thin glass capillary and the number of cells are detected by a laser beam. With this technique, the determination of the total number of suspended bacteria is complete within 15 min. Besides the total cell number, the size of the cells can be determined as well. With this infomation, a microbiological „fingerprint“ of each water sample can be generated, allowing rapid detection of changes or failures during drinking water treatment and distribution. With the use of specific dyes or antibodies, living intact cells can be distinguished from dead cells. Furthermore, specific pathogenic organisms can be detected as well.
Rethinking microbiological quality standards
Typical values for total cell numbers determined by Flow Cytometry are a few thousand cells per ml in a good groundwater up to a few million cells per ml in surface waters. Hygienically clean tap water and mineral water contain in general approximately 10‘000 to 200‘000 cells per ml (see figure). With regard to the current regulations of drinking water in Switzerland, which demand less than 300 colony forming units of aerobic, mesophilic microbes (those growing on culture media), the total numbers of cells as measured by Flow Cytometry call for a re-evaluation of the microbiological assessment of drinking water. There is also need for a change in the public perception of the presence of naturally occurring microbes in drinking water. Naturally occurring microbes in drinking water play an active role in maintaining „clean“ water by consuming even low concentrations of available nutrients and thus preventing the growth of known pathogenic organisms leading to water borne diseases such as Cholera, Typhus and Ruhr.