Flow cytometry (FCM) can now be officially used for the quantification of microbial cells in drinking water. The new analytical method - developed at Eawag and extensively tested both in Switzerland and abroad - has been incorporated into the Swiss Food Compendium (SLMB) by the Federal Office of Public Health (FOPH). FCM provides much more realistic results than the conventional method, in which bacterial colonies are grown on agar plates. The results demonstrate that even good-quality drinking water harbours 100 to 10,000 times more living cells than the conventional plate count method would suggest.
For over 100 years, the method used to assess the microbiological safety of drinking water has remained essentially unchanged: bacteria present in water are allowed to grow on solid nutrient media (incubated at a warm temperature), and the colonies formed are then counted. The intestinal bacteria Escherichia coli and enterococci serve as indicators of faecal contamination. At the same time, the heterotrophic plate count (HPC) is determined as a measure of general microbiological quality. This method quantifies all the microorganisms present which can reproduce at temperatures of around 20-45°C (mesophilic). According to the global standard, the number of colonies formed should not exceed 300 per millilitre.
Cell counts significantly underestimated
The cultivation-based method has two major drawbacks: it is time-consuming - results are only available after 3-10 days in the case of the HPC - and only a fraction of the living cells actually present in samples are counted. This is because the method only detects those bacteria which can grow and form colonies under the specified conditions - generally 0.01-1% of the total. Thus, the limit of 300 colony-forming units per millilitre (CFU/mL) also specified in the Swiss Ordinance on Food Hygiene (HyV) is based on a significant underestimate of the actual number of microorganisms present. The cultivation of E. coli and enterococci does, however, normally yield reliable results. (Total cell counts for different types of water are shown in Fig. 1.)