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Flow cytometry quantifies microbes in drinking water in minutes

Swiss researchers have discovered that flow cytometry can now quantify microbial cells in drinking water—and do so in minutes rather than days. Stemming from work at aquatic research institute Eawag (Dübendorf, Switzerland) and extensive tests both in Switzerland and abroad, the optical techinque has been incorporated into the Swiss Food Compendium (SLMB) by the Federal Office of Public Health (FOPH; Bern, Switzerland).

For over 100 years, the conventional method used to assess drinking water quality 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 E.coli and Enterococci serve as indicators of fecal 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° to 45°C (mesophilic). According to the global standard, the number of colonies formed should not exceed 300/mL.

But the cultivation-based method is both time-consuming (in the case of the HPC, results are available after 3–10 days) and incomplete, as it counts only a fraction of the living cells present in samples. This is because the method only detects those bacteria that can grow and form colonies under the specified conditions—generally 0.01–1% of the total. So the limit of 300 colony-forming units per milliliter (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. Cultivation of E. coli and Enterococci does, however, normally yield reliable results.
 
 

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