ISO 7899-3:2025

Water quality — Enumeration of intestinal enterococci — Part 3: Most probable number method

Scope

This document specifies a method for the enumeration of intestinal enterococci in water, including Enterococcus faecalis, Enterococcus faecium, Enterococcus durans, Enterococcus avium, Enterococcus gallinarum, Enterococcus hirae, Enterococcus casselifavus. The method is based on the growth of target organisms in a liquid medium and calculation of the “most probable number” (MPN) of microorganisms by reference to MPN tables or using suitable MPN informatic programs.
This method can be applied to drinking water and bathing water (fresh or marine), together with other similar water types including those containing an appreciable amount of suspended matter, and allows the detection of enterococci at 1 colony-forming unit (CFU) per 100 ml with definitive results within (26 ± 2) h in the presence of heterotrophic bacteria in numbers as high as 1 × 106 per 100 ml of sample.
For bathing waters, fresh and marine, enterococci are best enumerated when samples are diluted 1:10.
The test specified in this document relies upon the detection of intestinal enterococci based upon expression of the enzyme ß-D-glucosidase and provides a confirmed result in 24 h without further testing of positive wells.
This document does not apply to bottled waters, for which the method has not been validated and therefore is outside the scope of this document, unless appropriate validation of performance of this method has been undertaken by the laboratory prior to use.

National adoptions