210‐min solid phase cytometry test for the enumeration of Escherichia coli in drinking water

A 210‐min‐test for the enumeration of Escherichia coli in drinking water is described, based on solid phase cytometry (SPC) and a two‐step enzymatic procedure for fluorescence labelling of single cells and small microcolonies. The test involves membrane filtration through a 25‐mm black polyester filter, induction of β‐glucuronidase in the retained target cells, fluorescence labelling with fluorescein‐di‐β‐ d‐glucuronide as an enzyme substrate and laser scanning of the membrane filter. Scan results can be confirmed on‐line by epifluorescence microscopy. Application to 149 naturally contaminated and uncontaminated well, tap, out‐of‐pump centre (distribution), surface and sewage‐spiked water samples indicated ≥ 90% agreement and equivalence with plate count methods, including Chromocult Coliform agar and m FC agar. In 5·4% of all samples examined, SPC detected between 1 and 11 E. coli per 100 ml, while the two plate methods yielded negative results. Cases of a negative SPC result but a positive E. coli count on both reference media were not observed. This test would primarily be useful for ‘emergency’ monitoring of drinking water when rapid results are crucial.