A 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 beta-glucuronidase in the retained target cells, fluorescence labelling with fluorescein-di-beta-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 greater than or equal to 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 mi, 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.