(2) A light source directly above the membrane filter. (A small fluorescent
lamp is ideal for this purpose).
b. The colonies are counted by using one of the two following procedures.
(1) Method 1. Remove membrane filter from the petri dish and dry it for
about 1 hour on absorbent filter paper. Using the magnifying apparatus and light source
(a above), count the colonies that have the characteristic metallic sheen (generally gold-
green). The dried membrane filter may be held flat by placing it between two 2- by
2-inch glass slides. This counting procedure is preferred to the one described in
(2) Method 2. Place the dish containing the absorbent pad and filter
membrane, minus the top of the petri dish, under the magnifying apparatus and light
source. Count the colonies that show the characteristic metallic sheen.
c Enter the colony count for each membrane filter in the space provided on DD
Form 686 (see figure 3-11). Next, determine and enter the coliform count per 100
milliliters of the water sample using the following computation:
No. of coliform colonies counted X 100.
No. colonies per 100 ml =
ml of sample filtered
3-11. OTHER BACTERIOLOGICAL TESTS
The bacteriological tests that have been discussed thus far are for determining
the total coliform count in water. This is the only determination that is done on a routine
basis. In special cases, however, it is sometimes desirable to conduct additional
investigations to determine a particular source of contamination. Two tests that may
also be conducted by the membrane filter technique, using modified procedures, are the
a. Fecal Coliform. The total coliform determination does not differentiate
between fecal coliform organisms (those found in the colon of warm-blooded animals)
and nonfecal forms (those found primarily in the soil and on plants). Although the
presence of any coliforms indicates contamination, a determination of whether the
contamination is fecal or nonfecal may be of value in detecting and correcting the
cause. The test for fecal coliform is essentially the same as for total coliform, with the