Environmental Engineering Reference
In-Depth Information
produce a negative cytochrome oxidase (CO) and positive β-galactosidase
(ONPG) reaction. Generally, all red, pink, blue, white, or colorless colo-
nies lacking sheen are considered noncoliforms by this technique.
The membrane filter technique uses a specially designed filter pad with
uniformly sized pores (openings) small enough to prevent bacteria from
entering the filter. A measured volume of sample is drawn through the filter
pad by applying a partial vacuum. The special pad retains the bacteria on its
surface, while allowing the water to pass through.
Note:
In the MF method, the number of coliforms is estimated by the
number of colonies grown. In order to use the MF procedure for chlori-
nated effluents, you must be able to demonstrate that it gives information
comparable to that obtained by the MTF test.
The MF procedure uses an enriched lactose medium and incubation tem-
perature of 44.5 ± 0.2°C for selectivity and gives 93% accuracy in differenti-
ating between coliforms found in the feces of warm-blooded animals and
those from other environmental sources. Because incubation temperature is
critical, submerge waterproofed (e.g., plastic bag enclosures) MF cultures in a
water bath for incubation at the elevated temperature, or use an appropriate,
accurate solid heat sink incubator. Alternatively, use an equivalent incubator
that will hold the 44.5°C temperature within 0.2°C throughout the chamber
over a 24-hour period while located in an environment of ambient air tem-
peratures ranging from 5 to 35°C.
Materials and Culture Medium
1.
m-FC media
—The need for uniformity dictates the use of dehydrated
media. Never prepare media from basic ingredients when suitable
dehydrated media are available. Follow manufacturer's directions
for rehydration. Commercially prepared media in liquid form (ster-
ile ampule or other) may also be used if known to give equivalent
results.
2.
Culture dishes
—Use tight-fitting plastic dishes because the MF cul-
tures are submerged in a water bath during incubation. Enclose
groups of fecal coliform cultures in plastic bags or seal individual
dishes with waterproof (freezer) tape to prevent leakage during
submersion.
3.
Incubator
—The specificity of the fecal coliform test is related directly
to the incubation temperature. Static air incubation may be a prob-
lem in some types of incubators because of potential heat layering
within the chamber and the slow recovery of temperature each time
the incubator is opened during daily operations. To meet the need for
greater temperature control, use a water bath, a heat-sink incubator,
