Figure 3-9. Place membrane filter on absorbent pad saturated with culture medium.
i. Place all membrane filters in the petri dishes and all petri dishes in the
incubator, grid-side down. Incubate at 35C + or--0.5 for 22 to 24 hours. A high level
of humidity (approximately 90 percent relative humidity) must be maintained in the
incubator. This can be accomplished by placing a wet sponge in the incubator.
Remove the cultures from the incubator after incubation.
3-10. COUNTING COLONIES
Coliform colonies are dark, usually with a gold-green metallic sheen (see
Figure 3-10. Typical coliform colonies.
a. The following equipment is most desirable in counting the coliform colonies:
(1) A wide field binocular dissecting microscope or a simple lens with
magnification of 5 to 10 times.