3. If typical colon-like red colonies have developed upon the plate
within this period, the confirmed test may be considered positive.
4. If, however, no typical colonies have developed within 24 hours, the
test cannot yet be considered definitely negative, since it not
infrequently happens that members of the B. coli group fail to form
typical colonies on Endo’s medium or lactose-litmus-agar plates, or that
the colonies develop slowly. In such case, it is always necessary to
complete the test as directed under “C” 2 and 3.