a relatively easy-to-isolate indicator organism whose presence suggests probable
contamination with pathogens, but whose absence offers reasonable assurance that no
pathogens are present.
c. Characteristics of Indicator Organisms. In order for an indicator organism
to give a true indication as to the presence of pathogens in a water supply, it must meet
several criteria. The criteria are that the indicator:
(1) Must be present when the water is contaminated--preferably in greater
numbers than the pathogens.
(2) Must respond to environment and water treatment processes in the
same manner as pathogens, having a similar life span.
Must be easily identified and counted by simple procedures.
Should be nonpathogenic itself (safe to the laboratory technician).
d. Coliform Bacteria as Indicators. A group of nonpathogenic bacteria known
as the coliform group has been selected as the indicator of fecal contamination of water
and, thus, the presence of pathogens causing waterborne diseases. The coliform group
is not a particular species of bacteria, but a group of several genera of bacteria that are
normal inhabitants of the intestinal tracts of human, and other warm--blooded animals.
(1) There are two definitions of the coliform group, depending upon which
test is being employed to detect them. They comprise all of the aerobic and facultative
anaerobic, gram--negative, non-spore--forming, rod--shaped bacteria which:
(a) Ferment lactose broth with gas formation within 48 hours at 35C
(multiple tube method); or
(b) Produce a dark colony with a gold-green metallic sheen on an
Endo-type medium with lactose within 24 hours at 35C (membrane filter technique.)
(See paragraph 3-7b for media.)
(2) The most important members of the coliform group are Escherichia coli
(E. coli), a normal inhabitant of the intestinal tracts of warm-blooded animals, and
Aerobacter aeroqenes, which is widely distributed in nature and is normally found on
plants and grains, in the soil, and to a varying degree in the feces of humans and
animals. The presence of either of these organisms in water is prima facie evidence of
contamination of the water with feces or soil. (Prima facie evidence of contamination
means that enough evidence is present to presume contamination.)
(3) The coliform group of bacteria was selected as the indicator for
pathogenic organisms in water for the following reasons: