The total and fecal coliform bacteria test is a primary indicator of
"potability" , suitability for consumption, of drinking water. It measures
the concentration of total coliform bacteria associated with the possible
presence of disease causing organisms.
Coliform bacteria are a natural part of the microbiology of the intestinal tract
of warm blooded mammals, including man. Coliform bacteria can also be
found in soil, other animals, insects, etc. The total coliform group is
relatively easy to culture in the lab, and therefore, has been selected as the
primary indicator bacteria for the presence of disease causing organisms.
Coliform bacteria are not pathogenic (disease causing) organisms, and are only
mildly infectious. For this reason these bacteria are relatively safe to work
with in the laboratory. If large numbers of coliforms are found in water, there
is a high probability that other pathogenic bacteria or organisms, such as
may be present. The PADEP requires public drinking water supplies to
demonstrate the absence of total coliform per 100 mls (about 4 oz) of drinking
water. At this time, there are no regulations governing individual water wells.
It is up to the private well owner to have his or her water tested.
Approved tests for total coliform bacteria include the membrane filter, multiple
tube fermentation, MPN and MMO-MUG ("Colilert") methods. The membrane filter
method uses a fine porosity filter which can retain bacteria. The filter is
placed in a petri (culture) dish on a pad with growth enrichment media (mEndo)
and is incubated for 24 hrs at 35 degrees C. Individual bacteria cells
which collect on the filter grow into dome-shaped colonies. The coliform
bacteria have a gold-green sheen, and are counted directly from the dish. Since
some other bacteria may develop a similar color, a confirmation test using more
specific media is required. The confirmation procedure requires an additional 24
to 48 hrs to complete the test for suspected positive total coliform tests.
The MPN (most probable number) method uses a test tube full of media with a
smaller inverted test tube inside which captures carbon dioxide gas released
from the growth of coliform bacteria. A series of dilutions and replicates are
set up, and those producing gas in 24 hrs at 35 degrees C are counted. A
statistical analysis is used to determine the most probable number of bacteria
Our laboratory uses a number of techniques including the membrane filter
method. The sample should be collected in a specially prepared, sterile whirl
pack bag for the test to be valid. The bags contain a small amount of sodium
thiosulfate to remove any chlorine present, and have been sterilized. Sample
collection should be done very carefully and directly into the bottle from the
tap to avoid contamination of the bottle from hands or a transfer vessel such as
a cup. The sample should be kept cool and delivered to the lab within 6 to 24
hrs for analysis.
Total coliform bacteria testing is a relatively inexpensive when compared to
the cost for the determination of the concentration or presence of viruses,
Bacteria are removed by disinfection and/or filtration. Filtration alone may
not be completely effective, but can improve the performance of disinfectants by
removing sediment that can shelter the bacteria. Methods of adding
chlorine to water include solution feeders for dry chlorine or liquid chlorine
or by feeding gas chlorine directly from 100, 150, or 2000 lb. cylinders. Gas
chlorination is recommended only for larger systems that can support the
services of a trained water treatment plant operator. Chlorine is normally dosed
to a concentration sufficient to maintain a free residual of at least 0.2 parts
per million (PPM).
Other disinfectants include iodine, ozone, ultraviolet light, and physical
methods such as boiling or steam sterilization. Chlorination is still the most
method in the United States, although recent concerns have been raised about the
reaction of chlorine with organic matter in water. Such a reaction can result in
the formation of
trihalomethanes, which are suspect carcinogenic compounds. For most
individual water supply systems, the most common form of treatment is