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 Giardia and
Cryptosporidium, 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 cells present.
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, Giardia, or
Cryptosporidium.
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 common disinfection
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
ultraviolet disinfection.
