Coliform Test

Coliform Tests

One of the oldest methods around for testing drinking water is done by testing for coliform presence in the water.  Coliforms are used as indicator organisms because they are commonly found in the intestinal tracts of all warm-blooded mammals.  Coliforms are defined as rod-shaped Gram-negative organisms that ferment lactose and produce gas at 35 degrees Celsius.  They generally are not the cause of disease but their mere presence is indicates that other pathogenic organisms might be present.  The pathogenic organisms that are common with fecal contamination are pathogenic bacteria, viruses, protozoa and parasites cause a variety of life threatening illnesses.

Water can become contaminated several different ways, which include run off from woodlands, pastures and feedlots, septic tanks and animal waste.  Domestic animals contribute the most to bacterial pollution because of how frequently they come in contact with the water.  High levels of fecal contamination also come from areas heavily populated with humans and animals. These are all contributing factors to fecal contamination and must be taken into consideration when deciding whether or not to test for contamination.  Since Coliforms are common in mammals and soil, agencies like the EPA use this as a standard test for drinking water contamination.

There are several ways to test for coliform bacteria and the most commonly used are the most probable number (MPN) method and the membrane filter (MF) method.  The MPN method is a presumptive test, which is generally performed first.  Fermentation tubes are filled with lauryl tryptose broth and inoculated with a sample of wastewater and then incubated at 35 degrees Celsius for 24 hours.  After a 48 hour period, a positive for coliform presence is given by observing the production of gas in the in inverted gas trapping tube then a second test, called the confirmed test is administered to test if the coliforms are fecal coliforms or not.  The MF method is more rapid but not as reliable as the MPN test.  The water is filtered through as sterile membrane and then transferred to a sterile Petri dish.  The dish is then incubated for 24 hours at 44.5 degrees Celsius.  Colonies produced by fecal coliform bacteria are blue and are counted and reported as the number of organisms per 100ml.

The test we will be using is the Pathoscreen field test developed by Hach Chemical Company.  This is a simple test designed to give results in a presence/absence fashion, which is also approved for use by the EPA.  This is a cheap, simple and quick field test that can be conducted without any extensive training and little to no equipment.  It simply tests for hydrogen-sulfide producing organisms by putting a sample of water in a bag and adding the media and sealing the bag.  It is then left to incubate for 24 hours and checked periodically for black spots to appear.  If black spots are observed in the sample at any point then this will give a positive for contamination and the water should be deamed contaminated and not drank.