Water Quality Tests

Various water quality tests are available to detect the number and types of microorganisms in waters and assist communities in keeping the microbial content of water supplies at a low level. These tests vary from the more sophisticated tests to the standard procedures that have been used for decades.

Gene probe tests. Among the most sophisticated tests for water bacteriology are those that employ gene probes. Gene probes are fragments of DNA that seek out and combine with complementary DNA fragments. Often the test is designed to test for the presence of Escherichia coli in water. This Gram-negative rod, usually found in the human intestine, is used as an indicator organism. If it is present, then it is likely that the water has been contaminated with human feces. The feces may contain microbial pathogens.

To use a gene probe test for E. coli in water, the water is treated to disrupt any bacteria present and release their nucleic acid. Then a specific E. coli probe is added to the water. Like a left hand seeking a right hand, the probe searches through all the nucleic acid in the water and unites with the E. coli DNA, if present. A radioactive signal indicates that a match has been made. If no radioactivity is emitted, then the gene probe has been unable to locate its matching DNA, and E. coli is probably absent from the water.

The membrane filter technique. The membrane filter technique uses a filtration apparatus and a cellulose filter called a membrane filter. A 100-ml sample of water is passed through the filter, and the filter pad is then transferred to a bacteriological growth medium. Bacteria trapped in the filter grow on the medium and form colonies. By counting the colonies, an estimate can be made of the number of bacteria in the original 100-ml sample.

The standard plate count. It is generally impractical to test for all pathogenic organisms, but the total number of bacteria can be calculated. One test is thestandard plate count. In this test, samples of water are diluted in jars containing 99-ml sterile water, and samples are placed in Petri dishes with nutrient agar or other nutritious medium (Figure 1 ). After incubation, the colony count is taken and multiplied by the dilution factor to obtain the total number of bacteria per ml of sample. 


Figure 1

The standard plate count procedure. A 1‐mL water sample is diluted in buffer solution, and various amounts are placed with nutrient medium into Petri dishes to encourage bacterial colonies to form. The colony count is multiplied by the dilution factor to yield the total plate count.

Indicator bacteria can be detected to give an estimate of pathogens. The most common indicator organisms in water bacteriology are the coliform bacteria. These are Gram-negative rods normally found in the intestine and typified by Escherichia coli. To test for the presence of coliforms, a standard plate count can be performed, with violet red bile agar used as the growth medium to encourage proliferation of the coliform bacteria.

The most probable number test. In the most probable number (MPN) test, tubes of lactose broth are inoculated with water samples measuring 10 ml, 1 ml, and 0.1 ml. During incubation, coliform organisms produce gas. Depending upon which tubes from which water samples display gas, an MPN table is consulted and a statistical range of the number of coliform bacteria is determined. The MPN test is very easy to perform and interpret, but it does not determine the exact number of bacteria as the standard plate count does.

In order to test for the presence of E. coli in water, a medium called eosin methylene blue (EMB) agar is used. On this medium, E. coli colonies become green with a metallic fluorescent sheen.

The biochemical oxygen demand (BOD). The extent of pollution in wastewater can be determined by measuring the biochemical oxygen demand (BOD). The BOD is the amount of oxygen required by the microorganisms during their growth in wastewater.

The BOD test is begun by noting the oxygen concentration in a sample of water before incubation. The water is then incubated in an air-tight, stoppered bottle for a period of about five days. A temperature of between 5° and 20°C is used. The oxygen concentration in the water is then noted again, and the difference in the dissolved oxygen is the BOD. A higher BOD indicates presence of a higher amount of organic matter. High BOD values are found in wastewater from agricultural communities, food processing plants, and certain industries.