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
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