Classical indicators in the 21st century--far and beyond the coliform.

Indicators have been used for many years to designate the microbiological quality of water. In 1914, the U.S. Public Health Service set a standard that required that drinking water show no evidence of coliform organisms (U.S. Treasury Department, 1914). Today, almost 100 years later, drinking waters in the United States must meet the standards established in the Total Coliform Rule, which requires that drinking water show no evidence of the presence of total coliform bacteria in 100 mL of water (U.S. EPA, 1989). However, as limitations with the use of coliforms have become apparent and the applications for indicator microorganisms have expanded, new indicators have been proposed and, in some cases, adopted, for specific purposes, as discussed in detail in a number of recent reports (i.e., National Research Council, 2004; World Health Organization, 2003).