A presence–absence (P–A) test providing sensitive and inexpensive detection of coliforms, fecal coliforms, and fecal streptococci in municipal drinking water supplies

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Abstract
A simple presence-absence (P-A) test was developed to provide a more economical and more sensitive method for conducting coliform analyses on municipal drinking water samples than the membrane filter (MF) technique. Over 90% of these samples routinely gave negative results by the MF method. A modified MacConkey broth, enriched to improve on acid and gas production by coliforms, was the isolation medium for the presumptive part of the P-A test. Parallel analyses of water samples were made by both the P-A and MF methods. Confirmatory tests established the reliability of the respective procedures to detect coliform bacteria. A statistical analysis of the results showed that the P-A test was more sensitive for detecting lower levels of pollution than the MF technique. Many of the confirmed positive P-A results came from P-A bottles that produced presumptive positive tests only after an extended incubation period of 2 to 5 days. The P-A test was about 5 times less expensive than the MF technique and by adding a few simple tests, the P-A procedure could give information on the presence of both fecal coliforms and fecal streptococci. For several samples, fecal streptococci were found by P-A tests in the absence of detectable coliform bacteria by either the MF or P-A methods of analysis.

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