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

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 ninety percent 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 two to five days.The P–A test was about five 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.