The influence of increasing numbers of nonindicator organisms upon the detection of indicator organisms by the membrane filter and presence–absence tests

Abstract

Municipal water samples were analyzed by membrane filter (MF) procedures for total coliforms, "background" counts, and 24-and 48-h, 35 °C plate counts. Presence–absence (P–A) tests were done on the samples for total coliforms and other indicator bacteria. The frequency of detection of indicator organisms by P–A tests was better than twice that recovered by MF analyses. When the 24-h plate count data were grouped into counting ranges of 0, 1–100, 101–1000, and > 1000, a marked inhibition effect was observed with total coliform MF recoveries on samples producing plate counts > 1000/mL. When the "background" and 48-h plate count results were placed in the corresponding counting ranges, inhibition of indicator organisms in the total coliform MF analyses was not observed. No inhibition effect was observed in the recovery of indicator organisms by P–A tests at any of the counting range levels. The presence of an apparent inhibition effect by high numbers of bacteria in a sample was shown to be influenced by the incubation period and type of count parameter, as well as the isolation technique for detection of indicator organisms.