Evaluation of the Anderson Baird-Parker direct plating method for enumeratingEscherichia coliin water

Abstract

The direct plating (DP) method for enumerating Escherichia coli in food was adapted for water analysis by membrane filtration and a standarized protocol was described. The DP method was found to give equal or better recoveries of E. coli than a membrane filtration method using 0.1% sodium lauryl sulphate agar; the repeatability of the DP method was markedly better. The necessity to transfer membranes from the non-selective medium tryptone soy agar (TSA) to the selective medium tryptone bile agar (TBA) after pre-incubation for 4 h was considered disadvantageous for practical purposes. A double-layer method, where the membrane filter is placed on a layer of TSA poured over TBA, with incubation in an incubator that automatically switches from 37.degree. to 44.degree. C after 4 h, was found to be an acceptable alternative. Recovery of E. coli and inhibition of competitive flora were equal or only slightly less than for the standard DP method.