Comparison and verification of four field-based microbiological tests: H2S test, Easygel®, Colilert®, Petrifilm™

Abstract

The UN defines water supplies as ‘improved’ or ‘unimproved.’ These indicators are easy to measure, but do not reflect water quality, which requires laboratory or field tests. Laboratory and test availability, expense and technical capacity are obstacles for developing countries. This research compares and verifies four low-cost, field-based microbiological tests: the EC-Kit (Colilert® and Petrifilm™ tests), the H2S bacteria test, and Easygel®, against a standard method (Quanti-Tray®). The objectives are to: (1) verify the accuracy of the four field-based tests, (2) study the accuracy of these tests as a function of improved and unimproved sources; (3) recommend a single microbiological test, if appropriate, based on accuracy and cost, and/or (4) recommend a testing combination, if appropriate, based on accuracy and cost. The tests of 500+ total water samples from Capiz Province, Philippines and Cambridge, MA indicate that two-tests systems gave better results than a single test. Both the 100-mL H2S test + Petrifilm™ and the 20-mL H2S test + Easygel® combinations yield promising results, in addition to being inexpensive. None of the field-based tests should be used on their own. We recommend further verification of a larger sample size and scale be undertaken before these testing combinations are recommended for wider use.

Microbial Monitoring of Surface Water in South Africa: An Overview

“…Therefore a certification mechanism must be implemented for the H 2 S strip test based monitoring, similar in scope and format to the South African Scoring System certification [110]. Recent data from the Philippines and the USA compared reliability of the Colilert TM system and the H 2 S strip test based that the medium of Venkobachar et al [37,111]. For the WHO-defined improved and unimproved sources of drinking water, the Colilert TM system correctly predicted faecal contamination in 75% of 521 samples taken from drinking water sources similar to the ones used in South Africa [111].…”

“…On the other hand, the H 2 S strip test performed best if the manually-prepared sampling kits were used and the sample size was equal to 20 mL [18,111]. In that case, 84% of 521 samples were correctly identified as faecally-contaminated in the H 2 S strip test, while the rate of false positive results was equal to 10% [111].…”

“…On the other hand, the H 2 S strip test performed best if the manually-prepared sampling kits were used and the sample size was equal to 20 mL [18,111]. In that case, 84% of 521 samples were correctly identified as faecally-contaminated in the H 2 S strip test, while the rate of false positive results was equal to 10% [111]. At the same time, only 6% of samples were incorrectly classified as faecally-contaminated and the H 2 S strip test increased the error of the faecal contamination detection to a lower extent than the Colilert TM system [111].…”

“…In that case, 84% of 521 samples were correctly identified as faecally-contaminated in the H 2 S strip test, while the rate of false positive results was equal to 10% [111]. At the same time, only 6% of samples were incorrectly classified as faecally-contaminated and the H 2 S strip test increased the error of the faecal contamination detection to a lower extent than the Colilert TM system [111]. These data rates will be further improved based on the addition of 0.5% of deoxycholate [18,109] and will have to be verified under South African conditions.…”

Rural water services in Guatemala: a survey of institutions and community preferences

“…The kit combines two tests: (1) the Cohlert 10 mL presence/absence test; and (2) the Petrifilm test that provides a quantitative count of E. coli colonies (see Chuang et al (2011) for a detailed description, evaluation and comparison of these tests in a developing country context). The Colilert test indicated that 69 out of the 118 water samples collected (58.5%) were contaminated with E. coli.…”