Are Presence/Absence Microbial Tests Appropriate for Monitoring Large Urban Water Supplies in Sub-Saharan Africa?

Abstract

Screening for fecal contamination via microbial water quality monitoring is a critical component of safe drinking water provision and public health protection. Achieving adequate levels of microbial water quality testing, however, is a challenge in resource-limited settings. One strategy for addressing this challenge is to improve the efficiency of monitoring programs. In African countries, quantitative microbial testing methods are commonly used to monitor chlorinated piped water systems. However, presence/absence (P/A) tests may provide an appropriate alternative for water supplies that generally show negative fecal contamination results. This study compares 1048 water quality test results for samples collected from five African urban water systems. The operators of the systems conducted parallel tests on the 1048 samples using their standard quantitative methods (e.g., most probable number or membrane filtration) and the Colitag™ method in P/A format. Combined data demonstrates agreement rates of 97.9% (1024/1046) for detecting total coliforms and 97.8% (1025/1048) for detecting E. coli. We conclude that the P/A test offers advantages as a simpler and similarly sensitive measure of potential fecal contamination for large, urban chlorinated water systems. P/A tests may also offer a cost-effective alternative to quantitative methods, as they are quicker to perform and require less laboratory equipment.

Keywords: Colitag™; monitoring; presence-absence; sub-Saharan Africa; urban water; water quality; water testing.

Figure 1

(a) Percentage of positive and negative samples for total coliforms according to P/A and quantitative diagnostic methods; (b) percentage of positive and negative samples for E. coli according to P/A and quantitative diagnostic methods. Due to the infrequent presence of fecal indicators in the distribution waters collected, naturally contaminated water sources were used to spike some of the samples; therefore, the percentage of positive results should not be used to draw conclusions about system water quality.