Evaluation of detection performance and cost-effectiveness of IDEXX Colilert-18 and ChromAgar for environmental surveillance of extended-spectrum beta-lactamase Escherichia coli in water samples from urban marketplaces in Blantyre, Malawi

In sub-Saharan Africa, extended-spectrum beta-lactamase (ESBL)-producing bacterial infections pose significant public health risks. The importance of ESBL transmission within the food chain, as well as the role of urban marketplaces in this relationship, is becoming increasingly recognized. ESBL Escherichia coli also represents a key indicator organism within global antimicrobial resistance surveillance strategies, including the World Health Organization Tricycle protocol. Therefore, evaluating simple, cost-effective, and scalable detection methods for ESBL-producing E. coli is crucial. In 2021, a modified Colilert-18 method and a conventional ChromAgar ESBL culture method were assessed for the detection of ESBL E. coli in water samples from four urban marketplaces in Blantyre, Malawi. ESBL E. coli were detected in water samples obtained from urban marketplaces using both ChromAgar ESBL culture (39% [n = 65/167]) and IDEXX Colilert-18 methods (76% [n = 127/167]). Using ChromAgar ESBL as a reference standard, the modified Colilert-18 method showed a sensitivity of 86% and a specificity of 30%. Agreement between the methods was low (Cohen’s kappa = 0.141, P < 0.001). The cost per test was lower for Colilert than for ChromAgar, indicating potential cost-effectiveness in primary ESBL E. coli detection. At the time of sampling, water collected from urban marketplaces was contaminated with ESBL E. coli, posing possible public health risks. Both the ChromAgar method and the modified IDEXX method offer valuable tools for microbiological surveillance of ESBL E. coli in urban markets. However, further studies must be conducted to optimize their use and determine additional considerations to factor into cost-effectiveness.