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.IMPORTANCEThis study highlights the role of urban marketplaces as hotspots for potential transmission of extended-spectrum beta-lactamase (ESBL) bacteria. The findings suggest that markets are critical sites that warrant for environmental surveillance, particularly in settings with limited water, sanitation, and hygiene infrastructure. We investigated the differences in detection performance and the cost-effectiveness of a modified IDEXX Colilert-18 method compared to conventional culture using ChromAgar. We found that the higher detection rate and lower cost of the Colilert-18 method point to its potential as a viable alternative for surveillance of ESBLs in water samples. These findings are particularly relevant for low-income settings, where routine environmental monitoring is constrained due to poor laboratory capacity and limited resources. Our study contributes to the evidence on cost-effective methods for environmental surveillance of antimicrobial-resistant bacteria and emphasizes the need for locally appropriate, scalable tools to support public health responses to antimicrobial resistance.