Screening Antibiotic-Resistant Escherichia coli in Wastewater and River Water Using a Novel Simple Phenotypic Antibiotic-Susceptibility Testing Method

Recently, wastewater treatment plants have been identified as potential reservoirs of antibiotic-resistant bacteria. Hence, it is important to monitor antibiotic-resistant bacteria in wastewater treatment plants. Currently available methods are laborious, time-consuming, and costly. In this study, a novel simple phenotypic antibiotic-susceptibility testing method for Escherichia coli was developed. A growth curve for E. coli in the presence of an antibiotic was established by monitoring the fluorescence intensity of a fluorogenic substrate specific to E. coli using a microplate reader. The antibiotic-resistant E. coli to total E. coli ratios for wastewater and river water samples were determined using fluorescence intensity measurements. The lethal concentrations of antibiotics to E. coli could be roughly estimated using the method. The lethal concentrations were dependent on the antibiotic type rather than differences between the wastewater treatment plants and were higher for samples from primary clarifiers than secondary clarifiers in wastewater treatment plants. The lethal concentrations for river water decreased as the distance from the wastewater treatment plant discharge outlet increased. The novel simple phenotypic antibiotic-susceptibility testing method allows antibiotic-resistant E. coli concentrations in wastewater and river water to be determined rapidly with a high throughput and will allow effective and timely decisions to be made to control antibiotic-resistant E. coli.