ABSTRACT
The emergence of antimicrobial resistant (AMR) bacteria has been identified as one of the principal public health threats of the 21st century. The World Health Organization (WHO) has long recognized the threat of AMR bacteria and highlights environmental surveillance as a key step in understanding and combating the global rise of antimicrobial resistance. Here, we modified and validated an IDEXX defined-substrate assay commonly used for recreational water quality monitoring of E. coli to enumerate cefotaxime resistant E. coli in environmental reservoirs. We then applied this method to understand AMR trends in multiple environmental matrices over time. This modified IDEXX assay performed highly similarly to two widely accepted plating methods (TBX and MacConkey agar) for enumerating AMR bacteria in pure culture samples and environmental matrices, indicating it is a valid method for enumerating AMR E. coli in the environment. We detected AMR E. coli in urban surface water (63%, 15/24 samples), surface soil (35%, 8/23), and waterfowl feces (43%, 3/7). Sampling around a heavy rain event also revealed that concentrations of AMR E. coli and total E. coli co-vary over time in both surface water and surface soil. This novel method can reliably be performed outside of a laboratory setting and has very low equipment requirements, meaning it has tremendous potential to bolster global monitoring efforts, particularly in resource-restricted and highly rural settings.
Subject(s)
Anti-Infective Agents , Escherichia coli , Anti-Bacterial Agents/pharmacology , Bacteria , Soil , Drug Resistance, BacterialABSTRACT
Antibiotics are priority contaminants of emerging concern due to their pseudo-persistence in the environment and contribution to the development of antimicrobial resistance. In solution, antibiotics undergo (de)protonation reactions that affect their UV absorbance and, therefore, photolytic fate in natural and engineered systems. This study employed enhanced spectrophotometric methods to determine the acid dissociation constants (as pKa values) and molar absorption coefficients for 12 fluoroquinolone, 9 sulfonamide, and 7 tetracycline antibiotics of environmental relevance. Molar absorption coefficient heatmaps were generated for all 28 antibiotics at 200-500 nm and pH 1.8-12.2. The data in the heatmaps were deconvoluted to calculate pKa values and specific molar absorption coefficients at each wavelength. All antibiotics had at least one pKa value in the environmentally relevant range of 5.5-8.5, and pKa values were reported for methacycline, moxifloxacin, nadifloxacin, rolitetracycline, sulfadoxine, and sulfapyridine for the first time. Deprotonation of the carboxylic acid associated with pKa,1 (5.5-6.7) exerted the strongest effects on the UV absorbance of fluoroquinolones. For tetracyclines, deprotonation of the tertiary amine at pKa,3 (7.8-10.2) was responsible for major shifts in UV absorbance. Although sulfonamides have conserved pKa sites, no general trends were observed for the molar absorption coefficients. The structural similarity of fluoroquinolones and tetracyclines supported the potential for a class-based approach to identifying molar absorbance as a function of pH. Overall, the reported pKa values and specific molar absorption coefficients will serve as important resources for future studies on antibiotic fate in natural and engineered systems.
