Prevalence and characterization of Escherichia coli isolated from the Upper Oconee Watershed in Northeast Georgia

Abstract

Surface waters are important sources of water for drinking, industrial, agricultural, and recreational uses; hence, contamination of water by fecal, pathogenic, or antimicrobial resistant (AR) bacteria is a major environmental and public health concern. However, very little data is available on prevalence of these bacteria in surface water throughout a watershed. This study aimed to characterize Escherichia coli present in the Upper Oconee Watershed, a mixed-use watershed in Athens, GA, USA for potential pathogenicity and AR. E. coli were enumerated by colony counts, cultured by enrichment and direct plating, and characterized by phylo-groups, diarrheagenic pathotypes, and antimicrobial susceptibility. From the analysis, 99.3% (455/458) of the total samples were positive for E. coli resulting in 496 isolates. E. coli counts were as high as 1.2×104 CFU/100 ml, which is above the United States Environmental Protection Agency (U.S. EPA) threshold for recreational water (235 CFU/100 ml based on a one-time measurement). Phylo-groups B2 (31.7%; 157/496) and B1 (30.8%; 153/496) were the most prevalent among the isolates. Enteropathogenic E. coli (EPEC) (19/496) and Shiga toxin-producing E. coli (STEC) (1/496) were the only diarrheagenic pathotypes detected. AR was observed in 6.9% (34/496) of the isolates, 15 of which were multidrug resistant (MDR; resistance to two or more classes of antimicrobials). Tetracycline resistance was most often detected (76.5%; 26/34), followed by ampicillin (32.4%; 11/34), streptomycin (23.5%; 8/34), sulfisoxazole (23.5%; 8/34), and nalidixic acid (14.7%; 5/34). Results from this study showed that E. coli is prevalent in high levels in the Upper Oconee Watershed, suggesting possible widespread fecal contamination. The presence of pathogenic, AR E. coli in the watershed indicates that environmental water can serve as a reservoir of resistant bacteria that may be transferred to humans through drinking and recreational activities.

Fig 1. Map of water sampling sites in the Upper Oconee Watershed.

The map of the Upper Oconee Watershed in Georgia with the enlarged map of the study area. Sampling sites where AR E. coli were isolated are in red, where EPEC were isolated are in blue, where STEC was isolated is in green, and where both AR E. coli and EPEC were isolated are in purple (sites labeled). Other sites, where all E. coli isolates were pan-susceptible and non-pathogenic, are in yellow.

Fig 2. Seasonal distribution of E. coli in the Upper Oconee Watershed in colony forming units [CFU] per 100 ml.

X-axis represents each sampling season with the numbers in parenthesis indicating the total number of water samples. Y-axis represents the E. coli counts in log₁₀ CFU/100 ml. The threshold represents the EPA threshold for water quality for recreational purposes.