Elevated levels of antibiotic-resistant (AR) bacteria are routinely detected in surface water, whereas the dissemination of such bacteria across tributaries, rivers, or watersheds is often unknown. In a present study, one year of water quality monitoring was conducted to assess the sources, occurrence, and distribution of AR bacteria in a typical urban watershed, the Gwangju Watershed, Korea. For this study, a total of 828 Escherichia coli (E. coli) isolates obtained from 8 sites in stream water were tested for their resistance against 15 different antibiotics. Results revealed that while antibiotic resistance of the E. coli isolates showed no significant difference among sites, resistance rates to one and more antibiotics were always higher than those of non-antibiotic (below 50%), representing a high incidence of antibiotic resistance in the surface water. Among the antibiotics tested, the isolates were most resistant to tetracycline (50%), followed by carbenicillin (33%) and ampicillin (32%). However, the resistance rates showed no measurable difference between the isolates from the wastewater treatment plants and those from downstream sites, except for streptomycin (p < 0.05), indicating that untreated sewage discharge was not a primary source of the resistance. In addition, no significant difference in resistance rates was observed between summer and winter seasons. When the relationship between resistance rates in 828 E. coli isolates to antibiotics and those of multiple antibiotic resistance was further examined, a high correlation was found in streptomycin, carbenicillin, piperacillin, tetracycline, and ampicillin, thus suggesting that these antibiotics could be used as potential indicators for representing the resistance rate of E. coli isolates in surface water.

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