The antibiotic resistance (AR) patterns of 462 Escherichia coli isolates from wastewater, surface waters, and oysters were determined. Rates of AR and multiple-AR among isolates from surface water sites adjacent to wastewater treatment plant (WWTP) discharge sites were significantly higher (P < 0.05) than those among other isolates, whereas the rate of AR among isolates from oysters exposed to WWTP discharges was low (<10%).The aim of this study was to investigate the rates of antibiotic resistance among Escherichia coli isolates from a variety of sources, including wastewater treatment plants (WWTPs), surface waters (including those directly influenced by WWTP discharges), and oysters affected by WWTP discharges. This information will contribute to our understanding of antibiotic resistance in the aquatic environment and the potential environmental and public health risks associated with exposure to aquatic bacteria.This study was conducted on the central east coast of Australia on the Brisbane and Bremer Rivers and Cabbage Tree Creek, which are subtropical city-dominated systems with both estuarine and freshwater regions. Samples from each stage of the treatment process at five regional WWTPs (42 samples) were collected (Table 1), as well as samples from surface waters in the Brisbane River, including waters at sites adjacent to the investigated WWTPs (five sites referred to as point sources [PS]) and waters at sites distant from PS (six sites referred to as nonpoint sources [NPS]) (Fig. 1). Approximately 30 native oysters (Saccostrea commercialis) were collected at low tide from a small estuarine creek (Cabbage Tree Creek), approximately 300 m downstream from a WWTP discharge site (Fig. 1).The isolation of E. coli from WWTP and surface water samples was performed using membrane filtration (20). Oysters were washed and scrubbed in 70% ethanol (ChromAR [purity, 99.9%]; Mallinckrodt Chemicals) before shucking. The isolation of E. coli from oysters was carried out using a standard food technique (21).A total of 462 isolates were tested for sensitivity to six antibiotics by using the CLSI disk susceptibility testing method (5). The antibiotics chosen were ampicillin (10 g; Oxoid), cephalothin (30 g; Oxoid), nalidixic acid (30 g; Oxoid), sulfafurazole (300 g; Oxoid), gentamicin (10 g; Oxoid), and tetracycline (30 g; Oxoid). A one-way analysis of variance followed by a posthoc Tukey honestly significant difference means test was used to determine significant differences (P Ͻ 0.05) among sources of E. coli isolates for each antibiotic and among multiple-antibiotic-resistance (MAR) indices based on zones of inhibition. For MAR patterns, data were converted into a binary code (resistant or nonresistant) and differences (P Ͻ 0.05) among sources for each of the resistance patterns were evaluated by paired t tests for all combinations.The observed level of antibiotic resistance among all investigated isolates, 59%, was markedly lower than those demonstrated previously in similar studies, typically around 90% (11,16). It ...