The broad objective of this study is to assess the risk posed by contamination of natural waters with antibiotics on the emergence of antimicrobial resistance in the microbiota of exposed fish. There is mounting evidence that natural waters are contaminated with antibiotics, in the United States and internationally. These contaminants come in large part from human and animal excretions, as a large proportion of antibiotics administered to treat infections is excreted un-metabolized in urine and feces, and antibiotics are inefficiently removed by waste water treatments. At low levels, antibiotics are very potent signaling molecules influencing bacterial gene expression, facilitating mutagenesis and horizontal gene transfer, and modulating bacterial metabolismand virulence. We have a negligible knowledge of the consequences of chronic exposure to sub-inhibitory levels of antibiotics on the aquatic life that bathes in it. In the present proposal, we aim at testing the consequences of chronic exposure to sub-inhibitory levels of sulfamethoxazole  (SMX) on a fish species of economic importance in the US, the rainbow trout (Oncorhyncus mykiss). Sulfamethoxazole is used in human and veterinary medicine and is the most common antibiotic detected in natural waters. Juvenile rainbow trout will be exposed to sub-inhibitory concentrations of SMX for a period of six months. A non-exposed group will be added as a control. Acquisition of antimicrobial resistance will be assessed phenotypically by (i) quantifying the ratio of resistant to total CFU from the intestinal microbiota, andgenotypically by(ii) performing PCR on isolated resistant colonies to identify the presence of knownresistance genesand (iii) sequencing known target of SMX for mutations. Results from this study will contribute to assess the threat posed by contamination of natural waters with antibiotics on the potential for emergence of antimicrobial resistancein aquatic animals.  This will further our understanding of the generation of antibiotic resistance in the natural environmentand a threshold for activity of antibiotics.