Figure 4. Schematic representations of the four class 1 integrons and one class 2 integron sequenced from multiple antibiotic–resistant Escherichia coli isolates. The sat-1 gene, which codes for the streptothricin acetyl transferase, was not detected in any other E. coli isolate. The presence of the sat-1 gene cassette, in combination with the aadA gene, suggests that this class 2 integron is likely a derivative of the class 2 integron found on transpo- son Tn7 (33,34). The aadA gene was conserved among the class 1 and class 2 integrons, which suggests a possible selective mechanism for this cassette in enteric bacteria from natu- ral waters. The 2-kb integron-specific variable region–con- taining strain, which was isolated from the Presidio area, harbored the dihydrofolate reductase gene (dhfrXII) instead of the aadA gene (35). Overall, these results suggest that the irrigation canals and sediments associated with the Rio Grande are contam- inated by bacteria of fecal origin that contain antimicrobial resistance genes. Of 322 E. coli isolates, 32 (approximate- ly 10%) were resistant to multiple antimicrobial drugs. Five of these 32 E. coli isolates harbored class 1 and class 2 integron sequences. This study did not investigate the possibility that other integron-bearing nonfecal bacteria were present. The occurrence of integron-bearing E. coli in irrigation water is important since these organisms are known fecal contaminants, and the potential for lateral gene transfer exists. The results also indicate that integron- RESEARCH bearing strains may not always express the antimicrobial phenotype; thus, phenotype-based isolation of resistant organisms can underestimate the levels of resistant organ- isms. Studies are needed to identify whether integron- mediated antimicrobial resistance transfer does indeed occur within the irrigation canal sediments and on veg- etable surfaces, when they are irrigated with contaminated irrigation water. Acknowledgments We thank Anne Summers and Cynthia Liebert for providing the integron-positive strains and James Zhu for helpful discus- sions concerning the sequencing data. This work was supported by funds from the State of Texas ATP project 00517-0361-1999, the U.S. Department of Agriculture (USDA)/CSREES-IFAFS grant 00-52102-9637, the USDA CSREES grant 2001-34461-10405 and Hatch grant H8708. Mr. Roe conducted this study while an M.S. student in the Food Safety and Environmental Microbiology Laboratory in the Poultry Science Department at Texas A&M University. His research interests are in environmental microbiology and food safety. References 1. Ochman H, Lawrence JG, Groisman EA. Lateral gene transfer and the nature of bacterial innovation. Nature 2000;405:299–304. 2. Lucey B, Crowley D, Moloney P, Cryan B, Daly M, O’Halloran F, et al. Integronlike structures in Campylobacter spp. of human and ani- mal origin. Emerg Infect Dis 2000;6:50–5. 3. Zhao S, White DG, Ge B, Ayers S, Friedman S, English L, et al. Identification and characterization of integron-mediated antibiotic resistance among Shiga toxin–producing Escherichia coli isolates. Appl Environ Microbiol 2001;67:1558–64. 4. Briggs CE, Fratamico PM. Molecular characterization of an antibiot- ic resistance gene cluster of Salmonella typhimurium DT104. Antimicrob Agents Chemother 1999;43:846–9. 5. Mroz RC Jr, Pillai SD. Bacterial populations in the groundwater on the US-Mexico border in El Paso County, Texas. South Med J 1994;87:214–7. 6. Pillai SD, Widmer KW, Maciorowski KG, Ricke SC. Antibiotic resistance profiles of Escherichia coli isolated from rural and urban environments. J Environ Sci Health 1997;32:1665–75. 7. International Boundary and Water Commission. Water Bulletin #63: Flow of the Rio Grande and related data. El Paso (TX): The Commission; 1993. p. 132. 8. Scandura JE, Sobsey MD. Viral and bacterial contamination of groundwater from on-site sewage treatment systems. Water Sci Technol 1997;35:141–6. 9. Rayburn EL, Sternes KL, Weidenfeld RP, Pillai SD. Microbial pathogens in irrigation canals and associated sediments. In: Proceedings of the 101st General Meeting of the American Society for Microbiology. 2001. Orlando (FL): American Society for Microbiology; 2001. 10. Food and Drug Administration. Bacteriological analytical manual online. 8th ed. 2001. Available from: URL: http://www.cfsan.fda.gov/ ~ebam/bam-toc.html 11. National Committee for Clinical Laboratory Studies. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aero- bically. Vol. 10. Villanova (PA): The Committee; 1999. p. 33. Emerging Infectious Diseases • Vol. 9, No. 7, July 2003 825