Bacterial antibiotic resistance is an emerging and serious public health concern due to the compromised efficacy of antimicrobial agents used in the treatment of infectious diseases (Cohen, 1992; Neu, 1992; Martı´- nez and Baquero, 2002). embers of the Enterobacteriaceae family of bacteria are medically important as infectious agents, exhibit antibiotic resistance, and are present in large numbers in the animal gut (Paterson, 2002; Rupp and Fey, 2003). Antibiotics are extensively used in human and veterinary medicine, and in agricultural settings, for the treatment of infections, growth enhancement, and prophylaxis in food animals, potentially leading to selection of drug- and multidrug-resistant bacteria (Aarestrup, 1999; Barbosa and Levy, 2000b). In addition, antibiotic-producing microorganisms are found naturally in soil, suggesting intrinsic chromosomal antibiotic resistance originated in the soil in response to inhibitory environments generated by such antibiotic-producing microorganisms (George and Levy, 1983; Randal and Woodward, 2001).