INTRODUCTION Antibiotic usage is considered the major factor promoting the emergence, selection and dissemination of antimicrobial- resistant bacteria in both veterinary and human medicine. Globally most of the antimicrobial agents used for treatment of human infections, are also used in animal industry for therapy and control of infections (Chopra and Robert, 2001; Brayan et al., 2004; Miles et al., 2006). Tetracycline (Tet) is the drug of choice for treatment of bacterial infections and growth promotion, but its extensive use has contributed to the emergence of resistance (Robert, 1996, 2003). Genetic analysis of Tet-resistance have demonstrated that 36 genes have been participated to Tet-resistance which encode one of the three main mechanisms including the efflux pump, ribosomal protection, or direct enzymatic inactivation of the drug, but the efflux mechanism appear to be more frequent (Robert, 1996, 2003; Chopra and Robert, 2001; Aminove et al., 2002; Hartman et al., 2003; Pool, 2005). The determinants of Tet-resistance are normally located in mobile or potentially mobile elements which can be responsible for spreading of the resistance genes vertically between bacteria of different genera or horizontally between different bacterial strains and/or species within the same genus (Robert, 1996; Chopra and Robert, 2001; Goldstein et al., 2001; Kang et al., 2005). In Gram negative species, resistance to Tet is frequently due to acquisition of a resistance operon of Tet A family which is associated with conjugative plasmids. These plasmids mediate resistance to multiple antimicrobial agents and can result in the resistance to all or most of important antimicrobial agents (Johnson et al., 2004a, 2004b; Sherley et al., 2004). In poultry E. coli the resistance plasmids are frequently observed and Tet-resistance in isolates from poultry has been shown associated with the energy-dependent efflux * Corresponding Author. Phone: 98288058940; Fax: 982188058912; E-mail: tfalsafi@yahoo.com Annals of Microbiology, 59 (2) 199-205 (2009) The pattern, association with multidrug-resistance and transferability of plasmid-mediated tetracycline resistance in Escherichia coli isolates from the poultry in Iran Tahereh FALSAFI 1* Monir EBRAHIMI 1 , Ezat ASGARANI 1 , Vahid MIRTORABI 2 1 Department of Biology, Faculty of Sciences, Alzahra University, Vanak, Tehran, Iran 2 Veterinary Institute of the Agricultural Research Council, Ghom, Iran Received 6 October 2008 /Accepted 5 May 2009 Abstract - Objective of this study was to assess the pattern of plasmid-mediated tetracycline-resistance (Tet r ) in poultry Escherichia coli, to verify whether this resistance is associated with multidrug-resistance (MDR), and to know about the involvement of efflux via detection of tet (A) and tet (C). Seventy poultry E. coli-isolates were tested against tetracycline, β-lactams, quinolones/flouroquinolone, chloramphenicol, aminoglycosides, and trimethoprim-sulphamethoxazole (SXT) by disk-diffusion. Plasmid-DNA from Tet r -isolates were compared by electrophoresis and transferred to DH5α and Hb101 E. coli strains via electroporation and conjugation; presence of tet(A) and tet(C) was detected by PCR. Transformants and transconjugants were compared with donor strains in plasmids, restriction-endonuclease patterns, and presence of tet (A) or tet (C) Eighty-nine percent and 11% of the isolates were resistant and intermediately-resistant to Tet; of which 75% were multidrug-resistant to quinolones/flouroquinolone, SXT, chloramphenicol and/or β-lactams. Their plasmid profile was heterogenous but 72% contained a 100 Kb plasmid-band. Fifty-percent, 19%, and 3% of the Tet r -isolates amplified for tet(A), tet(C), and both of them, respectively. Transformants and transconjugants have acquired a large 100 Kb-plasmid with similar restriction-endonuclease patterns of the donor-strains, resistance to Tet, SXT, β-lactams, and the efflux determinants of Tet(A), or tet(C). Escherichia coli strains from poultry in Iran are heterogeneous, multidrug-resistant, contain a large transferable-plasmid conferring resistance to Tet, β-lactams, SXT and carrying the Tet efflux genes of tet(A) and/or tet(C). These data indicate that poultry E. coli strains in this region can be a reservoir for antimicrobial- resistance genes and can play a role in dissemination of resistance genes to other pathogenic and commensal bacteria in poultry industry and/or environment. Key words: Escherichia coli; poultry; tetracycline; multidrug-resistance; plasmid; efflux; Iran.