Incidence and characterization of integrons, genetic elements mediating multiple-drug resistance, in avian Escherichia coli

Antibiotic resistance among avian bacterial isolates is common and is of gr eat concern to the poultry industry. Approximately 36% (n = 100) of avian, pathogenic Escherichia coli isolates obtained from diseased poultry exhibit ed multiple-antibiotic resistance to tetracycline, oxytetracycline, strepto mycin, sulfonamides, and gentamicin, Clinical avian E. coli isolates were f urther screened for the presence of markers for class 1 integrons, the inte gron recombinase intI1 and the quaternary ammonium resistance gene qacE Del ta 1, in order to determine the contribution of integrons to the observed m ultiple-antibiotic resistance phenotypes, Sixty-three percent of the clinic al isolates were positive for the class 1 integron markers infI1 and qacE D elta 1. PCR analysis with the conserved class 1 integron primers yielded am plicons of approximately 1 kb from E. coli isolates positive for intI7 and qacE Delta 1. These PCR amplicons contained the spectinomycin-streptomycin resistance gene aadA1. Further characterization of the identified integrons revealed that many were part of the transposon Tn21, a genetic element tha t encodes both antibiotic resistance and heavy-metal resistance to mercuric compounds. Fifty percent of the clinical isolates positive for the integro n marker gene intI1 as well as for the qacE Delta 1 and aadA1 cassettes als o contained the mercury reductase gene merA. The correlation between the pr esence of the merA gene with that of the integrase and antibiotic resistanc e genes suggests that these integrons are located in Tn21, The presence of these elements among avian E. coli isolates of diverse genetic makeup as we ll as in Salmonella suggests the mobility of Tn21 among pathogens in humans as well as poultry.