Characterization of integrons in Escherichia coli of the normal intestinalflora of swine

Multiresistant Escherichia coli isolates of the normal intestinal flora of healthy fattening pigs were examined for the presence of integron class 1 b y XL (extra long) PCR, The class 1 integron was detected in 17 isolates ori ginating from 14 healthy animals on seven different farms. One isolate cont ained two class 1 integrons. The inserted gene cassettes were characterized by DNA sequencing and PCR, The ant(3 ")-Ia gene responsible for resistance to streptomycin/spectinomycin was inserted in all integrons detected. Fift een isolates contained this gene cassette as the only inserted cassette. Th ree isolates contained integrons with two gene cassettes. Two isolates cont ained integrons with the trimethoprim resistance gene dfr1 and one isolate contained the oral beta-lactamase gene upstream to the ant(3 ")-Ia gene. De tection of these three different resistance gene cassettes in bacteria from swine shows that cassettes occurring in integrons in human clinical isolat es also appear in bacteria of the normal intestinal flora of healthy swine. Two integron-harboring strains were obtained from each of three different animals. These strains were probably not clonal derivatives of each other, suggesting the existence of different multiresistance clones within the int estinal normal flora of one specific animal. The oral nucleotide sequence f ound in E. coli from swine differ by seven nucleotides from the oral nucleo tide sequence of the gene from the R-plasmid RGN238. The fact that these tw o sequences are not identical might indicate that the two genes have evolve d separately in different surroundings from the common ancestor, Transmissi ble plasmids of approximately 200 kb containing integron class I were detec ted in eight of the isolates when conjugation experiments were performed wi th E. coli DH5 as recipient strain. The transfer frequency ranged from 4 x 10(-4) to 6 x 10(-2) transconjugants per recipient cell. This study shows t hat the enteric commensals of domestic animals may be considered as a reser voir of integron-containing transmissible plasmids and gene cassettes that might be transferable to the pathogens of swine and to important zoonotic b acteria associated with the enteric flora of swine such as Salmonella typhi murium DT104.