Antibiotic resistance in Escherichia coli of the normal intestinal flora of swine

Twelve hundred enterobacterial Escherichia coli isolates of porcine origin were screened phenotypically for antibiotic resistance. The bacteria were i solated from 10 herds of swine with different histories of exposure to anti microbial agents for therapeutic purposes. The bacterial isolates were part of the normal bacterial flora of the intestines of the animals because the y were isolated from healthy individuals. The strains were tested for pheno typic antibiotic resistance against sulfonamides, trimethoprim, streptomyci n, ampicillin, neomycin, chloramphenicol, and tetracycline, Resistance agai nst streptomycin was found to be most common, followed by resistance agains t sulfonamides and tetracycline, The highest number of resistant bacteria w as found in herds where the use of antimicrobial agents was considered to b e high. A selection of multiresistant bacterial isolates were further genet ically characterized by hybridization with probes specific for the antibiot ic resistance genes; sulI, sulII; dfrI, dfrIIb, dfrIX, and the class A, B, C, and D tetracycline resistance determinants. A PCR was developed and used for detection of the strA-strB gene pair encoding streptomycin resistance in Gram-negative bacteria. The strA-strB gene pair was the most frequent re sistance determinant in the isolates examined. This study indicates that no npathogenic E. coli from swine may represent a considerable reservoir of an tibiotic resistance genes that might be transferable to pathogens.