Drug Resistance and R Plasmids in Escherichia coliStrains Isolated from Broilers

In 1978, 1,021 Escherichia colistrains were isolated from 105 field broilers (F) and 1,058 strains from 106 broilers in a zootechnical experiment station (Z), and their drug‐resistance patterns and the presence of conjugative R plasmids were compared. The resistance markers examined were tetracycline (TC), chloramphenicol (CM), streptomycin (SM), sulfonamides (SA), kanamycin (KM), and ampicillin (APC). The populations of individuals that excreted resistant strains were 100% in F and 58% in Z. Frequencies of isolation of drug‐resistant strains among the total isolates were 93% in F and 36% in Z, indicating that the resistant strains are a rather high proportion of the intestinal flora in F but are slightly less prevalent in Z. The resistance pattern to (TC.SM.SA.KM) was seen at the highest frequency in both groups. Conjugative R plasmids were demonstrated more frequently in field broilers (F). The results reflect the wide use of antibiotics in the livestock industry, resulting in the appearance of drug‐resistant strains mostly due to the presence of R plasmids.