Abstract
A total of 930 Escherichia coli strains, consisting of 307 strains isolated from cattle, 414 from swine, and 209 from chickens isolated during a period from 1976 to 1980, were examined for R plasmid with carbadox (Cdx) resistance. No Cdx-resistant strains were found among the strains of bovine and avian origin. All the bovine and avian strains were susceptible to 3.13 μg/ml of Cdx under anaerobic conditons. Of the 414 strains of porcine origin, however, 30 strains (7.2%) showed resistance to Cdx: All CdX-resistant strains were originated from six farms where Cdx was used for preventing swine dysentery or promoting piglet growth. Of the 30 resistant strains, 21 (70%), the isolates on a farm in 1980, had conjugative R plasmid carrying the Cdx-resistance determinant. The character of Cdx-resistance was invariably transmissible by conjugation to E.coli K-12 jointly with the resistance to such other drugs as streptomycin, spectinomycin and apmicillin. Then, pNV 13 with Cdx-resistance, a representative R plasmid isolated from a porcine strain of E. coli, was examined for the host range. It was conjugally transmissible to Klebsiella pneumoniae, Citrobacter freundii, Salmonella ryphi, Salmonella typhimurium and Shigella flexneri 3a, but not to Treponema hyodysenteriae. These transconjugants carrying pNV 13 showed resistance to Cdx; the minimum inhibitory concentrations of Cdx raged from 6.25 to 12.5 μg/ml under anaerobic conditons.
