Epizootiological studies on R plasmid with carbadox resistance.

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Abstract
A total of 930 Escherichia coli strains, 307 from cattle, 414 from swine and 209 from chickens, isolated from 1976-1980, 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 .mu.g/ml of Cdx under anaerobic conditions. Of the 414 strains of porcine origin, 30 (7.2%) showed resistance to Cdx. All Cdx-resistant strains originated from 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. Cdx resistance was invariably transmissible by conjugation to E. coli K-12 jointly with resistance to streptomycin, spectinomycin and ampicillin. pNV 13 with CdX-resistance, a representative R plasmid isolated from a porcine strain of E. coli, was examined for host range. It was conjugally transmissible to Klebsiella pneumoniae, Citrobacter freundi, Salmonella typhi, S. typhimurium and Shigella flexneri 3.alpha., but not to Treponema hyodysenteriae. These transconjugants carrying pNV 13 showed resistance to Cdx; the minimum inhibitory concentrations of Cdx ranged from 6.25-12.5 .mu.g/ml under anaerobic conditions.

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