Fluctuation of drug-resistant Escherichia coli strains in chickens.

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Abstract
Fluctuations of drug-resistant E. coli strains were investigated in 5 groups of chickens. The colonizing ability of resistant E. coli strains isolated from fluff of hatchers and the cloacae of day-old chicks before feeding was also investigated by typing of O-antigen. E. coli strains were isolated from the cloacae of chickens at regular intervals up to about 50 days after hatching. These chickens were fed diets devoid of antibiotic supplementation and resistant E. coli strains. These strains were isolated from the cloacae of all day-old chicks before feeding and from fluff of hatchers at high frequencies. Some of them had conjugative R plasmids. Moreover, most of the resistance patterns and serovars of E. coli strains isolated from fluff were identical with those of E. coli strains isolated from day-old chicks before feeding and such strains were found in about 1/2 of the E. coli strains isolated from chickens throughout the experiment. On the other hand, the resistance patterns and serovars of the E. coli strains isolated from day-old chicks before feeding varied with the source of chicks. Fluctuations of resistance patterns and serovars of E. coli strains and those of conjugative R plasmid-carrying strains also varied with the source of chicks. Resistant E. coli strains hardly decreased in any experimental group throughout the experiment.