The fate of orally administered ampicillin was studied in axenic lambs, in gnotoxenic lambs given a complex microflora and a mixture of ampicillin resistant and/or sensitive strains of , and in conventional lambs. In axenic lambs or animals with a sensitive micro-flora, antibiotic concentrations of 500-1600 μg ml were detected in the intestine, and most of the ampicillin passed through the small intestine and entered the large intestine, within 12-15 h of administration. These antibiotic concentrations were sufficient to decrease the numbers of ampicillin-sensitive from 10 – 10 bacteria ml to about 10-10bacteria ml by 8 h after ampicillin administration. Second and third doses of antibiotic had no further effect on the bacterial count. Administration of ampicillin to animals hosting ampicillin-resistant resulted in a significant inactivation of the antibiotic in the intestine. As might be expected there was little reduction in the numbers of these organisms. These results are similar to those observed in conventional lambs hosting resistant as the dominant colibacillary flora.


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