Effects of low concentrations of ampicillin in feed on the intestinal Escherichia coli of chicks

Ampicillin in low concentrations (1.7 and 5 g t^-1) was incorporated in the feed given to 1-d-old chicks for 2 weeks. This was sufficient to select, in the intestinal contents, resistant Escherichia coli strains for which the minimum inhibitory concentration of ampicillin was > 100 μg ml^-1. Different clones of E. coli were identified by their biotype, pattern of resistance to antibacterial agents and plasmid profile (designated P-P types). The experiment was repeated twice and the average proportion of ampicillin-resistant P-P types among 72 isolates of E. coli from chicks given feed containing 0, 1.7 and 5 g ampicillin t^-1 were 10%, 31% and 46% respectively.