Effects of therapeutical and reduced levels of antibiotics on the fraction of antibiotic-resistant strains of Escherichia coli in the chicken gut

Abstract Development of antibiotic resistance in the microbiota of farm animals and spread of antibiotic-resistant bacteria in the agricultural sector not only threaten veterinary use of antibiotics, but jeopardize human health care as well. The effects of exposure to antibiotics on spread and development of antibiotic resistance in Escherichia coli from the chicken gut were studied. Groups of 15 pullets each were exposed under strictly controlled conditions to a 2-day course of amoxicillin, oxytetracycline, or enrofloxacin, added to the drinking water either at full therapeutic dose, 75% of that, or at the carry-over level of 2.5%. During treatment and for 12 days afterwards, the minimal inhibitory concentration (MIC) for the applied antibiotics of E. coli strains isolated from cloacal swabs was measured. The full therapeutic dose yielded the highest percentage of resistant strains during and immediately after exposure. After 12 days without antibiotics, only strains from chickens that were given amoxici...