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Antibiotic Resistance in Gut Bacteria from Dairy Calves: A Dose Response to the Level of Antibiotics Fed in Milk

F. M. Langford^*, D. M. Weary and L. Fisher

^* Department of Veterinary Clinical Studies, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Veterinary Centre, Easter Bush, Midlothian, EH25 9RG, U.K.
Faculty of Agricultural Sciences, University of British Columbia, 2357 Main Mall, Vancouver, British Columbia, V6T 1Z4, Canada
Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, P. O. Box 1000, Agassiz, B. C., V0M 1A0, Canada

Corresponding author: D. M. Weary; e-mail: dan.weary{at}interchange.ubc.ca.

Dairy calves are commonly fed milk from cows treated with antibiotics. The concentration of ß-lactam antibiotic residues found in milk from treated cows was used to determine the range of concentrations of penicillin used in a dose-regulated experiment. Thirty-one Holstein calves were randomly assigned to milk with penicillin G added at concentrations of 0, 6.25, 12.5, 25, and 50 µl/kg. Fecal swabs were taken from each calf twice weekly. Resistance to penicillin was tested by measuring the zone of inhibition in bacterial growth around a disk impregnated with the antibiotic. Inhibition was greatest for bacteria from calves fed milk with no penicillin (2.89 ± 0.14 mm), and declined as the penicillin dose provided in the milk increased (to a low of 0.70 ± 0.10 for the 50 µl/kg treatment group). In conclusion, resistance of gut bacteria to antibiotics increases with increasing concentrations of penicillin in the milk fed to dairy calves.

Key Words: calf management • feeding • waste milk • health

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