MODE OF ACTION OF ANTIBIOTICS IN THE NUTRITION OF THE DAIRY CALF IV.Escherichia coli

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Abstract
A study was made on the development of drug-fastness in 2 strains of Escherichia coli pathogenic to calves. Low levels of chlortetracycline altered the growth curve of the bacteria, and repeated transfers in these low concentrations of antibiotic resulted in reestablishing a normal growth curve by the organism. The organisms which had become drug resistant were more susceptible to phagocytosis than their non-drug resistant sister cells even after 5 transfers in the absence of the drug. It was postulated that the feeding of nutritional levels of chlortetracycline to young calves contributes to the stimulation of the growth rate of the calves by reducing the resistance to phagocytosis of intestinal bacteria, thereby aiding the host defense mechanism.