Mode of Action of Antibiotics in the Nutrition of the Dairy Calf. III. Relative Effect of Age, Colostrum, and Chlortetracycline Feeding on Susceptibility of Intestinal Coliform Bacteria to Phagocytosis1

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Abstract
Coliform bacteria were isolated from the feces of chlortetracycline-fed and control calves. The ability of these microbes to resist the defense mechanisms of phagocytosis was measured in vitro. Bacteria isolated from the feces of chlortetracycline-fed calves were more susceptible to phagocytosis than bacteria from unsupplemented animals. Thus, a possible mode of action of antibiotics may be that of affecting intestinal bacteria in such a manner as to make them more susceptible to the body defense mechanism of phagocytosis. In a second experiment a study was made of the effectiveness of chlortetracycline in aiding the calf's defenses against intestinal coliforms when the passive immunity afforded by colostrum was absent. The phagocytic technique indicated that young calves deprived of colostrum were able to reduce the virulence of coliform bacteria, but the response apparently developed too late as all calves deprived of colostrum died. Feeding chlortetracycline increased the survival of calves that did not receive colostrum. However, the greatest rate of survival and highest mean phagocytic index occurred in the calves receiving both colostrum and chlortetracycline. In the first and second experiments the mean phagocytic index for the antibiotic-fed calves increased faster and reached a maximum at an earlier age than the mean phagocytic index for the calves fed no chlortetracycline. Copyright © . .