Mode of action of antibiotics in the nutrition of the dairy calf. IV. Phagocytosis of antibiotic-resistant strains of Escherichia coli.

The widespread use of antibiotics as therapeutic agents against pathogenic bacteria sometimes has presented a problem as the result of the development of drug resistance by the organism involved (Finland, 1956). Although the level of an antibiotic necessary to elicit a growth response in young animals is many times less than the therapeutic dose, drug-fast organisms have been found among the intestinal bacteria isolated from animals fed nutritional levels of an antibiotic (Radisson, Smith, and Ward, 1956b; Ingram et al., 1958). Developed resistance to antibiotics by microoorganisms may pose serious problems. The antibiotic may become ineffective against the microorganism toward which the drug is directed, or the organism which is inhibited may be replaced by another pathogen that is resistant to the drug. Radisson et al. (1956a) and MacFadden and Bartley (1959) showed that low level feeding of chlortetracycline to dairy calves aids the host defense mechanism of phagocytosis by reducing the virulence of the pathogen. Also, McKee and Houck (1943), Blair, Carr, and Buchman (1946), and Radisson et al. (1956b) observed that certain biological changes, such as in morphology and physiology, occurred in drug-resistant bacteria and that these changes were often accompanied by a reduction in virulence. This investigation was conducted to determine