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Thesis - Open Access

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Master of Science (MS)




The advent of antibiotics in animal nutrition placed new emphasis on the intestinal microflors. E.L.R. Stokstad (38) presented the following observations as indicating that the growth promoting effect of antibiotics was due to the action on the bacteria of the intestinal trace. “(1) Antibiotics and chemotherapeutic agents of widely varying chemical structure are effective. This precludes the possibility of their being incorporated into any growth essential for the animal. (2) The ineffectiveness of antibiotics in increasing growth in the germ free animal. (3) The ineffectiveness of aureomycin in increasing growth of the developing chick embryo. (4) The effect of sanitation on the magnitude of the antibiotic growth response.” The actual mechanisms of the growth promoting action arenot known. Some of the possibilities have been presented as follows (38): “(1) Increased bacterial synthesis of essential or stimulatory growth factors. (2) Inhibition of bacteria which compete with the host for essential nutrients. (3) Inhibition of microorganisms which are deleterious because they produce toxic compounds or damage the intestinal tissues.” It has been reported that sureomycin (19) did not stimulate growth of chicks in new quarters, and that penicillin and other antibiotics (2) failed to increase growth of chicks reared in clean quarters. Along with the non-response of germ free chicks to antibiotics these reports would tend to lend favor to the theory of the inhibition of deleterious organisms. Waibel at al (39) observed that for a two year period antibiotics gave an improved growth rate. During the next year the antibiotics no longer improved the growth rate. These authors propose the possibility that “harmful bacteria had been eliminated through the long-continued use of antibiotics.” Working with penicillin, bacitracin and autoclaved penicillin Elan at al (10) administered the antibiotics both orally and parenterally. These workers found that parenteral penicillin had no effect on the fecal microflora, autoclaved penicillin had little effect and bacitracin when orally or parenterally administered failed to have any effect on the fecal flora, yet all increased growth. These investigators stated that the antibiotics may have stimulated growth in some other manner than altering the number of intestinal organisms, and surmised that “the antibiotic molecule or a fragment of the same might not as a metabolite within the body of the bird.” Dixon and Thayer (S) obtained equally good or better growth response with intra-muscular injection of anreomycin and penicillin as the same antibiotics fed orally. In cases with the injected antibiotics the cecal microflora did not change from the control lot. From this they concluded that functional cecal organisms were not essential for growth promoting action of the antibiotics. That the antibiotics may possibly alter the metabolic rate of some intestinal organisms was demonstrated by Anderson et al (3). Working with the Warburg respirometer they showed that “penicillia enhanced the oxygen uptake of lactobacilli and aciduric type organisms.”

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South Dakota State University


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