Stimulation and Inhibition by Antibiotics of Intestinal Bacteria in Chicks

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Abstract
Chicks were fed antibiotics in a synthetic ration which contained limiting amounts of folic acid. The numbers of bacteria in the contents of different parts of the intestines of these chicks were determined at intervals. The feeding of the antibiotics increased the growth of the chicks. The increase in growth of the chicks was accompanied by changes in the numbers of coliform bacteria and lactobacilli in the intestinal tract. The antibiotics generally increased the numbers of coliform bacteria in all levels of the intestinal tract. Aureomycin was most effective in the upper levels of the tract; a combination of bacitracin and penicillin tended to be more effective in lower levels of the tract. In contrast to their effect on coliform bacteria, the antibiotics reduced the numbers of lactobacilli. The bacitracin and penicillin supplement virtually eliminated the lactobacilli in the contents of all levels of the tract. Aureomycin was less effective in depressing the numbers of lactobacilli. Thus, the antibiotics may have acted in these experiments to promote greater synthesis of growth factors by the coliform bacteria and to reduce the competition for folic acid or other essential compounds by the lactobacilli.