Abstract

The addition of certain antibiotics to the diet at low levels increases the rate of growth of young animals and the efficiency with which they utilize food as measured by the ratio between food intake and increase in body weight. This effect of antibiotics is not obtained in environments in which bacteria are absent, such as in the chick embryo or in "germ-free" chicks, and the effect may be reduced when the animals are kept under highly sanitary conditions. Evidently certain widely-distributed types of antibiotic-sensitive intestinal microorganisms interfere with the utilization of food. Aureomycin, terramycin, penicillin, streptomycin and bacitracin produce the "antibiotic growth effect" in chicks when added to the diet at levels as low as 1 to 20 parts per million. The effect with other species of animals depends on the antibiotic used; the "wide spectrum" antibiotics produce the most consistent effects over a range of species. A decrease in gut weight without a change in length has been reported to be associated with the antibiotic growth effect in chicks; Coates (50) has recorded a decrease of 18% in gut weight accompanying an increase of 9% in body weight. This gives rise to the speculation that a mild "early-sprue-like" condition, engendered by certain intestinal bacteria, may exist in so-called normal animals and that this condition is corrected by the dietary use of antibiotics which inhibit or eliminate the causative bacteria. Such a change might account for some of the "dietary" effects reported for antibiotics, including improvements in the utilization of food, sparing effects on the requirement for proteins and for certain minerals and vitamins, and anti-anemic effects under conditions which respond to folic acid. However, it is possible that other dietary additions may give rise to decreases in gut weight without increasing body weight (150a) The antibiotic growth effect may be obtained when the diet is supplemented with more than sufficient amounts of the known vitamins. The tissues of antibiotic-fed animals are of normal composition.

The practice of feeding antibiotics to farm animals is widespread. No untoward effects on public health have resulted; for example, detectable amounts of aureomycin are not found in meat even when the animals are fed antibiotics at levels far higher than usual; furthermore, many antibiotics are destroyed by cooking. In any event, the daily administration of small amounts of aureomycin for prolonged periods to children has not been found to have untoward effects. Resistant strains of pathogenic bacteria have not been reported to have made their appearance in animals as a result of feeding antibiotics. Reproduction in animals does not appear to be affected by feeding antibiotics, except that 1) hatchability of hen's eggs may be improved if the hens are on certain deficient diets, 2) the survival and early growth of young pigs and rabbits may be improved when an antibiotic is fed to the mother, perhaps because the number of pathogenic bacteria in the environment of the young animals is thus reduced.