Antibiotics in Animal Nutrition

Abstract

Growth responses to the feeding of antibiotics have been observed in chicks, turkeys, rats, mice, rabbits, dogs, pigs, calves, and human infants. Various antibiotics are different in their effectiveness with different species. Arsonic acids and surface active agents also have some effect in increasing growth in a few species, but less than antibiotics, and with some differences in the action. As possible explanations of the antibiotics'' favorable effect on growth, the author reviews tests of their influence on the organism''s utilization or requirement of dietary B12, other water-soluble vitamins, fat-soluble vitamins, protein, and minerals; also their effect on hepatic and renal disorders, hormonal disturbances, and radiation injury. It is considered that the action of antibiotics in increasing growth is confined to its effect on the bacteria within the intestinal tract. This opinion is based on the following observations: (1) The effective antibiotics and chemotherapeutic agents are of widely varying chemical structure; the possibility of their being incorporated into any growth essential for the animals is thus precluded. (2) Antibiotics are ineffective in increasing growth in the germ free animal. (3) Aureomycin is ineffective in increasing growth of the developing chick embryo. (4) The effect of sanitation on the magnitude of the antibiotic growth response is the final evidence. Little is known of the actual mechanisms of growth stimulation. The ability of antibiotics to increase synthesis by the intestines of the known water-soluble vitamins is well established. These growth effects are observed only when diets are inadequate and do not account for the growth responses obtained in the presence of large excesses of the known vitamins. Evidence that antibiotics act by preventing the uptake of essential vitamins by intestinal bacteria is unconvincing. It appears that certain bacterial species prevalent in the intestinal tract are deleterious. Differences in responses to antibiotics observed at various laboratories reflect variations in the prevailing level of infection, and these are not eliminated by the customary sanitation practices employed in animal experimentation. Numerous studies have been made on the intestinal microflora in an attempt to correlate changes in the bacterial population with the antibiotic growth response, but definite conclusions cannot be drawn from the results of these studies. The morph. changes in the intestine of the chick wrought by feeding antibiotics resemble the effects of maintenance in germ-free conditions.