The Mode of Growth Promotion by Antibiotics

Abstract

Historical developments leading to the use of antibacterial agents in animal feeds and the generally accepted concepts about their growth promoting action are reviewed. Growth-promoting concentrations of these agents modify the microflora or their products within the gastrointestinal lumen and probably in the host's immediate environment. Consistently demonstrable quantitative changes in populations of intestinal species have not been documented because these are predominately anaerobic and probably could not be quantitated by the culturing methods employed. The intimate relationships between the gastrointestinal mucosa and its anaerobes more recently shown by electron microscopy are summarized. During the 25 years since feed use of antibiotics was introduced, the average enhancement in rate of growth has remained relatively constant. Thus the mechanisms of growth promotion appear unrelated to factors causing antibiotic resistance or its transfer between microorganisms. Since antibacterial agents cause the gastrointestinal tract of animals to take on attributes of the germ-free state, comparisons of anatomic and other features of germ-free and conventional animals are stressed. These comparisons are discussed in their possible relation to nutrient requirements, postulated mechanisms of growth promotion, and the influence of antibacterial agents and other dietary factors on bacterial production of toxins from bile acids and nitrogenous substrates. Tentative estimatesof requirements for nutrients which microbial processes impose by enhancing the mass of intestinal tissues and the rate of cellular renewal are compared for germ-free and conventional animals. The biological properties of ammonia shown in studies with antibacterial agents, urease immunization, arginine deficient diets and exposure to gaseous ammonia are discussed in relation to the gastrointestinal and respiratory tracts, and the gaseous environment surrounding animals. The author cites over 220 reviews and original articles and has included concluding remarks which summarize his interpretation of the available evidence.