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Effect of Antibiotics on Growth of the Immature Rat^1,2,

Departments of Medicine, Surgery and Pathology, Emory University School of Medicine, Atlanta, Georgia 30322

The purpose of this study was to quantify the growth-promoting effect of a mixture of antibiotics for rats eating diets deficient in protein or an essential amino acid. Male albino weanling rats (70 to 80 g weight, 4 weeks old) were fed (a) a control diet containing all other required nutrients and varying amounts of casein (0 to 27%), or (b) a purified amino acid diet containing all other required nutrients and varying amounts of valine (0 to 70 µmoles/g diet), threonine (0 to 69 µmoles/g diet) or tryptophan (0 to 8.6 µmoles/g diet), with and without an oral antibiotic supplement consisting of neomycin sulfate (10 mg/100 g body weight/day), bacitracin (500 units/100 g body weight/day), and polymyxin B sulfate (1 mg/100 g body weight/day). At suboptimal intake of casein, valine, tryptophan or threonine, rats eating antibiotic-enriched diet showed up to 3 times greater daily body weight gain (BW) than rats eating a similar diet without antibiotics. The growth-promoting effect of antibiotics can be expressed as percent sparing of specified nutrient (casein or individual amino acid), defined as below:
where nutrient intake_{0 ab} or nutrient intake_ab represents that intake of casein or of a particular amino acid which is required to produce a specific BW in antibiotic-free or antibiotic-supplemented group, respectively. The percent sparing was inversely proportional to the dietary content of casein or limiting amino acid. For diets containing 10% to 25%, 25% to 50%, 50% to 75%, and 75% to 100% of the daily requirement of the limiting nitrogenous nutrient, sparing on the average was >80%, 60%, 20%, and <10%, respectively. Data on daily food intake of ad libitum fed rats, and data from an experiment with tube-fed rats, showed that the growth-enhancing effect of antibiotics was independent of changes in food intake.

KEY WORDS: • antibiotics • protein • growth

¹ This investigation was supported by U.S.P.H.S. grants AM15736 and RR00039.

² Part of this study was presented at the Annual Meeting of the Federation of the American Societies for Experimental Biology, Anaheim, CA, April 1976. Federation Proc. 35, 742, 1976 (abstract).

Manuscript received 16 September 1975.