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Growth of Germ-Free and Conventional Chicks: Effect of Diet, Dietary Penicillin and Bacterial Environment¹

Germ-free Animal Research Unit, Walter Reed Army Institute of Research, Washington, D. C.

Chicks hatched and reared in the absence of bacteria and fungi grow 15 to 25% faster than conventional chicks on the same autoclaved casein-starch or soybean meal-corn diet. The increased growth rate is not due to improved feed efficiency but is related to the fact that the germ-free chicks eat more than the conventional chicks.

Better growth was obtained when potassium and calcium were provided in the diet in the form of potassium monophosphate and calcium carbonate rather than as calcium phosphate.

In repeated experiments over a two-year period using two different diets no growth response to dietary penicillin supplements was observed either in the germ-free or in the conventional chicks which were kept in an animal room where chicks had not been raised previously. A growth response to penicillin was obtained only after the animal room was deliberately "infected" with intestinal contents from chicks reared in premises where a growth response to antibiotics occurred regularly. The growth-promoting effect of dietary penicillin is shown to be due to a reversal of a transmissible growth-depression condition.

² Department of Microbiology, School of Medicine, University of Pennsylvania, Philadelphia, Pa. Present address: Department of Microbiology, Temple University School of Medicine, Philadelphia, Pa.

³ Present address: Department of Microbiology, Vanderbilt University School of Medicine, Nashville, Tennessee.

Manuscript received 9 July 1958.

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