NCS mice gained weight rapidly when fed a gluten diet deficient in several amino acids, but their weight gain on the same regimen was very much retarded if they were given antibacterial drugs, even for a short period of time. This retardation of growth could not be entirely corrected by supplementing the gluten diet with lysine and threonine.

The decrease in growth rate brought about by antibacterial drugs could probably be traced to the alteration in intestinal flora resulting from drug treatment. The intensity and duration of both types of changes were related to the dose of drug administered, and to the length of the treatment period.

Whatever the nutritional regimen, treatment with penicillin caused a retardation of weight gain in NCS mice. The retardation was more pronounced, and longer lasting, when the animals were fed semisynthetic regimens (containing casein or gluten) than when they were fed crude diets (pellets) containing natural materials of ill defined composition. These differences probably had their origin in the fact that the changes in fecal flora induced by the drugs were profoundly influenced by the composition of the diet.

Antibacterial drugs which retarded weight gain of Swiss NCS mice, in contrast increased weight gain in ordinary Swiss mice raised under usual conditions. It is probable that this difference in response to the antibacterial drugs resulted from the fact that ordinary Swiss mice have a much more complex intestinal flora than NCS animals.

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