Abstract
Mice of two strains (a laboratory strain of white mouse and a fancy strain of coloured mouse) were fed on a proprietary cubed diet and were given to drink tap water, or water containing 26 mg per litre of one of the following antibiotics: chortetracycline hydrochloride, ampicillin, or tylosin tartrate. In the first experiment growing mice were given these diets for 8 weeks from 4 weeks of age. Ampicillin produced weight increases in both strains (significant in the white mice), chlortetracycline produced an increase in the coloured mice and a reduction in the white mice, while tylosin reduced weight increases in both strains. The results of fortnightly weighings of the control mice and those receiving ampicillin showed that in the coloured mice the effect was marked at an early age, while in the white mice it tended to spread over the growing period. In a second experiment with breeding mice, using does of both strains, ampicillin increased litter size in both strains (significantly in the case of the coloured mice), chlortetracycline increased litter size in the white mice only, and tylosin did so in the coloured mice only. All three antibiotics increased litter birth weights in coloured mice (ampicillin significantly), but reduced them in white mice. Weight increases from birth to weaning of selected groups of 8 mice showed that antibiotics had little effect in the white mice, and that ampicillin and tylosin improved the rate of gain in the coloured mice (ampicillin significantly).