The Response of Chicks to Several Antibiotics in Different Diets and Environments

ANTIBIOTICS appear to exert their greatest effect with chicks reared in an “old” environment according to data reported by Coates et al. (1951), Hill et al. (1953), Lillie et al. (1953), Morrison et al. (1954) and others. Nevertheless Waibel et al. (1954) and Libby and Schaible (1955) noted that, with the continued feeding of antibiotics in a given environment, the response obtained gradually decreased. This would indicate that, in an “old” environment, chicks do not grow at a maximum rate (1) because of the presence of bacteria detrimental to the well-being of the chick or (2) because bacteria which might be beneficial to the chick are suppressed in a highly infected environment. It has been noted that the response to antibiotics in our laboratory was variable. At first it was surmised that the differences in response to antibiotics reflected small variations in the vitamin content of the diets fed. In …