A study was made of the effect of
antibiotics on growth of chicks and on intestinal absorption of
fats and
carbohydrates. Around the 8th day of life, chicks fed an
antibiotic-free
casein-
sucrose diet developed a transitory syndrome of malabsorption of
fats and
carbohydrates, associated with disturbance of the efficiency of feed utilization and poor weight increase. Administration of
virginiamycin, at a level of 20 ppm, suppressed this period of malabsorption and resulted in improved feed conversion and increased
weight gain. The temporary growth depression and malabsorption were not observed in disinfected rooms in new quarters. Under these conditions
virginiamycin did not stimulate growth nor was the efficiency of feed utilization improved by the
antibiotic. However, the growth-depressing flora could be introduced to the new quarters by feeding each bird 50 mg of fresh feces collected from chicks in old quarters. Both the intestinal absorption and the growth-promoting effect of
virginiamycin were influenced by the type of
carbohydrate in the basal diet, and have been found to be most pronounced when
sucrose was fed as the sole source of
carbohydrate. The malabsorption was less obvious when
cornstarch was substituted for
sucrose. In this case
virginiamycin had only a limited effect on growth and on feed conversion. The present investigations suggest that
antibiotics stimulate growth of chicks by their antibacterial action against Gram-positive microorganisms which interfere with the absorption of nutrients. Furthermore, the growth-promoting effect seems to be most pronounced during a limited period of a few days around the 8th day of life.