The Mode of Action of Antibiotics in the Nutrition of the Dairy Calf. II. Effect of Aureomycin Administered Orally to Young Dairy Calves on the Sensitivity of Intestinal Bacteria to Phagocytosis

At least a part of the effect of antibiotics on growth is due to their effect on the physiology of bacteria in such a manner as to make the bacteria more susceptible to the body defense mechanisms and thereby less detrimental to their host.It is likely that under different environmental conditions certain species of bacteria may be more affected than others.Editor.In a preceding study, Radisson et al. (34) showed that standard bacteriological techniques alone are inadequate for determining the mode of action of certain antibiotics on the intestinal flora of young dairy calves.Also, evidence was presented that the ingestion of certain antibiotics may cause changes in the physiology of the bacteria of the intestinal tract.The most striking and consistent physiologic change is the increase in the resistance of intestinal bacteria to certain antibiotics following feeding of low levels of these antibiotics.Drug-fastness of the illtestinal microorgauisms has been reported by others (~, 9, 10, 11, 18, 35, 39).Such aH increase in resistance of bacteria to antibiotics has beeH considered one of the dangers of feeding antibiotics for a long period of time.IIowever, antibiotic-resistant organisms generally have a slower rate of metabolism and less virulence than do sensitive strains of these same organisms (33).Also, sensitive organisms have been reported to decrease in virulence after exposure to antibiotics in vitro and in vivo (7,8,13,17).In view of these considerations, an experiment was initiated to study the effects of aureomycin administered orally to young dairy calves on the virulence of intestinal bacteria as measured by the sensitivity of these bacteria to the host defense mechanism of phagocytosis.