Indirect consequences of low-level use of antimicrobial agents in animal feeds

Antibiotics may be classified as "therapeutic" or "feed" depending on their use for treatment of disease or for promotion of growth in animals. The mechanism of growth promotion is largely unknown, but it has been shown that the specific feed additive causes defined lesions in the cell wall of Escherichia coli. This effect may sensitize the bacteria to the action of therapeutic drugs. Damage to the bacterial cell wall may also cause the bacteria to be more susceptible to the body defense mechanisms. One form of stress may be defined in terms of a disturbance of intestinal flora brought on by relocation, group mixing, change of diet, or travel. Prolonged feeding of antimicrobial agents to animals has not been shown to be responsible for the current high level of R factors in the intestinal tract of man.