Antibiotics as growth promotants: Mode of action

Recent concerns about the use of growth-promoting antibiotics in pig diets have renewed interest in the immunologic and growth-regulating functions of the gastrointestinal (GI) tract. The numerically dense and metabolically active microbiota of the pig GI tract represents a key focal point for such questions. The intestinal microbiota is viewed typically as a beneficial entity for the host. Intestinal bacteria provide both nutritional and defensive functions for their host. However, the host animal invests substantially in defensive efforts to first sequester gut microbes away from the epithelial surface, and second to quickly mount immune responses against those organisms that breach epithelial defenses. The impact of host responses to gut bacteria and their metabolic activities require special consideration when viewed in the context of pig production in which efficiency of animal growth is a primary objective. Here, we summarize the working hypothesis that antibiotics improve the efficiency of animal growth via their inhibition of the normal microbiota, leading to increased nutrient utilization and a reduction in the maintenance costs of the GI system. In addition, novel molecular ecology techniques are described that can serve as tools to uncover the relationship between intestinal microbiology and growth efficiency.