Alternatives to in-feed antibiotics in pigs: Evaluation of probiotics, zinc or organic acids as protective agents for the intestinal mucosa. A comparison of in vitro and in vivo results

Weaned pigs are susceptible to frequent infectious diseases. Antibiotics have been used over decades to reduce pathogen infections, but many microorganisms are becoming resistant to antibiotics. Thus, there is an urgent need to find alternatives to in-feed antibiotics. Up to now, various compounds have been used as alternatives, giving promising but sometimes contrasting results. In recent years, the in vitro models of cell culture have been proposed for the screening of various compounds and understanding their mechanisms of action. The intestine is a complex system with a continuous cross-talk among epithelial cells, the local immune system and microflora. Alternatives to in-feed antibiotics may interact with any of these constituents. This review presents data on the protective effects of probiotics, zinc salts and organic acids, used as alternatives to in-feed antibiotics, against pathogen induced damage and inflammatory processes in the intestinal cells and the underlying mechanisms, with an attempt to compare in vitro and in vivo results. All the alternatives tested resulted in effective protection against pathogens by a series of interactions with intestinal cells and pathogens, including the inhibition of adhesion and invasion of pathogens, interference in signalling pathways induced by pathogenic bacteria, maintenance of the epithelial cell cytoskeleton and junctional structure and modulation of the host immune response.