Antibiotics as growth promotant was discovered in the 1940s, when it was observed that chicks improve in growth when fed bacterial shells of Streptomyces aureofaciens from which antibiotics had been extracted. Because the amount of antibiotic that can provide growth enhancement was extremely small, the effect was regarded as largely nutritional by producers and authorities in the food industry16. In the years to follow, other countries also allowed the use of antibiotics in animal feeds. Subsequently, however, when the emergence of antibiotic resistance was recognized as an increasing risk, the use of growth promoters became the focus of numerous regulatory interventions, and bans on growth promotants were often enacted on particular classes of antibiotics. To date, different countries have different lists of approved and banned growth promoter antibiotics in their respective livestock industries.
HOW DO SUBTHERAPEUTIC LEVELS OF ANTIBIOTICS PROMOTE GROWTH?
Although repeatedly proven in various studies, the mechanism of action for the enhancement of growth of subtherapeutic levels of antibiotics remains unclear. Among the hypotheses tested are the following:
- Stimulation of intestinal synthesis of vitamins by bacteria.
- Reduction in total numbers of bacteria in the intestinal tract with a lowering of competition between microorganisms and host animals for nutrients.
- Inhibition of harmful bacteria which may be mildly pathogenic or toxin-producing.
- Inhibition of bacterial urease.
- Improved energy efficiency of the gut.
- Inhibition of bacterial cholytaurin hydrolase activity.
- Nutrient sparing.
- Improved nutrient absorption from morphological changes to small intestinal epithelium.
- Modification of intestinal enzyme activity.
- Reduced immune stimulation.
- Modification of rumen microbial metabolism.
(From Giguere et al., 2006)