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Growth Promotion

antibiotics for growth promotion copy.jpgAntibiotics 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:

  1. Stimulation of intestinal synthesis of vitamins by bacteria.
  2. Reduction in total numbers of bacteria in the intestinal tract with a lowering of competition between microorganisms and host animals for nutrients.
  3. Inhibition of harmful bacteria which may be mildly pathogenic or toxin-producing.
  4. Inhibition of bacterial urease.
  5. Improved energy efficiency of the gut.
  6. Inhibition of bacterial cholytaurin hydrolase activity.
  7. Nutrient sparing.
  8. Improved nutrient absorption from morphological changes to small intestinal epithelium.
  9. Modification of intestinal enzyme activity.
  10. Reduced immune stimulation.
  11. Modification of rumen microbial metabolism.


(From Giguere et al., 2006)

 

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Antimicrobial Growth Promotants

The use of subtherapeutic doses of antibiotics as growth promotants was yet another unintentional discovery.  In 1948, animal nutritionist Robert Stokstad and biochemist Thomas Jukes of the Lederle company, were then extensively working on a variety of vitamin B12 which was believed to be the “animal protein factor” that can enhance the growth of chickens.  Because the Lederle laboratories (the laboratory where the very first tetracycline, chlortetracycline, was discovered) uses vats of Streptomyces auerofaciens for the production of the antibiotic aureomycin, they utilized its cellular remains after the antibiotic had been extracted because they found that this contain substantial amounts of Vit B12.  They found that chicks receiving supplements of Streptomyces auerofaciens fermentation grow 24% more rapidly than those receiving liver extract, another source of this vitamin.  They later realized that this observed growth enhancement was not because of the vitamin, but due to the minimal residues of antibiotics left in the bacterial carcasses.  This opened a whole new market for antibiotics to the multi-billion dollar industry that it is today; a surprising offshoot from the vitamin research that had no direct investment return to the laboratory.