Use of organic acids as a model to study the impact of gut microflora on nutrition and metabolism

Approvals for the use of nontherapeutic antibiotics in animal feed are fast disappearing worldwide. The primary effect of antibiotics is antimicrobial; all of the digestibility and performance effects can by explained by their impact on the gastrointestinal microflora. Among the candidate replacements for antibiotics are organic acids, both individual acids and blends of several acids. Like antibiotics, short-chain organic acids also have a specific antimicrobial activity. Unlike antibiotics, the antimicrobial activity of organic acids is pH dependent. Organic acids have a clear and significant benefit in weanling piglets and have been observed to benefit poultry performance. Organic acids have antimicrobial activity; however, there appear to be effects of organic acids beyond those attributed to antimicrobial activity. Reductions in bacteria are associated with feeding organic acids, which are particularly effective against acid-intolerant species such as E. coli, Salmonella and Campylobacter. Both antibiotics and organic acids improve protein and energy digestibilities by reducing microbial competition with the host for nutrients and endogenous nitrogen losses, by lowering the incidence of subclinical infections and secretion of immune mediators, and by reducing production of ammonia and other growth-depressing microbial metabolites. Organic acids have several additional effects that go beyond those of antibiotics. These include reduction in digesta pH, increased pancreatic secretion, and trophic effects on the gastrointestinal mucosa. Much more is known about these effects in swine than in poultry. There appears to be more variability in detecting an organic acid benefit in comparison to that observed with antibiotics. Lack of consistency in demonstrating an organic acid benefit is related to uncontrolled variables such as buffering capacity of dietary ingredients, presence of other antimicrobial compounds, cleanliness of the production environment, and heterogeneity of gut microbiota. Additional research can clarify the role of these factors and how to minimize their impact.