What they found is that the efficacy of acids against the coli- form bacteria follows this order: benzoic > fumaric > lactic > butyric > formic > propionic acid. The proliferation of most pH sensitive bacteria (E. coli, salmonella, and C. perfringens) is minimised below pH 5 while acid-tolerant ones survive. This means organic acids promote the proliferation of beneficial bacteria which are acid-tolerant such as the Lactobacillus sp. and Bifidobacterium sp., creating the preferred microbio- ta-balanced or eubiotic state in the gut. Based upon the envi- ronmental pH and pKa values, organic acids in their undisso- ciated form can diffuse across the bacterial cell membrane and dissociate inside the cell, releasing H+ ions and decreas- ing intracellular pH. This inhibits bacterial replication and growth, leading to bactericidal effects. The efficacy of organic acids varies depending on the target organism. Gram positive bacteria (e.g., C. perfringens, Ente- rococcus spp., Streptococcus spp.) are mainly susceptible to MCFA, while Gram-negative bacteria (e.g., E. coli, Campylo- bacter jejuni, Salmonella spp.) are more sensitive to SCFA. The lipophilic nature of MCFA allows them to have a strong- er antibacterial activity against Gram-positive species, whereas the presence of lipopolysaccharide in the Gram-negative cell wall confers resistance to these species. What is also observed is that SCFA in pigs increases the rela- tive mRNA expressions of intestinal development-related genes, including IGF-1, IGF-IR, GLP-2, and GLP-2R, indicating the benefits of SCFA on gut morphology and development. Taking an example, butyric acid was seen to promote intesti- nal epithelial integrity as measured through its effects on in- creasing the relative mRNA expression of tight junction – of- fering an intestinal barrier function.
Modulation of microflora Here the researchers further explained how organic acids modulate intestinal fermentation patterns. Taking for exam- ple, formic acid, the most studied; formic acid or its salts in- creases acetic acid and decreases lactic acid concentrations in both ileum and colon contents. This indicates a shift in the composition of intestinal flora and a modulation of microbial fermentations with more nutrients or metabolites (like ace- tate) available to the host. Based on their antimicrobial mode of action, organic acids can inhibit the growth of undesired pH-sensitive microorganisms like Enterobacteriaceae, while promoting the proliferation of beneficial bacteria tolerant to lower pH. In this view, organic acids can be an optimal tool to control the dysbiosis characterised by coliforms overgrowth and Lactobacilli depression, typical of weaning time in pigs. SCFA reduce the pH and promotes the proliferation of benefi- cial bacteria; thus, offering a probiotic effect in the colon. What the researchers also found is that, besides individual or- ganic acids, blends of organic acids including SCFA, MCFA, and acids salts increase the levels of acetic, propionic, and bu- tyric acid produced by microbial fermentation of carbohy- drates in the large intestine. Overall, organic acids have the potential to modulate the microflora populations and conse- quently microbial metabolites production along the GIT.
Digestive system
villus height pH of GIT
Pathogenic bacteria Gastric retention time Pancreatic secretion
Increase feed utilization Improve mineral absorption Improve nutrient digestibility
Immunoglobulins Immune organ weights WBCs
Antibody production Serum protein and albumin
Performance and health indices
Improve liver health Improve thyroid hormones Intermediates in citric acid cycle
Immune system
Enhance mineral utilization Increase weight gain Improve meat and carcass quality
stress
Antioxidant activity Antimicrobial activity Mitigate ammonia emission & urea excretion
Improve production of fatty acids and antioxidant capacity
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Stimulate immune system & enhance immune response
Improve productive and reproductive performances
▶ GUT HEALTH | DECEMBER 2020
Provision of energy Organic acids can have a metabolic role by improving the intestinal mucosa trophism and modulating general metabolism, particularly for SCFA. In the large intestine of pigs, SCFA is rapidly absorbed by the colonic epithelium to play a pivotal role in the intestinal and general energy metabolism. Butyric acid is almost completely oxidised within the mucosa, serving as preferred energy source for the colonocytes, whereas propionic acid is collected by the liver where it is converted into glucose, and acetic acid is instead used by peripheral tissues. Sorbic acid added to weaning piglets increases growth performance through the modulation of lipid metabolism and the enhancement of insulin-like growth factor system, particularly relevant for the GIT development. In addition, the researchers point out that pigs can utilise fumaric acid as energy source with an efficiency close to that of glucose. The researchers also gave an overview of the various applications of acidifiers in animal nutrition as highlighted in Figure 1.
Factors influencing efficacy Several factors influence the efficacy of acidifiers; factors such as physical and chemical form (acid, salt, coated or uncoated), odour and taste, solubility, pKa value, minimum inhibitory concentration, site of action, dietary composition, and feed buffering capacity. Coating, for example, ensures the slow release of acids throughout the GIT, and curb the effects of odour.
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