Since January 2006, the use of antibiotic growth promoters (AGPs) in feed for food producing animals has been banned in the EU. This has compromised not only animal performance, but it has also made ensuring food safety more difficult.
Salmonellas have been recognised as important pathogens and Salmonella enteritidis (primarily in birds) and Salmonella typhimurium (primarily in pigs) have accounted for the majority of cases of human salmonellosis.
Furthermore, the ban on AGPs has also lead to various digestive disorders and the occurrence of intestinal bacterial dysbiosis, an unbalanced bacterial flora in the intestinal tract. This may be further exacerbated by heavily contaminated feeds, stress, medicines, poorly digestible feed ingredients and unbalanced diets, and may result in functional disturbances, such as constipation or diarrhea, vitamin deficiencies (vit.B12, vit.K) or chronic inflammatory diseases. This may also compromise a proper functioning of the immune system through the lymphoid tissues in the intestinal mucus.
The concept of a eubiotic approach is to overcome these digestive disorders by ensuring a healthy gastro-intestinal microflora, rather then killing the entire intestinal microflora with antibiotics.
Necrotic enteritis
The use of AGP’s in feed minimized disease outbreaks. Since they have been banned, enteric diseases, such as necrotic enteritis caused by Clostridium perfringens, have increased in poultry flocks.
C perfringens is a normal member of the microbial community in the gastro-intestinal tract of animals and humans, particularly in the hind-gut. It is suggested that colonization of poultry by C perfringens is an early event in the life of birds, and can be transmitted within the integrated broiler chicken operation, starting at the hatchery. Conditions that promote excessive growth of C perfringens in the chicken upper intestine are the typical digestive disorders caused by stress, excessive use of antibiotics, and a sub-acute status of coccidiosis.
In particular, Eimeria species that colonize the small intestine, such as Eimeria maxima and Eimeria acervulina, are known to predispose to necrotic enteritis. The use of high protein diets or diets high in non-starch polysaccharides derived from small grains, also strongly influences necrotic enteritis incidence in broilers.
The disease occurs mostly in broiler chicks starting at the age of two weeks, and the acute form leads to increased mortality in broiler flocks. This can account for 1% losses per day, for several consecutive days during the last weeks of the rearing period. In the sub clinical form, damage to the intestinal mucosa caused by C perfringens leads to decreased digestion and absorption, reduced weight gain and an increased feed conversion ratio.
Sub clinical necrotic enteritis is also reported in countries where AGPs are still in use. From our own experience (broiler trial, Australia, 2009), we noticed that virginiamycin at 40 ppm in the starter period gave a numerical advantage, however, there was no (further) effect on animal performance using virginiamycin at 20 ppm after the second week of production when C perfringens usually starts colonizing the small intestines (see Table). Therefore, it can be suggested that, during the period when C perfringens becomes invasive, virginiamycin at low dosage is no longer effective.
Use of organic acids
The use of organic acids and salts is a long standing concept. Their antimicrobial action is mostly due to the pH changes of the environment in which the microorganism occurs.
All microorganisms need optimum conditions for their growth, including an optimum pH level. Bacteria are known to prefer a pH near to neutral values (pH 6.5-7.5)), yeast prefer lower pH values and moulds have the widest range of acceptable pH. Various experiments have revealed the efficacy of organic acids, to successfully replace AGPs without a significant reduction in performance. Formic-, propionic-, lactic-, acetic-, fumaric- and citric acids emerge as prospective growth promoters, but more consistent and cost-effective performance can be obtained through acid combinations.
The use of organic acids to acidify swine diets is a common practice to reduce post-weaning diarrhea in piglets. However, the use of organic acids and their salts in the poultry industry is still at an early stage. A proper application of organic acids in poultry production must improve protein digestion and nutrient retention, resulting in a better feed efficiency and increased growth and, ultimately, a more cost-effective production of meat and eggs.
Organic acids can demonstrate eubiotic effects in various ways. Dissociated organic acids release protons that result in a lowering of the pH. Most pathogenic microorganism (Salmonella, E coli) stop growing at a pH lower than 4.5, except for lactic acid producing bacteria (Lactobacilli, Streptococci, Bifido). The organic acid anions, when dissociated, are charged and not lipid permeable. However, some have a destructive effect on the outer membrane of gram-negative bacteria. These gram-negative bacteria have an outer cell membrane consisting of a lipopolysaccharide (LPS) surface stabilized by calcium and magnesium ions. This membrane provides an effective barrier against a variety of antimicrobials.
Molecules that can disrupt the integrity of this membrane are termed permeabilizers. Typically, sorbic-, lactic- and citric acids are strong chelators and interfere with these metals from the cell membrane and increase the permeability of the LPS-layer of these gram-negative bacteria, allowing a better diffusion of the other organic acids or even provoking leakages. It is recommended to use those permeabilizers along with organic acids as this will increase their effectiveness. Typically, this is the combined use of organic acids with lactic acid. In addition to its antimicrobial property due to the lowering of the pH, lactic acid also functions as a permeabilizer of the gram-negative bacterial outer membrane and may act as a potentiator of the effects of other organic acids.
Undissociated organic acids have a lipophilic character and have the ability to pass through the cell membrane of gram negative bacteria and enter the microbial cell. There they will dissociate and lower the pH of the plasma. The cell must eliminate the H+ across the cell membrane to restore the pH gradient (proton motif force). This requires large amounts of energy and will lead to the death of the cell.
At a low pH, more of the organic acid will be in the undissociated form. Consequently, antimicrobial activity of organic acids is indisputable at low pH (gastric environment), but uncertain at pH above 6 (intestinal environment).
Eubiotic concept of SCFA enriched in lauric acid
The antibacterial activity of Short Chain Fatty Acids (SCFAs) in an acidic environment against gram-negative pathogens has been long known. The most prominent natural fats rich in Medium Chain Fatty Acids (MCFAs) are coconut oil and palm kernel oil with less than 10% C8 (caprylic acid) and C10 (capric acid), but high proportions of C12 (lauric acid) and C14 (myristic acid). The pKa’s of MCFAs are close to 5 and are therefore more suitable to be active in the intestines than SCFAs. Previous studies have shown that lauric acid (C12) is the most inhibitory saturated MCFA against gram-positive organisms and has the highest antimicrobial activity against C perfringens, followed by myristic and capric acid.
The mode of action of MCFAs is not fully understood. In their undissociated form they may diffuse into the bacterial cells and dissociate within the protoplasm, thereby leading to intracellular acidification.
Another possible mechanism of action may be a physical or functional alteration of the gastrointestinal colonization site of pathogens in chicks. But MCFAs may also alter the outer membranes of bacteria as they are used for incorporation in the membrane as phospholipids leading to membrane fluidity and even leakages. As such, they may also prevent colonization, or they may have a direct inhibitory effect on the expression of virulence factors necessary for colonization. MCFAs have been shown to decrease Salmonella invasion in intestinal epithelial cells.
It is expected to see an added effect when combining SCFAs and their salts with MCFAs. This was confirmed in a broiler trial in an independent research institute in Poland, where this combination resulted in significant (p<0.05) performance improvements. Moreover, another broiler trial in Australia showed that this combination can be used as an alternative to antimicrobial growth promoters.
