While the search for alternatives to antibiotic use in livestock can be traced back to the late 1960s, the issue accelerated in recent years and continued even as products were removed from the market. Adjustments following the withdrawal of these substances in livestock production have been sometimes difficult. While many replacement solutions have been proposed by the feed additive industry, it has not been easy to replace products that have proven to be generally effective for the last 50 years.
As consensus begins to develop among the scientific community on this subject, a few approaches stands out for efficacy, and technological and economical feasibility, particularly the use of organic acids and the use of essential or botanical oils.
While antibiotics were proven to be effective in improving poultry production, their use came under pressure as an increasing number of consumers feared their inclusion in animal feed rations would lead to antibiotic-resistant bacteria that are pathogenic to humans. Organic acids provide a natural alternative, reducing production of toxic components by bacteria and causing a change in the morphology of the intestinal wall that reduces colonization of pathogens, thus preventing damage to the epithelial cells (Langhout, 2000).
Organic acids as alternatives
Organic acids have been used successfully in pig production for more than 25 years and continue to be the alternative of choice. While much less work has been done in poultry, we can still confirm today that organic acids are very efficacious, provided their use is adapted to the physiology and anatomy of poultry. Organic acids (C1-C-7) are widely distributed in nature as normal constituents of plants or animal tissues. They are also formed through microbial fermentation of carbohydrates mainly in the large intestine, and are found in sodium, potassium or calcium form.
Mode of action
Over the years, it was thought that a pH reduction of the gastrointestinal tract (GIT content was the mode of action. But new research has proven differently. Research in the food preservation field has brought clear explanations on the mode of action of organic acids on bacteria and numerous trials have shown that the concept works both in pigs and poultry. The mode of action of organic acids on bacteria is related to:
Undissociated organic acids entering the bacterial cell.
Bacteria membrane disruption (leakage, transport mechanisms).
Inhibition of essential metabolic reactions (ex. glycolysis).
Stress on intracellular pH homeostasis (normal bacteria pH is ± neutral).
Accumulation of toxic anions.
Energy stress response to restore homeostasis.
Chelation as permeabilizing agent of outer membrane and zinc binding.
The key basic principle on the mode of action of organic acids on bacteria is that non-dissociated (non-ionized) organic acids can penetrate the bacteria cell wall and disrupt the normal physiology of certain types of bacteria that we call "pH-sensitive," meaning that they cannot tolerate a wide internal and external pH gradient. Included among those bacteria we have E. coli, Salmonella spp., C. perfringens, Listeria monocytogenes, Campylobacter spp.
Upon passive diffusion of organic acids into the bacteria, where the pH is near or above neutrality, the acids will dissociate and lower the bacteria internal pH, leading to situations that will impair or stop the growth of bacteria. At the same time, the anionic part of the organic acids that cannot escape the bacteria in its dissociated form will accumulate within the bacteria and disrupt many metabolic functions, leading to osmotic pressure increase incompatible with the survival of the bacteria.
It has been well-demonstrated that the state of the organic acids (undissociated or dissociated) is extremely important to define their capacity to inhibit the growth of bacteria. As a general rule, we need more than ten to twenty times the level of dissociated acids to reach the same inhibition of bacteria, compared to undissociated acids. Too often, "in vitro" assays showing the antibacterial capacity of organic acids are done at a low pH, making sure that the acids are not dissociated. At a pH below 3.0-3.5, almost all organic acids are very efficacious in controlling bacteria growth.
Organic acid selection
The antimicrobial activity of organic acids is related to reduction in pH and its ability to dissociate, which is determined by the pKa value of the respective acid and the pH of the surrounding environment. The organic acids are lipid soluble in undissociated form. The more the undissociated form of organic acids, the better the efficacy. Hence the selection of organic acid is very critical in determining the efficacy of the product. The undissociated form of the organic acid penetrates the cell membrane of bacteria. Inside the cell, the acid dissociates according to internal pH: acid<-> anion +H+ (proton). The H+ (proton) decreases pH in the cell. The bacteria use its energy resources trying to remove the protons and dies. Anions of organic acids deactivate RNA transferase enzyme, which damage the nucleic acid multiplication process and eventually result in death of the organism.
Effects of organic acids
Many authors have studied the effects of organic acids on animals, trying to find an explanation on their mode of action as a growth promoter. Their findings are more related to experiments in pigs but could be partially extrapolated to poultry. Among the explanations, some still believe that the GIT content pH change is important even if recent and not so recent publications are showing differently, with very high acid levels in the feed or water (Table 1). Many are underestimating the capacity of the animal to maintain its GIT environment homeostasis in order to warrant the normal functioning of all digestive functions. Also, the strong buffering capacity of the feed prevents any significant GIT pH modification. Logically, organic acids added to feeds should be protected to avoid their dissociation in the crop and in the intestine (high pH segments) and reach far into the GIT, where the bulk of the bacteria population is located.
More likely, the organic acids in poultry might play a direct role on the GIT bacteria population, reducing the level of some pathogenic bacteria (ex. C. perfringens) and mainly controlling the population of certain types of bacteria that compete with the birds for nutrients.
Performance with less worry
With the use of organic acids in poultry we expect an improvement in performance similar or better than the antibiotic growth promoters, without the public health concern, a preventive effect on intestinal problems like necrotic enteritis and a reduction of the carrier state for Salmonella spp. & Campylobacter spp.
Use of essential oils
Contrary to organic acids, a wealth of research has been done on the use of essential oils, herbs, and botanicals in poultry production, both as a growth promoter and a disease prevention product. The scientific and popular press uses a lot of different names (plant extracts, phytogenic additives, etc.), so to have a better understanding of what we are working with, here are some definitions:
Essential oils: Any of a class of volatile oils obtained from plants, possessing the odor and other characteristic properties of the plant, used chiefly in the manufacture of perfumes, flavors and pharmaceuticals (extracts after hydro distillation).
Herbs: A flowering plant whose stem above the ground does not become woody and persistent. A plant valued for its medical properties, flavor, scent or etc.
Botanicals: Drugs made of a plant, as from roots, leaves, barks etc.
Essential oils or plant extracts can be used as appetite stimulants, aroma, stimulants of saliva production, gastric and pancreatic juices production enhancer and antioxidant. However there is not a real clear demonstration of the impact of these factors on chicken performance.
The antibacterial property of essential oils is the most widely studied area, in human nutrition, food preservation and animal production. Because the control (modulation) of the GIT microflora is the most important aspect in replacing antibiotic growth promoters, we will concentrate on this aspect.
Differing structures and effects
Plants contain hundreds of substances having different properties but essential oils are composed mainly of nine groups (and many sub-groups) of molecules that are of interest to us (Table 2). There are many chemical constituents but no two oils are alike in their structure and effect. One must distinguish between non-purified plant extracts containing numerous different molecules interacting and pure active compounds, either extracted from plants or synthesized (nature identical). According to the plant chosen, one or more active compounds are dominant and the quantity found will differ according to factors like plant variety, soil, moisture, climate, time of harvest and other factors.
It is counter productive to test every plant that can have interesting properties. Concentrating on the active compounds and selecting the right plants or the right synthetic molecules is easier and will be more acceptable on a regulatory point of view.
Nutritionally, metabolically and toxicologically, we have a clear interest in using as low of levels of essential oils as possible in animal nutrition. Essential oils are extremely potent substances. They can lead to feed intake reduction, GIT microflora-disturbance, and accumulation in animal tissues and products.
Most essential oils are generally recognized as safe (GRAS) but they must be used properly and cautiously because they can be toxic (allergens), potent sensitizers and their odor and/or taste may contribute to feed refusal. They are also very volatile and will evaporate rapidly, leading to large variation in concentration in the finished products.
Actions of essential oils
It is extremely difficult to generalize on the mode of action of essential oils (EO) on bacteria and yeasts because each has different properties and each type of microorganism has a different sensitivity. Generally, Gram+ bacteria are considered more sensitive to EOs than Gram- bacteria because of their less complex membrane structure.
The consensus on the mode of action of EOs is that these compounds influence the biological membranes of bacteria. The cytoplasmic membrane of bacteria has two principal functions: barrier and energy transduction, which allow the membrane to form ion gradients that can be used to drive various processes; and formation of a matrix for membrane-embedded proteins influencing the ATP-synthase complex.
Essential oils work by: inhibiting the growth of the bacteria; sharply reducing intracellular ATP pool through a reduction of ATP synthesis and increased hydrolysis; education of the membrane potential, which is the driving force for ATP synthesis; the membrane becoming more permeable to protons; and reduction of the bacteria internal pH.
Technical problems with use
The use of organic acids and essential oils in the feed industry can potentially be a source of problems: corrosion, worker's safety, handling, vitamin stability in premixes, environmental concern, and stability of products.
It has been demonstrated that when both OA and EO are protected in a special matrix, the quantity required to achieve maximum performance in poultry can be reduced drastically. The active ingredients can be delivered into the intestine directly where the bulk of gastrointestinal bacteria are located. Without protection, organic acids are readily dissociated in the first part of the chicken GIT and are rendered useless. Essential oils are very rapidly absorbed in the duodenum and cannot interact with the microflora.
In summary, there is a general consensus on the efficacy of organic acids as the best alternative to antibiotic growth promoters. Essential oils also have an effect as a replacement of antibiotic growth promoters but they can act in synergy with organic acids, both for their growth promoting effect and prevention of specific intestinal diseases.