Introduction
The benefits of antibiotics used as growth promoters(AGPs) were discovered during the
1940’s and have largely contributed to the rapid expansion of the poultry industry worldwide. The benefits of using antibiotic growth promoters include lower production cost leading to lower market prices and a more plentiful supply of food commodities. The AGPs function by modifying the intestinal micro flora targeting mostly the gram-positive organisms responsible for many of the health problems observed in poultry and other farm animals. The basic mode of action of these products involves direct inhibition of the microbiota growth which not only increases the net supply of nutrients to the host but also alleviates the negative effects of the metabolites produced by these bacteria many of which may be potential pathogens. The response to such products, however, is variable and may, to a large extent, be dependent upon the environment in which the animals are raised, and the diet offered to them. Whatsoever the response elicited may be, the primary objective of using the AGPs in diets of food producing animals was to keep the microbial load as low as possible to minimise the challenge while reducing the reliance on the “therapeutic usage of antibiotics” for keeping the flock healthy and this logic still holds enough validity provided it can be ensured that the antibiotics used to promote growth of the animals do not lead to residual effects in the final product entering the human food chain.
Why gut acting growth promoters are so important?
The small intestine of the newly hatched chick is sterile. Bacteria from the environment, the mother, and the diet begin to colonize the GI tract almost immediately and their numbers increase in a logarithmic scale. Aerobic and facultative anaerobes including Escherichia coli, lactobacilli, and streptococci colonize first which by reducing the local environment facilitates the establishment of the obligate anaerobes like the Bacteroides, Bifidobacterium, and Clostridium. The animal prefer to select only those bacteria which serve to protect them from the pathogens. In poultry, Enterococci and Lactobacilli are the dominant species in the crop, duodenum, and ileum during the first week of life, whereas coliforms, Enterococci, and Lactobacilli are present in high numbers in the ceca. Then a highly complex group of mostly obligate anaerobes begins to take over the ceca, whereas lactobacilli take over the crop, duodenum, and ileum. The intestinal microflora are established and stable by the third week of life.
The benefits and costs of the intestinal microflora
An established micro flora resist colonization by pathogenic and other non-indigenous microbes, a phenomenon also known as competitive exclusion and stimulates development of intestinal host defences, including the mucus layer; the epithelial monolayer; and the laminapropria. The sub-mucus layer is extremely important as all products aimed at either growth promotion of the animals or to prevent and treat any disease related to the intestinal health target this area first. The commensal bacteria present in chicken gut produce short chain fatty acids such as acetate, propionate and butyrate which contribute significantly to the energy supply of the animals. These short chain fatty acids also control the number of the undesirable bacteria in the caecum and by improving the villi structure increase the absorptive surface area of the gut.
Though apparently beneficial, the intestinal microflora sometimes costs a lot to the host animal too. The process of attachment of any bacterial species to the mucosal layer of the host animal elicits an immune reaction. The antibodies, cytotoxic and helper T cells and phagocytic cells combat not only the pathogens but also interact with the beneficial bacteria. Being within the physiological limit this response, which is nothing but an inflammatory process, does not cause any apparent harm, but an overt reaction which might result from an uncontrolled growth of the so called beneficial bacterial species eat on the nutrients the animal consumes to produce enough protective inflammation and the result is a suppressed growth and feed efficiency. Additionally, the presence of the microflora in numbers anything in excess of what the host system needs to protect itself takes a toll in the form of competition for nutrients and production of toxic amino acid catabolites, decreased fat digestibility, and the requirement for increased mucussecretion and gut epithelial cell turnover. The whole idea of the above description is to point out towards the fact that it is not always true to have an uncontrolled lot of microflora in the intestine even if the flora is not that much pathogenic and seemingly there is a need to restrain the growth of these inherent inhabitants of the gut.
Manipulating the intestinal microflora and moving away from the AGPs
Ideally, the goal should be to let the microflora grow up to a certain level which seems optimum for a given production system and then maintain the flora at that level. This can be achieved through dietary manipulations by adding certain additives which keep the inflammatory process within the physiological limit. This objective has so far been achieved quite successfully by the AGPs which could prevent the attachment of the bacteria, irrespective of their effects on animals in terms of pathogenicity, on the mucosal surface of the small intestine and probably had the concern of developing resistance against antibiotics been not raised then AGPs would probably be the best choice in yielding profits from any farm animal operation even today. As a matter of fact, the stand of banning the inclusion of the AGPs in the dietary regimens of the food producing animals forced the industry to drive all its research efforts to find out a solution about to combat a situation when virtually no antibiotics would be present in the dietary regime of the food animals with practically no change in the rearing system from what it is now.
Diet – the first control point in controlling intestinal microbial growth
Nutritionists often neglect the fact that the diet is a source of nutrients for the microflora as much as it is for the target animal. With an optimum digestion under normal condition the resident bacteria get little opportunity to have easy access to the rapidly digestible starch, protein, and other soluble nutrients. However, if digestion is compromised, more starch and protein reaches the lower gut which not only facilitates overgrowth of the local microbiota as a whole but also bring about a change in the pattern of species dominance. The basic strategy to prevent this from happening is to shift the process of digestion to the anterior gut and this can be achieved easily by usage of exogenous enzymes which facilitate release of the “caged” nutrients from the trapping factors (phytates, complex carbohydrates etc.) to make them available for digestion and absorption much before they reach the hind gut bacteria. The other strategy might be to activate the “ileal brake” to slow down the process of nutrient flow down through the gut so that they get exposed to digestive enzymes for a longer period of time and the amount reached to the hind gut bacteria gets reduced drastically. Using specifically designed fibres which can slow down the digestive passage rate without increasing the digesta viscosity might be one option that the nutritionists can take resort to.
Necrotic enteritis (NE), a major enteric disease of poultry, represents an ideal case where diet becomes one of the major contributing factors that predisposes the birds to enteric diseases. Diets formulated with cereals like wheat, barley, oats and rye which are high in non-starch polysaccharides (NSPs) help in proliferation of C. perfringens and predispose the birds to NE. However, corn-based diets, with comparatively lower amounts of soluble NSPs, do not favour NE to take place. Probably, the NSPs result in increased viscosity of digesta and a reduction of nutrient digestibility, thereby favouring the growth of mucolytic bacteria, like C. perfringens. Apart from the NSPs and cereal based diets, protein-based diets can also be one of the predisposing factors for NE, depending on the type of the protein present in diet. High level of animal proteins like fishmeal, meat meal or meat-bone meal in diet favours growth of C. perfringens. If the bird fails to digest these animal proteins completely, the undigested fraction, upon reaching the hind gut becomes the substrate for growth of C. perfringens owing to presence of certain amino acids like methionine and glycine. The condition gets exacerbated in presence of some pre-existing conditions like coccidiosis. The growth of C. perfringens may be favoured in the ileum of broiler chickens fed with animal fats.Animal fats with lower digestibility serves as a source of diacylglycerol in the gut which by accentuating the protein kinase pathway may induce inflammation and facilitate the α-toxin mediated pathogenicity of C. perfringens.
Future innovation
Substantial research has been conducted over the past few years to evaluate the potential of alternative antimicrobial agents for replacement of antibiotic growth promoters. Some natural compounds such as lactoferrin, lysozyme, bacteriocins and antimicrobial peptides appear to result in beneficial effects. Lactoferrin, isolated from bovine milk was evaluated as a potential feed additive in early weaned piglets and significant positive effects on performance parameters were observed at a 2000ppm inclusion level. However, due to its relatively high production costs, practical use in animal nutrition is currently not feasible. Lysozyme (1,4-beta-N-acetylmuramidase) is an enzyme exhibiting antibacterial properties. It is present at low concentrations in animal products such as milk, hen eggs and also in many tissues. Recently published results confirmed that dietary addition of lysozyme improved growth performance of young piglets and it could also be considered as an alternative to antibiotic growth promoters. Bacteriocins and antimicrobial proteins have also attracted attention as potential substitutes, but some regulatory issues, particularly in the EU, in addition to their high production costs are factors that might prevent their practical application in the near future.
The reality – it is too difficult to sustain productivity without antibiotics
Research works are ample to prove that in absence of antibiotic growth promoters it is possible to raise broilers with great efficiency. One of the easiest ways to achieve this target is to use therapeutic doses of antibiotics under prescription but this probably will increase the likelihood of the emergence of resistant human pathogens. As far as non-antibiotic measures are concerned any single strategy will not fully compensate for their removal and all the strategies discussed so far elicit their effect indirectly and hence slowly. This is likely to impact the production system especially in the underdeveloped or developing countries with less developed hygiene and production practices. Therefore, to ensure a dietary regime without antibiotics a stricter bio-security program and focussed vaccination strategy to be adopted along with prudent management practices are mandatory.
The final call
Neither food safety nor the profitability can be compromised.
Of the 20 most serious bacterial infections exhibiting problems with antimicrobial resistance in human medicine, 12 are in no possible way related to antimicrobial use in food-producing animals as these bacteria cannot be acquired via the food chain. For the rest 8, the chance of developing such resistance is < 1%.
Chickens raised for the organic market without antimicrobials have been shown to have a prevalence of Campylobacter almost three times greater than that of conventionally grown chickens.
By enhancing intestinal strength and minimizing gastrointestinal ruptures during evisceration and processing, and by reducing the shedding of the likely human pathogens such as Salmonella spp. and Campylobacter spp. the use of antimicrobial feed additives in animal feeds ultimately enhances the safety of the final product for the consumer.
So, a prudent decision has to be made while approaching a feeding regime free of antibiotics considering all the corners covering the feed manufacturing, bird husbandry, bio-security programs and vaccination strategies so that the possible areas of leakages are sealed the challenges imposed on the bird health is minimized.
The conclusive remarks
Complete withdrawal of antibiotics from the feeding regimens of poultry is possible if certain fundamental criteria related to hatchery hygiene, farm bio-security, feed milling technology and farm management issues are met with.
The occurrence of the inherent immune challenges even in absence of any enteric challenge per se cannot be ruled out and in an antibiotic free era the producer should be ready to face a wider variability in performance indices despite inclusion of some of the best alternative additives.
To keep the therapeutic usage of antibiotics at the minimum usage of some antibiotics with virtual ‘zero’ absorption from the gut may be considered.
If the authority is still not convinced, then a mandatory withdrawal of antibiotics for a period of 72 h may be the other way out.
A comprehensive strategy designing to save the poultry industry’s profit while ensuring the human food safety is the need of the hour.
by Dr Sudipto Haldar, Agrivet Research and Advisory
Source: Think Grain Think Feed
