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To avoid poultry from different types of pathogens mainly antibiotics are used in past, because these antibiotics act as antimicrobial agent. These antibiotics also improve the digestion due to the effect on intestinal micro flora. But now day's antibiotics are replaced by organic acids as individual acid or blend of acids because they also have antimicrobial activity. The antimicrobial activity of organic acids is pH dependent. When organic acid containing diet fed to the bird`s then acid intolerance species such as E.coli, salmonella, and campylobacter diminish. 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. By the addition of these organic acids pH of digesta decreases, increase pancreatic secretion and also have effect on intestinal mucosa. Organic acids have benefits 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. Number of experiments conducted to check the effects of organic acids on growth, FCR, meat yield and carcass, so the researchers concluded that acidifiers also have positive effect on the above said parameters.
Antibiotics used against microbes earlier but now a days it is banned in January 2006, so alternatives to antibiotics are of great interest (Waldroup et al.,2003). Alternatives include acidifier, probiotics, prebiotics, enzymes, herbal products and immune-modulators. Organic acid strongly act as a salmonella control agent in both feed and water (Ricke, 2003; Yang et al., 2007; Van Immerseel et al., 2005; Gunal et al., 2006).
Certain studies shows that the use of acidifier in broiler diet improve the bird`s growth performance, reduce chance of diseases and also reduce management problems and improve F.C.R (Valdemirolla and Sourdjiyska, 1996; Runko et al., 1997; Jin et al., 1998; Gunal et al., 2006; Islam et al., 2008; Ao et al., 2009; Bolling et al., 2000; Esteiveet al., 1997; Jin et al., 1998; Denil et al., 2003; Afsharmanesh and Pourreza, 2005; Nezhad et al., 2007; Moghadam et al., 2006; Abdel-Fattah et al., 2008).
Organic acid increase the lactobacilli population which improve the digestion of protein and energy (Runho et al., 1997). Microbes compete with the host for nutrient, so nutrient absorption increases by reducing the microbial population with the help of acidifier, so the digestion of protein and energy improve (Dibner and buttin, 2002; Kirchgessener and Roth, 1988). Benefits of organic acids depend upon the form of administered organic acid (protected or unprotected), uncontrolled variables such as buffering capacity of ingredients, presence of any other microbial agent, cleanliness of production environment, and heterogeneity of microbes.
Antimicrobial effects changes from one acid to another and is dependent on concentration and pH (Chaveerach et al., 2002).
Organic acid reduces the colonization of microbs on intestinal wall, thus preventing damage to the epithelial cells (Langhout.2000; Green and Sainsbury, 2001; Deitch et al., 1995; Denli et al., 2003; Naidu, 2000; Chaveerach et al., 2004; Adams, 2004; Samik et al., 2007).
Microbes of small intestine reduce the digestion of fat and fat soluble vitamins due to the deconjugating effects of bile acids (Engberg et al., 2000).
Earlier short chain acids used against pathogenic bacteria in food products (Cudjoe and Kapperud, 1991; Van Netten et al., 1994).
All oil seeds and cereal grains are rich in P content but in phytate for which is unavailable to the poultry birds. The availability of P increases by the use of phytase (Edward, 1993; Biehl et al., 1993; Biehl and Baker, 1996; Gordo and Roland, Sir., 1997), citric acid (Bolinget al, 2000b; Boling-Frankenbach et al, 2001; Rafacz et al., 2003; Snow et al., 2004; Brenes et al., 2003;
Liem et al., 2008) and vitamin D compounds (Edward, 1993; Biehl et al., 1993; Biehl and Baker, 1996; Snow et al., 2004).
Recent studies showed that citric acid degrades aflatoxins in the ration (Méndez- Albores et al., 2007; Méndez-Albores et al., 2009).
Benzoic acid and its salts are both act as common food preservatives and as feed additives in fur animals (Polonen et al., 2000).
Organic acids in the chicken intestine may contribute a certain amount of energy to the host bird (Jamroz et al., 2002).
Organic acids have been used for more than 30 years against bacterial and fungal destruction of feedstuffs (Giesen, 2005; Freitag, 2007).
Thus the main objective of the addition of organic acids is to improve protein digestibility, growth rate, meat yield, FCR, immune system and also act as antimicrobial agents.
Mechanism of Antimicrobial Activity of Organic Acids
In the absence of oxygen which is act as terminal electron acceptor fermentative bacteria produce organic acids, but they are differ according to the type's acids production. Because the oxidation and reduction of molecules must be coupled, anaerobic bacteria may produce several acids. The simplest process of fermentation is the conversion of sugar to lactate, and many lactobacilli, streptococci, lactococci and enterococci in the presence of sugar. In the scarce condition of sugar these bacteria are also capable to carry on the fermentation process that produces acetate, formate and ethanol, so ATP production can be enhanced (Hsiao & Siebert, 1999). Those bacteria which produce butyrate typically utilize the butyrate hydrogenases as a mechanism of reducing equivalent disposal. In the presence of hydrogenase the transfer of interspecies hydrogen to a methanogen decreases the need for dehydrogenase activity so the acetic acid production typically is increased (Russell & Diez-Gonzalez, 1998). In the stagnant environment bacteria can also utilize fatty acids, but the growth of these bacteria is very slowly, and fermentative environments are typically acidic.
How do organic acids work?
The antibacterial activity of organic acids is related to the reduction of pH, as well as their ability to dissociate (Cherrington et al., 1991 and Russell, 1992). In the undissociated form organic acids are lipid soluble in which they are able to enter microbial cell. Once an acid enter into the cell releases the proton in more alkaline environment, reduces the intracellular pH. This influences microbial metabolism inhibiting the action of important microbial enzymes and forces the bacterial cell to use energy to release protons, leading to an intracellular accumulation of acid anions. This totally depends on the pH gradient. The acid anion seems to be very important for the antibacterial effect of acidifier and their salts. Organic acids can also act as antibacterial without significantly decreasing the pH in GIT. Lactic acid bacteria are more resistant to organic acids due to their growth at relatively low pH ((Russell & Diez-Gonzalez, 1998). As carbon chain and concentration increases the antibacterial effect also increases.
Where do organic acids work?
Organic acids can act as antimicrobial agent both in the feed and in the GIT of the bird. Pathogenic bacteria e. g. Salmonella enters into the GI-tract through crop. The environment of the crop according to the microbial composition and pH seems to be very important in relation to the resistance to the pathogenic bacteria. High amounts of lactobacilli and low level of pH in the crop decreases the entrance of Salmonella in the crop (Hinton et al., 2000). The antibacterial effect of dietary organic acids in chickens is mainly take place in the upper part of the digestive tract (crop and gizzard) (Thompson and Hinton, 1997).
Effect on mold growth
Virtually all animal feeds contain molds and their spores. The favorable conditions for mold include moisture level more than 12% warm temp., presence of O2 and prolonged storage time. Mostly molds are toxigenic and produced mycotoxins. These mycotoxins cause significant in feed ingredients. Several management tools such as mold inhibitors and drying of grains are available to control mold contamination. Organic acids are also used against mold growth such as acetic acid, propionic acid, or blends of acids. The relative efficacy of these acids at various concentrations is also valuable (Pelhate, 1973).
Effect in growth
By simple diffusion acidifiers enter into bacteria, decreasing the pH of the cells which causes the bacteria to divert metabolic energy from growth and multiplication and can eventually result in cell death. This effect is specific to certain organic acids and is pH-dependent because organic acids have a specific antibacterial effect at a low pH which may help to reduce overall bacterial numbers or modify bacterial species distribution in the gut after passing through the GI tract. This ultimately benefits the birds by improving their health status and also nutritive value of the diet (Dibner and Buttin 2002).
Effective for intestine histomorphology
Organic acids in nondissociated form can penetrate in the cell wall of bacteria and disrupt the normal physiology of certain bacteria. Organic acids also reduce the pH of digesta, increase pancreatic secretion and also have effect on mucosa of GIT. Acidification with various organic acids reduce the production of various toxic agents by bacteria and colonization of pathogens on intestinal wall, thus preventing damage to epithelial cells and also improve the digestibility of proteins, Calcium, phosphorous, zinc and magnesium. Organic acids also act as substrate in the intermediary metabolism (Denli et al., 2003).
Effective against heat stress
High ambient temperature causes significant economic losses in the broiler industry due to decreased body weight, poor FCR and increasing mortality. Heat-stress causes respiratory alkalosis by decreasing the partial pressure of C02 in blood. Therefore, acidifiers used against the negative effects of heat stress and to improve broiler performance by altering acid-base balance. Furthermore, acidified water is expected to be more effective than dietary acidification, since organic acid intake is decreased depending on the reduction in feed consumption during heat stress.
Effective in absorption
The use of organic acid inhibits the growth of harmful bacteria, while beneficial lactic acid bacteria and digestive enzymes remain active instead of others. Lactic acid has a role in the absorption of vitamins D and K, and also helps in the formation of soluble salts of calcium and iron respectively. Citric acid is also very helpful in the utilization of phosphorous in chicks (Boling et al., 2000b).
ORGANIC ACIDS IN ANIMAL NUTRITION
Different organic acids are used in poultry diet which plays different role in bird`s body.
The effects of citric acid in broiler rations are very limited. When 2% citric acid added in feed then coliform bacteria increased in the small intestine (Vogt et al., 1981).The use of citric acid at dietary concentrations up to 1% in relation to the caecal colonisation with Salmonella typhimurium and carcass contamination following oral challenge. The number of birds colonised with Salmonella typhimurium was actually increased following supplementation with citric acid as compared to control, which indicates that citric acid may not be reliable with respect to the prevention of Salmonella colonisation of the caeca (Waldroup et al., 1995).
The inclusion level of 0.4% and 0.8% Luprosil-NC decreased the coliforms bacteria and
E. coli in the small intestine without any changing in intestinal pH. When 0.4% Luprosil-NC was added to the diet then a reduced number salmonella on post chill broiler carcasses observed (Izat et al., 1990).
Increasing amounts of fumaric acid (0.5, 1, 0 and 2.0%) did not offer protection from caecal Salmonella colonisation or carcass contamination following oral challenge with Salmonella typhimurium (Waldroup et al., 1995).Fumaric acid also did not offer protection from caecal Salmonella colonisation or carcass contamination following oral challenge with Salmonella typhimurium (Waldroup et al., 1995).
Formic acid alone or a combination of formic acid with propionic acid (68% formic acid and 20% propionic acid) at concentrations of 0.6% was effective with respect to the prevention of infection with Salmonella kedougou (Hinton and Linton, 1988) and Salmonella gallinarum. The addition of 0.36% calcium formate and 0.25% formic acid significantly reduced levels of Salmonella on prechill carcasses (Izat et al., 1990).
Benzoic acid at concentrations of 0.2% may have a positive influence on growth (Engberg, 2001).
Microbial adaptation to acids
Microbial adaption to acids is an important survival strategy for many prokaryotes and eukaryotes. This adaption is mainly due to the structural genes and specific tolerance mechanism. Inducible acid tolerance has been revealed in many gram-negative and gram-positive microorganisms. The unifying concept is that the microorganism under siege will sense a deteriorating environment and undergo a programmed molecular response by which specific, stress-inducible proteins are synthesized. These proteins presumably act to prevent or repair macromolecular damage caused by the stress. Some stress proteins are induced by a range of stress conditions, whereas others are induced in response to a specific stress (Bearson et al., 1997). Furthermore, different microorganisms have developed different survival strategy against acids (Lin et al., 1995). There is a correlation between pathogenicity and the response of enterobacteria to acid stress (Bearson et al., 1997). The presence of short chain fatty acids in GIT or in the diet might contribute to the enhancement of the virulence of S. typhimurium by increasing acid tolerance (Known and Ricke, 1998). The growth at pH 5 than pH 7 causes a significant change in membrane fatty acids, which is responsible for the adaption mechanism of Streptococcus mutants (Quivey et al., 2000). So, the changing in unsaturated/saturated ratio with lower growth pH is responsible for the membrane fatty acids composition.
Although bacteria under in vitro conditions are known to adapt to acids, it is not known whether this also occurs in GI-tract, when organic acids are fed to chickens.
By the addition of organic acids in the poultry diet, population of salmonella and other pathogenic bacteria decreases so these organic acids act as antimicrobial agents. Organic acids become more beneficial by using correctly along with nutritional, managerial and biosecurity measures. Short chain fatty acids also have specific anti-microbial activity which is pH dependent. Both organic acids and antibiotics improve the protein and energy digestibility, reduced pH of digesta, increase pancreatic secretion and also have tropic effects on intestinal mucosa. They also improve carcass yield, body weight gain and FCR. When the blend of organic acids (formic and propionic acids) are used then the performance of broiler improved because they are also act as growth promoters.