During the past 50 years, the poultry industry has progressed in several areas of nutrition, genetics, engineering, management and communications toward maximizing the efficiency of growth performance (weight for age and feed conversion) and meat yield. Today, however, the poultry industry must focus more attention toward addressing public concern for environmental and food safety.
As in many other industries, the global paradigm is shifting from an emphasis on efficiency to one of public security. Nothing demonstrates this paradigm shift more clearly than the issues concerning the use of antibiotic growth promotants.
For the past four decades, antibiotics have been added to poultry feed to improve growth performance and protect birds from the adverse effects of pathogenic and non-pathogenic enteric microorganisms.
Despite the abundance of scientific data presented on the benefits of antibiotics, there is limited information about how antibiotics promote growth, and specifically how they affect the physical attributes of the animal and the microbial populations that reside within the gastrointestinal tract. Now, antibiotics have come under increasing scrutiny by some scientists, consumers and government regulators because of the potential development of antibiotic-resistant human pathogenic bacteria after long use (Phillips, 1999; Ratcliff, 2000).
Consequently, the poultry industry must develop alternatives to antibiotic growth promotants, or at least substantially reduce the amount of antibiotics used to maintain efficient poultry production and produce safe poultry meat and egg products.
Some of these alternatives may include significant changes in husbandry practices or the strategic use of enteric microflora conditioners, such as directfed microbials, enzymes and novel carbohydrates.
A greater understanding of how sub-therapeutic levels of feed-grade antibiotics promote growth is necessary before these alternative strategies can be effectively employed. This paper will review the mechanisms of antibiotic action to promote growth in poultry and compare it to a promising non-pharmaceutical alternative, mannan oligosaccharide. We will concentrate this discussion on turkeys because they are particularly susceptible to enteric disease and the industry has limited options to use pharmaceutical enteric conditioners in comparison to the broiler industry. Most of this information is based on research recently conducted at North Carolina State University.Antibiotic growth promotants and mechanisms of action
Feeding sub-therapeutic levels of antibiotics to production farm animals has been practiced since the late 1940s and currently serves to significantly improve the economic profitability of poultry production. The value of antibiotics to the rearing of farm animals was initially realized when veterinarians first utilized penicillin left over from World War II casualty stocks for the intra-mammary treatment of bovine mastitis (Gustafson and Bowen, 1997). Moore et al. (1946) then reported an improvement in growth when the antibiotic streptomycin was fed to young chicks. However, the ultimate value of these observations was not realized until 1949 when Stokstad and co-workers inadvertently fed the fermentation by-products of chlortetracycline as a vitamin B12 source and managed to increase body weights and decrease feed consumption of their chickens. Antibiotics have been shown to improve the growth and feed efficiency of broilers (Woodward et al., 1988; Miles et al., 1984) and turkeys (Salmon and Stevens, 1990; Waibel et al., 1991), decrease flock variability (Miles and Harms, 1984), and increase the intestinal digestion and absorption of carbohydrates and fats (Eyssen and De Somer, 1963a,b).
The mechanisms by which antibiotics influence the gut microflora and growth performance of poultry are not fully understood, but there are several proposed modes of action. First, antibiotics control and limit the growth of microbes (Clostridium perfringens) known to be detrimental to poultry (Truscott and Al-Sheikhly, 1977). Second, growthpromoting antibiotics limit the growth and colonization of numerous non-pathogenic species of bacteria in the gut, including lactobacilli (penicillin), bifidobacteria (ampicillan), bacteroides (clindamycin) and enterococci (kanamycin) (Tannock, 1997), and this may reduce the production of antagonistic microbial metabolites, such as ammonia (Zimber and Visek, 1972), which adversely affect the physiology of the host animal.
Dietary inclusion of antibiotics also reduces weight and length of the intestines in poultry (Visek, 1978; Postma et al., 1999). Stutz et al. (1983) reported reduced amounts of lamina propria, lymphoid tissue, reticulo-endothelial cells, intestinal weight, and moisture in germ-free birds. Wostmann et al. (1960) compared penicillin-fed birds to germ-free birds and found that conventional birds consuming the antibiotic treatment had reduced amounts of ileal lamina propria and reticulo-endothelial components almost similar to levels seen in the germ-free birds. A thinner intestinal epithelium in germ-free or antibiotic-fed animals may enhance nutrient absorption (Visek, 1978) and reduce the metabolic demands of the gastrointestinal system. ‘Thinning’ of the gastrointestinal tract walls may be due to inhibition of microbial production of polyamines and volatile fatty acids, known to increase enterocyte turnover rate and activity.
This increased net energy committed to maintaining the luminal tissue comes at the expense of more productive purposes, such as muscle accretion (Bedford, 2000). The minimization of gastrointestinal bacteria may also ease the competition for vital nutrients between the bird and the microbes (Ferket, 1991). Finally, antibiotics may reduce the adverse effects of immunological stress on growth performance by lowering the enteric microbial load.
Over-stimulation of the host immune system by the resident microflora could impair the optimum growth and performance of the bird (Cook, 2000; Klasing, 1988).
Originally defined as biological substances produced naturally by one microorganism that inhibit the growth of other microorganisms, antibiotics now include those substances created synthetically to possess certain antimicrobial abilities. Antibiotics function by altering certain properties of bacterial cellular metabolism resulting in impaired growth or death. Some antibiotics interfere with the building and maintenance of the cell wall, while others interrupt proper protein translation at the ribosomal level. Because of their elevated rate of growth and proliferation, bacteria are vulnerable to antibiotics that target active cellular metabolism. Limiting the growth and proliferation of certain bacteria and inhibiting the production of various toxins restricts the influence that the microbe has upon the host organism. This enables the host to grow and perform better than if grown under normal challenge conditions.
The modern poultry industry relies heavily upon several distinct classes of antibiotics for growth promotion and therapeutic treatment, including the macrolides, fluoroquinolones, beta-lactams, glycopeptides and streptogramins among others. Poultry producers traditionally have utilized only a small number of antibiotics for sub-therapeutic growth promotion. Virginiamycin, a streptogramin, and bambermycins, a member of the flavophospholipol group, are two of the more commonly encountered antibiotics utilized in turkey feeds.
Virginiamycin (Stafac®, Pfizer Animal Health, Inc., Exton, PA) is a streptogramin antibiotic fed to poultry to increase rate of weight gain and improve the efficiency of feed conversion. Produced by a mutant of the microorganism Streptomyces virginiae (Cocito, 1979; Van Dijck, 1969), virginiamycin consists of two subunits (M and S) that act synergistically to inhibit protein synthesis by the 50S ribosomal subunit in Gram-positive bacteria (Parfait et al., 1978; Cocito, 1973; Cocito, 1979). The M unit is a polyunsaturated cyclic peptide usually found in greater concentrations than the cyclic hexadepsipeptide S subunit (Bycroft, 1977).
Both units function bacteriostatically when separated but operate as a bacteriocidal agent when joined as a single unit. Virginiamycin is active against a broad spectrum of Gram-positive gastrointestinal microorganisms including Clostridium, Streptococcus, Lactobacillus, Bifidobacterium and numerous other species. Currently, virginiamycin is approved for food animal use within the United States at an inclusion rate of 10 g to 20 g per ton of feed for turkeys (Feed Additive Compendium, 1999). However, bans have been enacted on the use of virginiamycin within the European Union since July, 1999 because of the possible development of antibiotic resistance in bacteria due to antibiotic feeding in animals destined for human consumption (Ratcliff, 2000).
Numerous studies have reported the benefits of feeding virginiamycin to poultry during certain disease challenge situations or normal grow-out conditions. Superior weight gains were observed in turkeys fed diets containing 22 ppm virginiamycin from 1 to 70 days of age as compared to birds fed diets without the antibiotic (Salmon and Stevens, 1990). Body weights and feed to gain ratios for male and female turkeys fed 22 ppm virginiamycin during multiple trials conducted between 1983 and 1987 were improved significantly over those of the control-fed birds (Waibel et al., 1991). Similar improvements in body weight gain and feed efficiency by feeding virginiamycin were reported by Buresh et al. (1986). Miles et al. (1984) reached similar conclusions with work in broiler chickens fed virginiamycin, while extensive data have been accumulated on the effects of virginiamycin on pathogen reduction in broilers. Virginiamycin has not been shown to improve livability during Salmonella typhimurium infection (Abou-Youssef et al., 1982), although it has been shown to significantly reduce mortality and lower intestinal lesion scores when fed to broilers infected with Clostridium perfringens (George et al., 1982).
Virginiamycin should not have been expected to decrease infection resistance to Salmonella, a Gram-negative organism, because it possesses mainly a Gram-positive spectrum of activity.
Bambermycins (Flavomycin®, Hoechst AG, Frankfurt, West Germany), previously known as moenomycins, are a common form of Gram-positive antibiotic growth promotant fed to turkeys.
Bambermycins are produced by the microorganism Streptomyces bambergiensis in aqueous nutrient medium under aerobic, submerged conditions (Glasby, 1992). Bambermycins are also produced, although less extensively, by S. ederensis, S. geysiriensis and S. ghanaensis. It is an amorphous, white powder structurally characterized by an acid polysaccharide containing a lipid portion and phosphate ester groups with a molecular weight of approximately 68,000 to 70,000 kilodaltons (Glasby, 1992). Bambermycins are actually a mixture of four active components belonging to a group of phosphorus-containing glycolipid antibiotics with an antimicrobial spectrum similar to that of penicillin (Waldroup et al., 1970). These compounds inhibit the cell wall synthesis by either irreversibly blocking the incorporation of the UDP-Nac-muramylpentapeptide molecule into the cell wall structure (Huber and Nesemann, 1968) or disturbing the cross-linking of the peptidoglycan structure itself.
Growth promotion data on poultry fed bambermycins is plentiful in the literature. Caston and Leeson (1992) reported heavier and superior weight gain in turkeys fed Flavomycin® although males were somewhat more receptive to this response than females. This study correlated the observed increase in body weight gain with an increase in feed consumption. Heavier final weights and eviscerated carcass weights were also associated with the feeding of Flavomycin® in turkeys by Leeson (1984a). Waldroup et al. (1970) reported that bambermycins increased the body weights and feed efficiency of male and female turkeys. General improvements have also been reported by a number of other researchers while feeding bambermycins to poultry (Salmon and Stevens, 1990; Firman and Kirn, 1989; Waldroup et al., 1985). Studies involving birds undergoing bacterial pathogen challenge while consuming a diet including bambermycins have proven less successful than its use for growth promotion.
Feeding bambermycins to broiler chickens had no effects on body weights, duration of Salmonella shedding, tissue recoverability of Salmonella, or total number of resistance patterns (George et al., 1982). However, bambermycin-fed broilers experimentally infected with pathogens showed reduced shedding of Clostridium perfringens and Salmonella enteritidis, but not Campylobacter jejuni (Bolder et al., 1999).Antibiotic resistance
The use of growth promoting antibiotics has been criticized by many scientists, politicians, the media and the general public for their possible role in the occurrence of antibiotic-resistant microbes.
Numerous reports have been issued concerning the effects of agriculture-related antibiotics on the emergence of antibiotic resistance in human pathogens (Swann, 1969; SCAN Report, 1999).
Although a complete ban on the use of subtherapeutic doses of antibiotics in animal feed has not yet been enforced in many countries, this day may eventually come. There is growing evidence that indicate the relationships and the risks involved with the use of growth promotant antibiotics in animal and poultry feeds and bacterial resistance in human disease therapy.
Public concern about antibiotics stems from the emergence of vancomycin-resistant enterococci during the early 1980s. Although the occurrence of vancomycin-resistant Staphylococcus was expected, the appearance of vanA resistance genes in enterococci was a new threat (Gustafson and Bowen, 1997). Vancomycin was introduced in 1958 and had become a major contributor to antibiotic therapy until resistant isolates began to show up shortly after widespread use became commonplace.
Vancomycin is seen as one of the last and strongest antibiotic weapons used for fighting persistent clinical pathogens. Researchers began to suspect that the use of glycopeptides (same family which includes vancomycin) in farm animals might be linked with the onset of resistance in humans. The specific vanA gene cluster that encodes for vancomycin resistance has been isolated from Enterococcus faecium in farm animals destined for human consumption (Bates et al., 1994; Klare et al., 1995). The vanA gene cluster encodes for 1) the alteration of the vancomycin binding site, 2) the removal of endogenous binding sites, and 3) the regulation of the expression of resistance genes (Mazel and Davies, 1998). However, contrasting studies relate antibiotic resistance build-up to the over-reliance on antibiotics in human medicine (Mathews, 2001). Two areas shown to harbor the greatest sources of resistant microbes in humans are hospitalization scenarios and the misuse of prescription drugs by doctors and patients alike (U. S. Congress, Office of Technology Assessment and Committee on Drug Use In Food Animals, 1995). Bernick (1999) estimates that as little as 10% of the problems observed with microbial antibiotic resistance originate from the use of antibiotics in livestock practices.
The antibiotic backlash began with the banning of the glycopeptide antibiotic avoparcin in Denmark in 1995, followed by a complete ban by the entire European Commission in April, 1997. This ban was proposed as a ‘precautionary principal’ because of possible avoparcin cross-resistance with vancomycin. In January 1998, Denmark began its own campaign to ban the use of virginiamycin. Virginiamycin is a member of the streptogramin antibiotic class and was banned because of the possibility of future cross-resistance problems affecting the efficacy of human streptogramins like quinupristin/ dalfopristin (Synercid ®, Pfizer Animal Health, 1999). In 1999, the EU Agriculture Commissioner Franz Fischler initiated a European Union-wide ban outlawing not only the use virginiamycin, but tylosin, spiramycin and zinc bacitracin as well, disregarding reports by his own scientific fact-finding committee (SCAN Report, 1999).
Wide use of antibiotic growth promotants in poultry is one reason the public is placing some blame for antibiotic resistance of potential pathogens on the poultry industry. This blame may be partly justifiable. Antibiotic resistance has been displayed by field Escherichia coli isolates from commercial turkey farms in North Carolina, including resistance to Enrofloxacin (Fairchild et al., 2001). Although there are no specific claims that growth promotant antibiotics control disease (Gustafson and Bowen, 1997), the debate over resistance seen among Gram-negative bacteria, such as E. coli and Salmonella, has generated the strongest objection to antibiotic use (Scioli et al., 1983; Gustafson and Bowen, 1997). It has been reported that antibiotic resistance of indigenous E. coli of poultry has remained at a relatively high level since the 1950s (Gustafson and Bowen, 1997).
The actions of the EU made clear the risks that animal producers face in the future regarding the use of antibiotic growth promoters. Trade with the European Union, along with the uncertain future of feeding antibiotics in the US poultry industry, makes it prudent to seek alternatives to antibiotics as growth promotants. The antibiotic ban necessitates the need for more study and investigation of alternative growth promotion therapies as well as a more intimate understanding of how current growth enhancers operate.Mannanoligosaccharides: A nonpharmaceutical alternative to antibiotic growth promotants
In contrast to the mode of action of most antibiotics and carbohydrate fermentation sources, mannan oligosaccharides (MOS) and possibly other oligosaccharides serve as alternate attachment sites for Gram-negative pathogens, thereby preventing attachment onto enterocytes and subsequent enteric infection. Adherence of the pathogenic microbe to the enterocyte cell wall is thought to be a prerequisite for the onset of infection (Gibbons and Van Houte, 1975). For example, it has been shown that the organism Vibrio cholerae is incapable of initiating disease unless it is able to attach to the cell wall, despite the presence of large numbers of bacteria present (Freter, 1969). Adhesion leads to bacterial growth, entrapment and formation of mixed colonies, the entrapment of nutrients for growth, the concentration of digestive enzymes and toxins onto enterocytes, and the possible prevention of antibody attachment to the pathogenic cell (Costerton et al., 1978).
The cell wall of the yeast organism consists of carbohydrates and proteins in the form of chained and branched structures of glucose, mannose, and N-acetylglucosamine (Ballou, 1970). Mannan oligosaccharides, derived from mannans on yeast cell surfaces, act as high affinity ligands, offering a competitive binding site for the bacteria (Ofek et al., 1977). Pathogens with the mannose-specific Type- 1 fimbriae adsorb to the MOS instead of attaching to intestinal epithelial cells and, therefore move through the intestine without colonization. Newman (1994) reported that the presence of dietary MOS in the intestinal tract removed pathogenic bacteria that could attach to the lumen of the intestine in this manner. Mannose was shown by Oyofo et al. (1989a) to inhibit the in vitro attachment of Salmonella typhimurium to intestinal cells of the day old chicken. Then Oyofo et al. (1989b) provided evidence that dietary D-mannose was successful at inhibiting the intestinal colonization of Salmonella typhimurium in broilers.
In an effort to confirm the inhibitory effects of MOS on pathogen colonization reported in previous research, Spring et al. (2000) screened different bacterial strains for their ability to agglutinate mannan oligosaccharides in yeast cell preparations (Saccharomyces cerevisiae, NCYC 1026). Five of seven strains of E. coli and 7 of 10 strains of Salmonella typhimurium and S. enteritidis agglutinated MOS and Sac. cerevisiae cells.
However, strains of S. choleraecuis, S. pullorum, and Campylobacter did not lead to agglutination. They also determined the effect of MOS on cecal fermentation parameters, cecal microflora, and enteric pathogen and coliform colonization in chicks.
After 3-day old chicks were orally challenged with 104
CFU of S. typhyimurium 29E and received 4000 ppm dietary MOS (Bio-Mos®, Alltech Inc., Nicholasville, KY), cecal S. typhimurium 29E concentrations decreased from 5.40 to 4.01 log CFU/g, (P< 0.05) at day 10. A similar study using S. dublin as the challenge pathogen resulted in a decrease in the number of infected birds by day 10 from 90% to 56% (P<0.05). Dietary MOS supplementation also reduced the concentration of cecal coliforms, although less significantly (P<0.10) than with the Salmonella challenges. Dietary MOS supplementation had no effect on cecal concentrations of lactobacilli, enterococci, anaerobic bacteria, lactate, volatile fatty acids, or cecal pH.
The effects of hen age, Escherichia coli, dietary MOS and bambermycins on poult performance from 1 to 21 days were studied previously by Fairchild et al. (2001). Day-of-hatch BUTA (BIG-6) male poults were gavaged (1 mL) with 1 x 108
CFU/mL E. coli composed of four serotypes or sterile carrier broth. A mixture of the same E. coli cultures was added to the drinking water (1 x 106
CFU E. coli/ mL drinking water) on a weekly basis to ensure a continuous bacterial challenge. Within each E. coli split plot treatment group, poults from hens of different ages (33 and 58 wk of age) were fed diets containing 1 kg MOS/tonne feed (Bio-Mos®, Alltech Inc., Nicholasville, KY) and 2 g bambermycins/ tonne feed, alone and in combination, in a randomized complete block design. At weeks 1 and 3, one bird per pen (n=128) was chosen randomly for bacterial sampling of liver and intestinal tissue for coliforms, aerobic bacteria and Lactobacillus spp.
Individual BW and feed consumption by pen were recorded weekly and poult mortality was recorded daily. Escherichia coli isolates from tissue samples were O stereotyped. Under E. coli challenge, dietary MOS and bambermycins improved (P<0.05) poult BW and BW gains. When poults were not challenged with E. coli, dietary MOS improved (P<0.05) poult growth during week 2 while dietary bambermycins improved (P<0.05) poult growth through week 3. Cumulative 3 week BW gains for unchallenged poults were improved (P<0.05) by both MOS and bambermycins. Two of the four E. coli stereotypes administered were recovered in cultures of tissue samples. Several stereotypes were recovered that were not administered. This work demonstrates that dietary MOS can improve the overall performance of poults, especially when they are faced with an E. coli challenge, as well as traditionally used antibiotics.
Improved performance has also been reported in turkeys receiving dietary MOS. Savage and Zakrzewska (1996) reported a significant increase in weight gain in Large White male poults fed a diet containing 0.11% MOS (Bio-Mos®, Alltech Inc., Nicholasville, KY). Numerous other studies have also reported performance benefits associated with feeding yeast cultures to growing poultry (Hayat et al., 1993; Bradley, 1994; Bradley and Savage, 1995).Comparative studies of antibiotic growth promotants and MOS in commercial turkeys
In comparison to chickens, turkeys are relatively sensitive to enteric challenges because of their rapid growth rate and longer growth period. Because of regulatory restrictions, poor availability of prophylactic and therapeutic pharmaceuticals makes any enteric challenges that do arise difficult to avoid or treat. Therefore, non-pharmaceutical enteric conditioners, such as MOS, are of particular interest to the turkey industry. A series of turkey experiments were conducted at North Carolina State University to compare the effects of growth promotants (virginiamycin and bambermycin), commonly used in commercial turkey production and MOS, on growth performance and enteric conditions.
STUDY 1: MOS VERSUS VIRGINIAMYCIN AND BAMBERMYCIN FOR MALE TURKEYS
The objectives of the first study were to compare the effects of MOS, virginiamycin, bambermycin and their combinations on growth performance, enteric microflora metabolites, and dietary energy and protein utilization. We hypothesized that MOS would promote growth by maintaining a normal stable gut microflora, while the antibiotics would elicit their positive effects by altering microflora fermentation. Hybrid® Large White male poults were randomly assigned to 48 pens (9 m2
/pen) on day of hatch and reared until 140 days. There were eight replicate pens per dietary treatment with 20 birds per pen placed at the start of the trial, and randomly reduced to 12 birds per pen at 12 weeks of age.
Dietary treatments were randomly assigned to each of 48 pens within blocks of 12 pens each. Each treatment was replicated twice in each block of pens.The six dietary treatments as follows:
1) Control: typical US corn/soybean meal diet;
2) MOS (Bio- Mos®, Alltech, Inc., Nicholasville, KY) (1 kg/ton to 6 wk then 0.5 kg/ton);
3) BM (Flavomycin®, Hoechst Roussel Vet, Warren, NJ) (2 g/ton active ingredient);
4) VM (Stafac®, Pfizer, Inc., Exton, PA) (20 g/ ton active ingredient);
5) MOS+BM (MOS and BM dietary inclusion rates as in 2 and 3); and
6) MOS+VM (MOS and VM dietary inclusion rates as in treatments 2 and 4). All diets met or exceeded NRC (1994) nutrient recommendations for turkeys (Table 1). All feed was pellet-processed and fed in crumble form until the birds were 6 weeks of age, and subsequently as 8 mm pellets.
|Table 1. Composition of basal diets fed to male turkeys from 1 to 140 days of age.|
|1Premixes provided sufficient vitamins and minerals to exceed NRC (1994) requirements.|
Dietary supplementation with MOS, BM and VM resulted in improved BW and F/G (Table 2). All three dietary supplements improved 20 wk BW.
However, there were no additional improvements in 20 wk BW when either of the growth promotants was fed in conjunction with the MOS. Birds fed MOS or VM had significantly increased BW at wk 20 with MOS-fed birds exhibiting the best market weights. Birds fed BM, VM, and MOS+BM had increased 12 wk BW, but these differences disappeared at 15 weeks. All three additives (MOS, BM, and VM) and the combinations MOS+BM and MOS+VM improved F/G for 0-3 weeks of age. From 3-6 weeks of age, the VM and MOS+BM treatment improved F/G, suggesting a possible synergistic action between the two compounds.
MOS and VM also improved F/G from 15-18 weeks of age. VM, MB, and MV improved cumulative F/G for 0-6, 0-12 wk, and 0-18, but this difference was lost during the last two weeks of the trial because of increased variability due to uncontrolled feed wastage as the large birds became more crowded within the pens. There were no significant treatment effects on mortality rate.
At 12 weeks of age, three birds per pen were randomly chosen from the control, MOS, BM and VM treatments to obtain samples for measurement of the physical characteristics of the gastrointestinal tract and microbial metabolite content in the jejunum and ceca. The duodenum, jejunum and ileum segments were measured for total weight and length, and then mucosa and muscularis weights from 10 cm sections from each segment were determined according to a method described by Palmer and Rolls (1983). Jejunal and cecal digesta were analyzed for volatile fatty acid (VFA) concentrations (acetic, propionic, and butyric), pH, lactic acid concentrations and ammonia. Finally, apparent metabolizable energy of the feed (adjusted for nitrogen) (AMEN) was determined by analyzing the feed and ileal contents for dry matter content, total nitrogen and gross energy.
The growth promotion effects of the antibiotic treatments were associated with significant effects on weights and lengths of intestinal segments. In contrast, MOS had significant effects on neither relative intestinal weights nor differences in mucosa and muscularis tissue mass (Table 3).
Virginiamycin-fed turkeys exhibited decreased weights of duodenum, ileum, ceca and colon as compared to the control-fed birds, but BM had marginal effects on the weights of these gut segments.
Similar effects on the intestine mass by antibiotics were observed by other researchers (Dafwang et al. 1985; Stutz and Lawton, 1984; Hill et al., 1957). Henry et al. (1986, 1987) reported a 19% and 14% decrease in intestinal weight in broiler chicks from the dietary inclusion of VM and BM, respectively, primarily due to a visible thinning of the intestinal tract wall. Feeding penicillin and Aureomycin also has been shown to decrease the intestinal weights of chicks (Pepper et al., 1953; Coates et al., 1955). Antibiotics limit microbial population numbers and their production of toxins and by-products (primarily from Gram-positive bacterial species) in the lumen, they reduce the competition with the host for vital nutrients, and they enhance the absorption and utilization of nutrients due to a thinning of the intestinal wall (Visek, 1978; Waldroup et al., 1970; Caston and Leeson, 1992; Waibel et al., 1991; Buresh et al., 1986).
Gordon and Bruckner-Kardoss (1958) stated that the presence of the normal microflora in the lumen of the intestines of conventionally reared animals imparts an inflammatory effect on the cells lining the intestinal wall. After comparing conventional and germ-free chicks as a model to understand the effects of antibiotic growth promotion, Visek (1978) concluded that enteric microflora not only increases intestinal mass (small intestinal weight, lymphoid tissue, and reticuloendothelial cells), but tissue turnover rate increases up to 40% due to the presence of intestinal microbes (Visek, 1978). Indeed, small intestinal tissue is the most rapidly regenerating tissue in the body (LeBlond and Walker, 1956), which represents a significant metabolic load if its rate of turnover increases for any given reason.
|Table 2. Effects of mannan oligosaccharides (MOS), bambermycins (BM), and virginiamycin (VM) on the body weights and feed/ gain of male turkeys1.|
|abMeans within a column differ significantly (P<0.05). There were no significant differences in poult starting weights at 1 day of age (60 g).|
1Values represent means of 8 replicate pens containing 20 birds per pen reduced to 12 birds per pen at 12 wk of age.
2Cummulative feed/gain is adjusted for mortality losses.
3Standard error of the mean with 39 degrees of freedom.
The effect of VM on intestinal segment weights was more attributed to a decrease in the muscularis rather than mucosa in each segment. Thus, VM exerts its effects mainly within the underlying circular and longitudinal muscle layers and not the epithelial layer. In contrast to the results of our study, Gordon and Bruckner-Kardoss (1958) reported a greater impact of antibiotics on the lamina propria (mucosa) region than on the underlying muscular region (muscularis). The authors concluded that the decrease in microflora numbers reduced the need for lymphatic tissue in the lamina propria. In contrast, the BM treatment resulted in increased relative duodenal weight primarily associated with the mucosa, indicating a slight stimulatory effect on duodenal lymphatic tissue. However, the effect of VM on intestinal muscularis we observed in our study is reasonable.
Increases in gut motility occur during gastrointestinal distress, resulting in flushing or diarrhea (Vispo and Karasov, 1997). In several animal species, interruptions of the gastrointestinal ecosystem can result in hypertrophy of the muscular layer that is associated with a 3 to 4-fold increase in the contractile strength of the muscle fibers (Johnson, 1994). In the present study, a reduced microbial stimulus provided by the use of VM may have prevented this ‘accretion’ of intestinal muscle mass, thus resulting in diminished muscularis weights in antibiotic-fed birds.
|Table 3. Effects of mannan oligosaccharides (MOS), bambermycins (BM) and virginiamycin (VM) on the intestinal segment weights and lengths and wet weights of small intestinal mucosa and muscularis of male turkeys1.|
|a-cMeans within a row differ significantly (P<0.05).|
1Measurements made on a wet weight basis and represent average values of 3 sampled birds per pen from 8 replicate pens per treatment.
2Standard error of the mean with 25 degrees of freedom.
In comparison to the well-documented effects of antibiotics, little is known about the effects of dietary MOS on the intestinal tissue of poultry. Dietary MOS had only a marginal stimulatory effect on the physical attributes of the intestines of turkeys. Intestinal lymphoid tissue may be more developed or active due to MOS (Newman, 1994; Cotter, 1997), which may explain the slight thickening of the intestinal tract observed in MOS-fed turkeys in the current study. However, definitive data are lacking with respect to the effects of MOS on the immune system of male turkeys. The reported positive effects of MOS are attributed to a competitive binding of pathogenic microbes, including E. coli and Salmonella, thus suppressing their colonization in the gut. Suppression of disease-causing bacterial colonization by MOS may not be associated with a decrease in the stimulation of mucosal lymphatic tissue and enteric motility.
Although the ceca are the primary sites of gut microflora fermentation, noteworthy treatment effects were not observed in this study. Regardless, our interest was mainly associated with fermentation in the jejunum because of this organ’s function with digestion and nutrient absorption. The effects of dietary MOS and the antibiotics on microflora fermentation products in the jejunum are presented in Table 4. In comparison to the control treatment, the antiobiotics increased jejunal digesta pH (VM more significantly than BM). This increase in pH was associated with a reduction in total VFA concentration of the digesta, mainly attributed to significant reductions in propionic acid, followed by moderate reductions in the other VFAs. Antibiotics, including clindamycin, bacitracin and vancomycin, reduce bacterial fermentative activities as indicated by reduced fecal levels of short chain fatty acids (Cummings, 1995). Dietary MOS also reduced total VFA concentration significantly. In contrast to VM and BM, however, MOS decreased jejunal digesta pH. Although not statistically significant, MOS and the antibiotic treatments also reduced jejunal digesta ammonia content. There were no significant treatment effects on lactic acid concentration.
As hypothesized above, the reduction of small intestinal tissue weight that we observed in the antibiotic-fed turkeys was accompanied by a reduction in microbial fermentation products.
Although these fermentation products may not be the direct cause of the increased intestinal tissue weight, they are an indicator of the microflora population and its ecosystem.
Volatile fatty acids arise from the fermentation of carbohydrates by glucose fermenting bacteria and the fermentation and deamination of proteins by proteolytic bacteria (Hudson and Marsh, 1995) and may actually provide benefits to the host animal.
These benefits of VFAs include 1) use of acetic acid as a metabolic fuel for muscle, kidney, heart and brain tissue, 2) utilization of propionic acid for gluconeogenesis by the liver, and 3) use of butyric acid as a major fuel source in the enterocyte (Cummings, 1995). The VFAs have also been shown to possess bacteriostatic and bacteriocidal properties (Barnes et al., 1979, 1980; Corrier et al., 1990) against organisms such as Salmonella and E. coli. Lactate, produced mainly by saccharolytic bacteria (Lactobacillus, Bifidobacterium, Enterococcus, Pediococcus, and Streptococcus) during the fermentation of carbohydrates, may serve to protect the animal from pathogenic bacteria (Salmonella, E. coli, Clostridium) by decreasing the pH of the hingut, thus impeding the growth of these unfavorable bacteria (Barrow, 1992). Broad-spectrum Gram-positive antibiotics, such as BM and VM would serve to limit the growth of these beneficial microbes and may result in a decrease in intestinal levels of lactic acid.
|Table 4. Effects of mannan oligosaccharides (MOS), bambermycins (BM) and virginiamycin (VM) on microbial by-product production characteristics in the jejunum and dietary AMEn of male turkeys at 12 weeks of age.1|
|a-bMeans within a row differ significantly (P < 0.05).|
1Measurements represent average values of 3 sampled birds per pen from 8 replicate pens per treatment.
2Standard error of the mean with 25 degrees of freedom.
3Apparent ME corrected for nitrogen retained was calculated using ileal contents.
Ammonia, produced by proteolytic bacteria such as Clostridium, Enterococci and Bacteroides, is partially responsible for many toxic cellular and physiological side effects. Even small quantities of ammonia can inflict severe cytopathic effects on the host colonic epithelial cells by reducing the lifespan of mucosal cells, interference with DNA synthesis, and by increasing the rate of cellular turnover (Macfarlane and Macfarlane, 1995).
Bacteroides is a major member of the normal hindgut microflora that plays a large role in the turnover of proteins in the lower gastrointestinal tract by facilitating the production of free ammonia.
Ammonia is easily absorbed back into the bloodstream of the host animal and converted to non-essential amino acids (e.g., glutamine) in the host (Vispo and Karasov, 1997). Since ammonia of uric acid origin is derived in part from waste ammonia in the bloodstream, utilization to nonessential amino acids via microbial assistance could be considered energetically wasteful (Vispo and Karasov, 1997; Karasawa et al., 1993). Dietary inclusion of MOS apparently reduced the production of ammonia and its adverse effects by enteric microflora.
Enteric conditioners, such as MOS and antibiotic growth promoters, ultimately enhance the efficiency of nutrient utilization by reducing the competition between the host and its intestinal microbial inhabitants. Without the microbial competition for energy and other nutrients, the host retains a greater amount of nutrients available for absorption and metabolism. In our study, dietary inclusion of MOS and VM resulted in over 2.5% better dietary energy utilization (AMEn) than the control or BM-fed turkeys (Table 4). These results agree with the improvements in feed conversion observed in the MOS- and VM-fed turkeys during this study, and the results observed by other researchers. Nelson et al. (1963) reported that antibiotics improved the absorption efficiency of calorigenic nutrients.
Buresh et al. (1985) demonstrated that VM significantly reduced the amount of energy required to produce a gram of weight gain, especially for poults consuming a restricted energy diet compared to birds consuming an ad libitum diet. Harms et al. (1986) reported that VM improved dietary energy utilization far greater in low energy content diets compared to diets rich in energy. Dietary MOS may also improve the utilization of dietary energy utilization, but likely by a different mechanism than antibiotic growth promotants.STUDY 2: MOS VERSUS VIRGINIAMYCIN AND A MOS-VIRGINIAMYCIN SHUTTLE PROGRAM FOR FEMALE TURKEYS
The primary objectives of the second study were to determine the effects of dietary MOS and VM on the performance and carcass parameters of female turkeys and to ascertain the advantages of a dual growth promoter shuttle program over the traditional use of antibiotic supplementation. The secondary objectives were to ascertain the effects of MOS and VM on jejunum villi morphology and immune function. Hybrid® commercial female poults were randomly assigned to 32 pens (9 m2/pen) on day of hatch and raised from 1 to 98 days of age.
There were eight replicate pens per treatment with each pen containing 20 poults at the start of the trial. Poults that died within the first 6 days of the study due to starve-out were replaced and mortality records were maintained beginning on day 7. The four dietary treatments were: 1) control, typical US corn-soybean meal diet; 2) MOS (1 kg/tonne to 6 wk then 0.5 kg/ton); 3) VM 20 g/tonne active ingredient); 4) MOS-VM (MOS 0-6 wk then VM 6-14 wk, same rates as in 2 and 3). All diets met or exceeded NRC (1994) nutrient recommendations for hen turkeys (Table 5). Dietary feed additives were added at the expense of washed builder’s sand to avoid any confounding dilution effects. Feed was pellet-processed and fed in crumble form until the poults were 3 weeks of age, and subsequently as 8 mm pellets.
|Table 5. Composition of basal diets fed to hen turkeys from 1 to 98 days of age.|
|1The premixes provided sufficient vitamins and minerals to exceed NRC (1994) dietary recommendations.|
The growth performance data for the hen study are presented in Table 6. Body weight of the MOS treatment group was found to be significantly depressed in comparison to the other treatments, including the MOS-VM shuttle group that was fed a different batch of same diet formulation. Upon analysis of all treatment diets, we discovered a sodium deficiency (0.06% Na versus the intended 0.18% Na) in the MOS pre-starter diet fed from 0- 3 week. Because of this discrepancy, the MOS treatment group had stunted body weights throughout the growth trial. Therefore, comparisons concerning MOS were made with the MOS-VM treatment from 0-6 wk of age, after which comparisons were made with the knowledge of the previous information in mind. Feeding VM resulted in significantly (P<0.05) heavier body weights during each observation period compared to control birds, and overall average of 8.6% improvement in body weight. Interestingly, the MOS-VM treatment group, which during the 0-3 wk period was consuming only MOS, had significantly (P<0.05) lower body weights than the control birds.
Due to unseasonably cold weather conditions, the poults were subjected to sub-optimal temperatures during the first 2 days of brooding. This early cold stress may have contributed to the poor early performance of the birds consuming the diets containing MOS. According to Harun et al. (1997), the most crucial and vulnerable stage of growth in young birds is during the transition period from metabolizing internal yolk stores to external feed nutrients. During this period, poults and chicks have poor thermoregulatory capabilities (Whittow, 1976) and thus they are dependent upon the availability of external thermal energy (Harun et al., 1997). When ambient temperatures are too low for the poult, survivability and growth potential of the poult is dependent on its ability to consume dietary energy or mobilize body reserves to support thermal regulation (Smith, 1990). As mentioned earlier, antibiotics may improve dietary energy utilization in turkey poults (Buresh et al., 1985; Buresh et al., 1986). In contrast, the immunostimulation caused by MOS (Savage et al., 1996; Newman, 2001) together with the cold stress may have placed the poult in an energetic dilemma of partitioning its limited energy reserves. Because immunostimulation has been shown to reduce feed intake (Klasing et al., 1988), MOS may have further compromised the energy intake of the thermaldeficient poult.
|Table 6. Effects of mannan oligosaccharides (MOS), virginiamycin (VM) and MOS-VM shuttle program on live weight and rate of gain in female turkeys1|
|a-cMeans within a row differ significantly (P<0.05).|
1Values represent means of eight replicate pens containing 20 birds per pen.
2Standard error of the mean with 26 degrees of freedom.
The VM-supplemented birds had significantly greater rates of body weight gain (Table 6) during the 0-3, 3-6, and 6-9 wk periods compared to control birds. In contrast, the MOS-fed birds had significantly lower rates of gain from 0-3 and 3-6 wk, after which the MOS group maintained the same rate of gain as that of control birds. MOS-VM birds had rates of gain equal to control birds at 0-3 and 3- 6 wk, followed by a significantly greater weight gain between 6-9 wk and again equal gain rates to control birds from 9-12 and 12-14 wk of age.
Significant treatment effects of feed conversion were observed throughout the experiment (Table 6). Period F/G was primarily influenced by treatment early in the growth phase. From 0-3 wk of age, turkeys fed VM consumed significantly less feed per unit weight gain than the control birds. MOS birds had significantly decreased F/G as compared to control from 0-3 wk of age, although MOS-VM birds were equal to control birds during the same period. Only the MOS-fed birds consumed significantly less feed per unit weight gain than the control birds during the 3-6 wk period. This apparent compensatory feed conversion response may have been a result of the reduced maintenance requirements associated with the lower body weights in MOS-fed birds in comparison to birds on the other treatments. Subsequently, no other MOS treatment effects were observed. In comparison to the control birds, cumulative F/G was significantly decreased among the VM-fed birds from 0-6, 0-9, and 0-12 wk of age and approached significance (P<0.10) from 0-14 wk of age. The F/G of birds fed MOSVM approached significance (P<0.10) from 0-9, 0-12, and 0-14 wk of age. Feeding MOS alone had no effect on cumulative F/G of the turkey hens in this study.
At the end of the trial, two hens per pen were randomly selected for carcass parts yield analysis. Neither MOS nor VM had any significant effect on carcass parts yield. Leeson (1984b) and Woodward et al. (1988) reported significant improvements in dressed carcass weights and carcass yields of broilers fed VM. Izat et al. (1990) reported improved dressing percentage, percentage yield, breast weight, and breast as a percent of postchill weight in broilers fed BM and bacitracin methylene disalicylate (BMD). However, Izat et al. (1989) failed to observe any differences in overall broiler dressing percentage or carcass parts when supplementing the diets with VM or BMD. A similar lack of effect on carcass composition and yield has been observed in turkeys fed VM (Salmon and Stevens, 1990; Ferket et al., 1995). Research pertaining to the effects of MOS on carcass characteristics and parts yield is lacking, and these data represent the most comprehensive study to date on the influence of MOS on the carcass development of turkeys. It was hypothesized that a decrease in pathogen challenge provided by MOS would result in improved nutrient utilization and allocation, leading to benefits in lean muscle gain. However, this was not evident during the current trial.
At 14 days of age, one bird per replicate pen was sampled for morphometric analysis of 5 cm sections of jejunum. Measurements of villus height, crypt depth, muscularis thickness, and goblet cell number were made at a magnification of 10X. A minimum of 15 measurements per slide were made for each parameter and averaged into one value per bird, which was then used for statistical analysis among treatment groups.
Morphological observations of jejunal brush border are presented in Table 7. MOS had the greatest effect on villi morphology. Although MOS did not affect villus height, a decrease in crypt depth approached significance and villi height:crypt depth ratio was significantly greater than the control or VM treatments. Turkeys receiving MOS also exhibited a thinner muscularis layer and increased numbers of goblet cells per mm of villus height as compared to control birds. As observed with older toms detailed above, the poults consuming VM had a significantly lower jejunal muscularis weight than the control treatment, while an increase in goblet cell count per mm villus height approached significance (P<0.10).
The mucus gel layer coating the surface of the intestinal epithelium is the first major barrier to enteric infection. Hence, the production of mucus, as indicated by the number of goblet cells, is an important feature in the protective scheme against pathogens. Feeding MOS resulted in an increased proliferation of goblet cells into the surface of the villus membrane. The innate immune system has developed to recognize key molecular structures of invading bacteria, including lipopolysacchharides, peptidoglycans and possibly the mannose structures in the cell walls of yeasts. Oligosaccharides containing mannose have been shown to affect the immune system by stimulating liver secretion of mannose-binding protein. This protein, in turn, can bind to bacteria and trigger the complement cascade of the host immune system (Janeway, 1993; Newman, 1994). Intestinal microbes might influence goblet cell dynamics via release of bioactive compounds or indirect activation of the immune system (Bienenstock and Befus, 1980). Contrary to expectations, dietary VM increased goblet cell numbers in the poults. Antibiotic therapy was expected to decrease host reliance on mucus secretion for protection. However, decreasing numbers of viable Gram-positive bacteria, such as lactobacilli and bifidobacteria, may increase the presence of Gram-negative species. An increase in these types of microbes may actually necessitate the need for more mucus production and hence more goblet cells (Edens et al., 1997).
|Table 7. Effect of virginiamycin (VM) and mannan oligosaccharides (MOS) on the intestinal morphology of the jejunum of 14-day old hen poults.1 |
|a,bMeans within a column differ significantly (P<0.05).|
1 Means are an average of 15 individual measurements per bird and eight birds per treatment at 14 d of age.
2 Standard error of the mean with 19 degrees of freedom.
Just as we observed an apparent interaction between treatment and environmental stress earlier, significant treatment interaction effects with acute immunological stress were also observed. To induce an acute immune stress at 21 days of age, one bird per pen was injected intraperitoneally with 3 mL of sterile Salmonella typhimurium strain SL 684 LPS (100 mg LPS/L) obtained from Sigma Chemical Co. (St. Louis, MO) in an 8.2 g NaCl/L solution. One tagged non-injected bird per pen served as control.
Eight hours after LPS injection, cloacal temperatures were taken from each non-injected and injected bird per pen. Twenty-four hrs postinjection, injected and non-injected birds were removed from each replicate pen and killed by cervical dislocation. Body, liver, spleen, bursa of Fabricius, and intestinal tract weights were recorded.
The effects of S. typhimurium LPS challenge on fever response and lymphoid tissue are presented in Table 8. LPS injection successfully induced a mild fever response as indicated by a 0.25 ºC increase in body temperature 8 hrs post-injection. Birds fed MOS showed a decreased fever response compared to control at 8 hrs post-injection. Nonchallenged birds fed VM had decreased intestinal and bursa weights that approached significance (P<0.10) compared to control birds. There were no other treatment related effects in non-challenged birds. Challenge with LPS increased liver and spleen weights compared to non-injected birds, but failed to have any effect on bursa and intestinal weights.
Challenge with LPS increased liver (2.78 vs. 2.44 g/100 g BW, P<0.05) and intestinal (3.67 vs. 3.33 g/100 g BW, P<0.05) weights of MOS-fed birds compared to control birds, but had no effects on spleen and bursa weights. LPS injection had no effects on lymphoid organ weights of birds fed VM.
We hypothesized that VM-fed birds would have less active immune response than controls or MOSfed birds, therefore exhibiting a decreased reaction to an acute immune response provided by the LPS challenge. The liver, spleen, bursa, and intestines all play an integral role in the inflammatory immune response as indicated by their increase in weight during such challenges (Roura et al., 1992). These organs are also involved in acute phase protein synthesis, lymphocyte activation, antibody production and antigen sampling (Abbas et al., 1997). Moreover, a fever response (increased body temperature) has been correlated with IL-1 release (Klasing et al., 1988). A clear immunologic stress response was observed in LPS-challenged birds, as indicated by a significant increase in cloacal temperatures, as well as liver and spleen weights.
|Table 8. Effects of mannan oligosaccharides (MOS) and virginiamycin (VM) on response to immunological stress in female turkeys.1|
|a-cMeans within a row differ significantly (P<0.05).|
1Values presented are means of 8 birds per dietary treatment.
2Standard error of the mean (degrees of freedom).
However, antibiotic treatment had no effect on the fever response or lymphoid organ weights of LPS challenged birds. Antibiotic treatment did affect intestine and bursa weights to a degree that approached significance (P<0.10) in non-challenged birds. We hypothesized that the bursa of Fabricius, which plays an active role in the production of immature plasma cells and in antigen sampling (Glick, 1979), would increase upon exposure to LPS and decrease in response to a reduction in microbial load due to treatment. While there was no increase due to LPS challenge, there was a decrease in nonchallenged birds fed VM. These findings agree with reports by Franti et al. (1971) that showed a decrease in bursa weights in chicks fed bacitracin. The decreased intestine weights in the VM-fed poults were comparable to previous reports (Henry et al., 1986; Pepper et al., 1953). Feeding MOS had little effect on the lymphoid organ weights of non-challenged birds. However, feeding MOS to LPS challenged birds resulted in increased liver and intestinal weights. Lymphoid function related to the liver and intestines may have been primed by the long-term feeding of the antigenic portions of the Saccharomyces cell wall (Ballou, 1970) and may have led to the ability of these tissues to mount a greater response to LPS challenge.
At 63 days of age, antibody response was evaluated by intravenously injecting two birds per pen with 1 mL of a 7% saline suspension of sheep red blood cells (SRBC). Blood samples were then collected from each bird at 7 and 14 days post- SRBC challenge. Fourteen days after the first SRBC injection, the birds were given a second injection. Blood samples were taken again at 7 and 14 days post-booster in order to quantify secondary response antibody titers. Total, mercaptoethanolsensitive (MES, presumably IgM), and mercaptoethanol-resistant (MER, presumably IgG) anti-SRBC antibody titers were determined using a microhemagglutination technique described by Yamamoto and Glick (1982) and Dix and Taylor (1996). Data presented in Table 9 are expressed as the log2 of the reciprocal of the highest dilution giving visible agglutination.
|Table 9. Effects of mannan oligosaccharides (MOS) and virginiamycin (VM) on the antibody response to sheep red blood cell (SRBC) mitogen challenge in hen turkeys.1|
|a-bMeans within column with different superscripts differ significantly (P<0.05).|
1Mean values are averages of 8 replicate pens testing 2 birds per pen.
2Birds were injected with SRBC starting at 9 wk of age.
3Second SRBC injection given at 11 wk of age.
*Standard error of the mean with 19 degrees of freedom.
Modifications of the gastrointestinal microflora due to antibiotic growth promoters such as VM were hypothesized to alter the systemic immunological functions of B and T lymphocytes. However, we found this hypothesis not true in our experiment, as have other researchers. Dafwang et al. (1985) showed little response of broiler anti-SRBC antibodies to diets supplemented with oxytetracycline, lincomycin, penicillin, bambermycins or tylan. Belay et al. (1992) reported no effect of dietary virginiamycin on the total, IgG, nor IgM antibody titers in 7 week old broilers. Therefore, the influence of enteric conditioners and their effects on gut microflora could be mainly limited to the mucosal immune complement and not the systemic portion of the immune system. Although the contribution of gut microflora to the development and physiological status of the humoral and cellular mucosal immune systems is well understood (Lu and Walker, 2001; Cebra, 1999), the effects on the systemic immune complement may be less dominant.
Studies with germ-free animals reveal that their acquired immune system is similar to that of conventional animals, although the overall number of lymphocytes is reduced and the proportions of IgM are increased compared to those of IgG (Lu and Walker, 2001; Crabbe et al., 1968). Interestingly, upon challenge with SRBC, the VM group tended to follow this pattern having a numerically, but not significantly, greater proportion of IgM to IgG antibodies compared to control birds.
In previous studies, MOS has been shown to have a positive influence on humoral immunity and immunoglobulin status. Savage et al. (1996) reported an increase in plasma IgG and bile IgA in poults fed diets supplemented with 0.11% MOS.
Although the current study failed to produce significant differences in total Ig, IgG or IgM production, levels of IgG were higher than in control birds. An increase in antibody response to MOS would be expected because of the ability of the innate immune system to react to foreign antigenic material of microbial origin. Portions of the cell wall structure of the yeast Saccharomyces contained in MOS has been shown to elicit powerful antigenic properties (Ballou, 1970). Therefore, some MOSimmune system cross talk would be expected.
This paper provides a comprehensive review of antibiotic growth promotants commonly used in the turkey industry and the potential of MOS as a nonpharmaceutical alternative. Dietary antibiotics clearly promote efficient growth and health of turkeys and their benefits to the turkey industry and the consumer. However, like many other technological wonders, there are potential dangers when they are not used properly. Public concern about the increasing threats of antibiotic-resistant pathogens has forced the poultry industry to consider ‘biologically safer’ alternatives. There is considerable evidence now that MOS is among the best alternatives to antibiotic growth promotants.
MOS may indeed elicit greater benefits than antibiotics if it is used strategically together with other non-pharmaceutical enteric conditioners, such as probiotics, fructooligosaccharides, bio-active peptides and certain herbs. Table 10 is a comparative summary of the attributes of antibiotic growth promotants and MOS. A general difference that is evident from this comparison is that MOS elicits its beneficial effects by allowing the animal to enhance its own defense mechanisms by blocking the colonization and contact by pathogens. In this reason, MOS functions as a probiotic (meaning for life) or symbiotic (meaning working with life). In contrast, antibiotics (meaning against life) functions by suppressing the proliferation and metabolism of microflora, including some pathogens; thus giving the host a competitive advantage for nutrients and partitioning of body resources. Now that we know more about the mode of action of MOS in comparison to antibiotics, our next challenge is to learn how to use it more strategically with husbandry practices and other feed additives.
Authors: P.R. FERKET, C.W. PARKS and J.L. GRIMES
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Department of Poultry Science, North Carolina State University, Raleigh, NC, USA.