Numerous studies have been conducted and highlighted the beneficial effects of using probiotic feed supplements to enhance the performance and stimulating the immune responses in poultry (Table 5). In general, food animals are often exposed to stress due to physiological (age, health status), psychological (weaning, post-weaning), and environmental (diet, management) factors. This may lead to dysfunction and an increase in permeability of intestinal protective barriers that often results in changes in the intestinal microbial composition (eg; Bifidobacteria, Lactobacilli) and an increase in susceptibility to enteric pathogens (Si et al., 2004). Probiotic groups such as Pediococcus and yeasts such as Saccharomyces are less commonly used in animal feeds. However, they can modulate the establishment of lymphocyte populations and IgA secretions in the gut and reduce translocation to mesenteric lymph nodes followed by E. coli ETEC infection (Lessard et al., 2009).
Poultry management practices such as high stocking densities, transportation, and nutritional imbalances or regimen may predispose stress that would ultimately effect the host's immune system and colonization of pathogen bacteria in the gut and thereby compromising the food safety (Virden and Kidd, 2009). Supplementation of probiotics in poultry diets has been considered an effective tool to maintain a healthy intestinal microbiota, thereby improving the growth performance and reducing the intestinal pathogens (Jin et al., 1996). Factors affecting the functionality or efficacies of probiotic supplementation are route of administration (vent, feed, water), and stage of life cycle (Timmerman et al., 2006). Probiotic supplementations can be administered through powders, liquid suspensions or sprays in feed or water, and in ovo methods where the shell membrane of the air cell is inoculated with the probiotic culture after 18 days of incubation for early gut colonization (Fuller, 2001).
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7.2. Role of probiotics - beneficial effects
Probiotic groups such as Lactobacilli and Bifidobacteria have been identified in poultry that would modulate the immune system of the host by stimulating different subsets of the immune system to produce cytokines (Christensen et al., 2002; Lammers et al., 2003; and Maasen et al., 2000). Dalloul et al. (2003a and 2003b) demonstrated that administration of probiotics results in the secretion of cytokines and changes in the lymphoid cells in the chicken gut that may ultimately provide immunity against Eimeria acervulina. However, a precise understanding about the effect of probiotics on the induction of systemic antibody response is not well established (Haghighi et al., 2005). Beneficial effects of probiotic supplementation in poultry is mainly attributed to competitive exclusion (CE); which has demonstrated protection against colonization of Salmonella, C. jejuni, pathogenic E. coli, and C. perfringens in chicks (Nisbet, 2002; Schnietz, 2005).
Use of Lactobacillus as a probiotic nutritional and health supplement is an increasing trend in the poultry industry as they modulate immune system of the host as well as increase overall performance including growth rate, feed conversion ratio and meat quality (Kalavathy et al., 2003; Mountzouris et al., 2007). Addition of Lactobacillus successfully lowered the mortality rate caused by necrotic enteritis in one-day old chicks (Hofacre et al., 2003). Lactobacillus supplemented poultry diets also significantly reduced S. Enteritidis recovery in neonatal chicks (Higgins et al., 2008). However, there was no significant reduction in S. Typhimurium when challenged with the same cultures (Higgins et al., 2007). Other probiotics groups such as Bacillus cereus var. toyo, Bacillus subtilis suppressed the persistence and colonization of S. Enteriditis and C. perfringens (La Ragione and Woodward, 2003). Broiler chicks fed with a mixture of probiotics (L. acidophilus, L. casei, Bifidobacterium thermophilus, and E. faecium) lowered C. jejuni populations (Willis and Reid, 2008).
7.3. Commercial probiotic supplements
Currently, there are several commercial probiotic supplements consisting benficial microorganisms alone or in combination of fermentable carbohydrates (prebiotic compounds) available in the market (Table 5). Probiotic microorganisms such as Lactobacillus spp., Enterococcus spp., Pediococcus, and Bacillus spp. are commonly found in the commercial supplements with Lactobacillus spp. as the predominant group.
Several studies were conducted on Lactobacilli spp. based commercial probiotics (FloramaxTM [FM-B11], Histostat-50®, Nutra-GloTM) in poultry (Higgins et al., 2005, 2007, and 2008). Dietary supplementation of Lactobacilli spp. in poults (7-day old) lead to increased weight gain of the poults (increase by at least 18g on 21st day) and effectively treated clinical enteritis caused by S. seftenberg when used along with therapeutic antibiotic regimes (Penicillin, Roxarsone, and Neomycin) (Higgins et al., 2005). Furthermore, incorporating Lactobacilli spp. probiotic cultures in 1-day old broiler chicks reduced the incidence and colonization of S. Enteriditis and S. Typhimurium due to phagocytic action of macrophages (Higgins et al., 2007, 2008). Probiotic supplements containing Bacillus spp. (Bio-Plus 2B®, Toyocerin®) have shown growth enhancing activities (live weight, feed conversion ratio, and fattening) in broilers and turkeys and increasing antibody response to Newcastle disease virus in broilers (Jadmus et al., 2000, Mahdavi et al., 2005; Sabatkova et al., 2008; Dizaji et al., 2009 and Rahimi et al., 2009). Dietary supplementation of the lactic acid strain, Pediococcus acidilacti (Bactocell®) had reportedly stimulated the immune function of the broiler chicks and thus significantly increased antibody levels against New castle disease virus (Alkhalf et al., 2010). Administration of commercial probiotic supplements based on competitive exclusion (Aviguard®) also significantly reduced colonization of multi-resistant pathogenic E.coli, Salmonella in broilers (Reynolds et al., 1998 and Nakamura et al., 2002).
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There are several commercial supplements (Poultry Star®, PremaLac®, PREEMPT, ProtexinTM) containing mixed defined and characterized probiotic cultures in the market. They have successfully demonstrated diverse benefits such as host protective effects from enteric pathogens (immune stimulation, increased VFA production, reduced colonization and competitive exclusion) and overall growth performance activities (improved body weight, feed conversion ratio) in poultry (Ayasan et al., 2006; Sterzo et al., 2007; Mountzouris et al., 2007, Pour et al., 2010). Mountzouris et al. (2007) reported that mixed defined probiotic culture (Biomin Poultry Star®) exhibited modulated composition and activities of cecal microflora and displayed higher specific microbial glyoclytic enzymatic activity in broilers. In addition to these beneficial effects, some commercial probiotics are well known to protect birds from Salmonella colonization by increasing by their establishment in the ceca after probiotic (PREEMPT) supplementation (Corrier et al. 1995 a, b; Nisbet et al. 1996 a, b;Martin et al., 2000; Chamber and Lu, 2002). Furthermore, commercial probiotics consisting of yeasts and beneficial bacteria (ProtexinTM, Lacto-Sacc or Lacto-Sacc Farm pak 2X) have been shown to improve laying performance such as egg production, egg weight, feed conversion ratio (Ayasan et al., 2006 and Zeweil et al., 2006) and immune-modulatory activities against avian influenza in broilers (Ghafoor et al., 2005).
Dietary supplementation of probiotic along with prebiotic compounds has also been to elicit several health benefits in poultry. EL-Banna et al. (2010) reported improved feed conversion rate by 4.2 % or 5.34 % when probiotic (BACTOCELL®) was supplemented with lactose or Myco® (Mannoseoligosachharide) in 1-day old broiler chicks respectively. Furthermore, such combinations (probiotic and prebiotic) also inhibited enteric pathogens such as S. Enteriditis, S. Typhimurium, S. Choleraesuis, C. jejuni, E. coli) (Sterzo et al., 2007; McReynolds et al., 2009).
Use of probiotics sometimes can pose adverse effects especially when they competitively exclude other indigenous beneficial microflora with the newly introduced probiotic culture (Eden, 2003). Probiotic often cause a transitory alteration in the indigenous gut micrflora especially when large numbers of probiotic bacteria are introduced (Eden, 2010). Results available from the previous literature on probiotic treatments often appear to be contradictory. This may be due to variation in the target pathogen, dietary supplementation, and duration of use. Disregarding the environmental and stress status of the animals and the experimental settings are also reasons for inconsistent results. In spite of numerous health benefits in poultry, there are inconsistent effects observed following probiotic administration (Turner et al., 2001). These inconsistent responses are similar or comparable to the effects observed followed by the administration of conventional antimicrobials. Several factors such as production environment (cleanliness, history of diseases in the farm, health status) (Catala-Gregori et al., 2007), source of probiotic, number of viable cells in the probiotic and their consistency, survivability and metabolic capacity in the host gut, probiotic's host specificity, influence of feed processing (e.g., steam conditioning and pelleting) on survivability of the probiotic in the final prepared diet, and differences in the experimental conditions can all play an important role in the effective responses observed following administration of probiotics (Williams, 1997).
8. FUTURE DIRECTIONS
A comprehensive knowledge should be established on the metabolites responsible for the effect of probiotic on host immune system in responding to the pathogenic bacteria. Considerable work remains to be done to determine the mechanism of action and optimum dose of the probiotics. Further research should focus on determining the mechanism of action, interactions in the host and host responsiveness to probiotics. Genetic evaluation of probiotic and gut microbiota would help in selection of appropriate probiotic supplements. Application of modern techniques to identify/evaluate the microbial communities and their growth requirements is likely to divulge new microbial responses that have benefit for the host (Ricke and Pillai, 1999). Applying modern analytical techniques can be of great value in understanding the bacterial-diet interactions and the role of different probiotic bacteria on animal health. These technological advances would allow the development of therapeutic treatments, novel technologies, management systems, and modified nutrition to optimize gut health and growth of the concerned host. Identification tools based on molecular methods utilizing total bacterial DNA or RNA targeted probes and development of profiling tools such as DGGE, % Guanine (G) + Cytosine (C), gene amplification protocols, and mRNA analysis can further increase the ability to assure validation before, during and after application (Ricke and Pillai, 1999; Hanning and Ricke, 2011).
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A combination of probiotics and naturally occurring components such as prebiotics, non-specific substrates, plant extracts, and microbial metabolites that act synergistically to improve the host health would be more appealing and may yield a new dimension in using probiotics in sphere of safe food practices. The beneficial effects of probiotics can be further enhanced by selecting more efficient strains or combinations of microorganism, gene manipulation, combination of probiotics and naturally occurring synergistically acting compounds such as prebiotics (Bomba et al., 2006). Synbiotics are nutritional supplements which contain a mixture of prebiotics and probiotics that act synergistically and deliver beneficial effects to the host by improving the survival and implantation of live microbial dietary supplements in the GI tract (Gibson and Roberfroid, 1995). Adding prebiotics to animal feed would further increase the efficiency of probiotic culture preparations by improving the survival of probiotic bacteria through the upper intestinal tract and thereby inducing beneficial effects (Roberfroid, 1998; Suskovic et al., 2001).
The majority of earlier studies concerned with beneficial effects of probiotics were difficult to interpret their findings due to one or few of following reasons; No statistical interpretations, poor experimental protocols, and undetermined validity and viability of the probiotic strain (Simon et al., 2001). Henceforth, a comprehensive and clearly defined experimental protocol with valid statistical analysis should be in place for better application of the results in future research studies.
The major focus is the summation of beneficial effects and their impact in poultry pre-harvest food safety with changes in gut dynamics. These changes may include immune system stimulation, modulating intestinal architercture with metabolic and physiological adjustments. A critical understanding of the inter-relationship of gastrointestinal physiology, microbiology and its effect on the host immune system is important in the selection of probiotics. Dietary supplementation of probiotics in poultry production has reduced the use of regular antibiotics, growth promoters and therefore can be viewed as potentially safe for growth promotion. Furthermore, probiotics have the potential to improve the preharvest food safety by reducing the enteric pathogen load. Nonetheless, none of these alternative strategies/products will become sufficient to control the impact of foodborne pathogens and be effective under a wide variety of conditions unless more is understood on specific mechanisms and their respective relationship with the avian host.