The AGP are the antibiotics that are used in poultry feed continuously at a lower level to improve growth and feed conversion and not for the purpose of any therapeutic reasons. Antibiotic compounds commonly used as growth promoters include Bacitracin, Penicillin, Virginiamycin, Flavomycin, Chlortetracycline, Oxyteracycline, Colistin sulphate, Doxycycline, Erythromycin, Aureomycin, Avilamycin, Tiamulin, Furazolidone, Lincomycin, Enrofloxacin and Neomycin sulphate (Chowdhury et al., 2009). In germ-free birds, the use of antibiotics as growth promoters are not of much value regarding performance enhancement, which reflects that their mode of action is primarily due to antibacterial activity instead of having any direct effect on physiology of the birds (Stanley et al., 2004; Dibner and Richards, 2005).
Principal mode of action of antibiotics is the regulation and maintenance of optimal balance of avian intestinal microflora (between gram-negative and gram positive organisms). The well balanced intestinal microflora contains more than 90% Gram positive bacteria (mainly Lactobacillus). During stress or digestive disturbances the number of pathogenic organisms, like Escherichia coli or other gram-negative organisms, is increased, leading to an imbalance of microflora. The gram-negative bacteria further colonize in the intestines, adhere to intestinal epithelium and cause inflammation of intestinal mucosa thus reducing the absorption of nutrients and in turn retarding the growth and productivity of birds. The AGP also has the ability to alter processes of mucin biosynthesis; and modifications in mucin dynamics influence the gut function and health and may change nutrient uptake. Hence, for eliminating the chances of imbalance under practical conditions, antibiotics are added to the feed or water as a prophylactic measure (Jones and Ricke, 2003; WHO, 2004). Use of anticoccidial drugs along with antibiotics as growth promoter increased the growth and immune status of chicken in contaminated environment (Lee et al., 2012). Antibiotics are given at sub therapeutic dosage for stabilization of the microflora of intestine and for improving the performance in general along with prevention of certain specific pathological
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conditions of the intestine. However, while using antibiotics as growth promoters it must be kept in mind that their use on long term basis must be avoided as it can give rise to resistance group of microorganisms i.e., gram negative bacteria (E. coli and Salmonella spp.) (Gustafson and Bowen, 1997; Dibner and Richards, 2005; Hassan et al., 2010; Seniya et al., 2012).
Salient features of AGPs:
modifies intestinal microflora and help to improve bird’s performance and health status;
have inhibitory effect on enzymes released by microorganisms and also on enzymes involved in microbial metabolism;
reduces the growth-depressing metabolites produced by microorganisms (Feighner and Dashkevicz, 1987; Huyghebaert et al., 2011);
addition of antibiotics to feed results in increased amino acid levels in the gut and improved nitrogen balance;
improves absorption of feed nutrients because of thinning of intestinal wall, feed conversion ratio, weight gain, performance and productivity. Increases egg production and hatchability;
reduces the damage caused by dietary fluctuations and destroy the harmful bacteria, keeping and minimize the adverse effects of dietary changes;
prevents exponential multiplication of common pathogenic bacteria (E. coli,
Salmonella spp., Streptococcus spp., Hemophilus etc.), reduces the incidences of non-
specific diarrhoea or enteritis of chicken (George et al., 1982; Brennan et al., 2003; Huyghebaert et al., 2011);
reduces stress and mortality in chicks by boosting body defense;
reduces the microbial use of nutrients (Snyder and Wostmann, 1987; Huyghebaert et al., 2011).
The use of antibiotics through feed or drinking water when the birds are not infected can be disadvantageous, in certain aspects. It can lead to the development of antibiotic resistant strains of pathogenic microorganisms like Staphylococcus aureus and Streptococcus spp. of bacteria. The continued use of antibiotics as routine feed additives may also result in the presence of antibiotic residues in poultry products. Certain antibiotics even as residues can cause allergic or hypersensitive reactions in consumers. The continuous application of antibiotics can suppress sensitive natural microflora in the gastrointestinal tract like
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saprophytes, commensals, non-pathogenic bacteria, fungi and yeasts or can show a compensatory growth and few can even increase their virulence.
Since 1999, some of the antibiotics growth promoters like spiramycin, tylosin, bacitracin and virginiamycin are banned in European Union due to development of resistance among bacteria of man and animals (McNamee et al., 2013). Apart from that, many other commonly used feed antibiotics had been banned by European commission due to development of resistant bacteria (Huyghebaert et al., 2011; Devirgiliis et al., 2013; Koluman and Dikici, 2013). At present use of antibiotics as growth promoter in food animals has become a great concern (Devirgiliis et al., 2013). World health organization along with World Organization for Animal Health (WOAH) encourages the health, agriculture, veterinary sector for reducing the injudicious use of antibiotics as growth promoters and further to decrease the spread of resistant bacteria (Aidara-Kane, 2012). Therefore, other non-therapeutic alternatives such as probiotics, prebiotics, symbiotics, antimicrobial peptides, enzymes, etheric oils, essential oils, eucalyptus oil, medium chain fatty acids, clay minerals, egg yolk antibodies, rare earth elements, recombinant enzymes and immunostimulants have been introduced as an alternative to the antibiotic growth promoters (Yang et al., 2008; Windisch et al., 2008; Nava et al., 2009; Huyghebaert et al., 2011; Wen and He, 2012; Tellez et al., 2012; Thacker, 2013; Mookiah et al., 2014).
Studies conducted concerning antimicrobials that are safer as alternatives for antibiotic replacement for interaction with the microflora of intestine, include prebiotics and probiotics, enzymes or dietary acidifications (Bedford, 2000; Patterson and Burkholder, 2003; Ricke, 2003; Diebold and Eidelsburger, 2006; Kocher, 2006; Niewold, 2007). Pedroso et al. (2013) reported that withdrawal of antibiotics or using alternatives to antibiotic growth promoter could alter the microbial flora of body.