CHAPTER-II

REVIEW OF LITERATURE

Growth Promoter – Growth promoters are agents added to animal feeds in order to enhance the feed conversion efficiency in terms of increased formation of muscles, fats and body growth in feed-lot animals, including poultry. (Vani Prasad and Koley, 2006)

Pharmacology of Growth Promoters in general

Probiotics

The word “probiotic” comes from the Greek words “pro” and “biotic,” meaning “for life” (Gibson and Fuller, 2000). Probiotics have been defined as “live microorganisms of non-pathogenic and non-toxic nature, which when administered through the digestive route, are favorable to the host’s health” (Kabir, 2009). The joint Food and Agriculture Organization of the United Nations (FAO) and World Health Organization (WHO) Working Group defined probiotics as “live micro-organisms which when administered in adequate amounts confer a health benefit on the host” (FAO/WHO, 2002). Certain species of bacteria, fungi and yeasts belong to group of probiotics. Existing probiotics can be classified into colonizing species (Lactobacillus sp., Enterococcus sp. and Streptococcus sp.) and free noncolonizing species (Bacillus and Saccharomyces cerevisiae). The definite mechanism through which probiotics may improve the defence and performance of chickens remains unclear, but some possible mode of actions have been proposed:

(1) maintaining a healthy balance of bacteria in the gut by competitive exclusion (the process by which beneficial bacteria exclude potential pathogenic bacteria through competition for attachment site in the intestine and nutrients) and antagonism (inhibit the growth of pathogenic bacteria by producing for example lactic acids), (2) promoting the gut maturation and integrity,(3) modulating the immune system and preventing inflammation (4) improving the metabolism by increasing digestive enzyme activity and decreasing bacterial enzyme activity and ammonia production, (5) improving feed intake and digestion (as a result from the improved microbial balance in the gut), and (6) neutralizing enterotoxins and stimulating the immune system (Khaksefidi and Rahimi, 2005; Haghighi et al., 2006; Kabir, 2009; Brisbin et al., 2010; Sugiharto, 2016). In terms of immune responses, different species and/or strains of probiotics may have different immunomodulatory activities due to the ability of probiotics to induce cytokine production, which leads to modulation of innate and adaptive immune responses (Brisbin et al., 2010; Sugiharto, 2016).

Their effect on production results in improvement of the function of the immune system (Kabir et al., 2004)and by exhibiting significant influence on morpho-functional characteristics of intestines. These effects lead to growth of broiler chickens,improvement of feed conversionand reduced mortality (Mohan et al., 1996). Schneitz et al. (1990) developed the first successful commercial probiotic (under the tradename Broilact), which was a mixture of 22 anaerobic bacilli and 10 facultative anaerobes isolated from healthy adult hens. Broilact was shown to prevent colonization and infection of chicks by S. Enteritidis, S. Typhimurium, Campylobacter jejuni, and E. coli O157:H7 (Edens, 2003; Singh et al., 2015). Kalavathy et al. (2003) found that a supplementation of twelve Lactobacillus strains in broiler diets improved the body weight gain, feed conversion rate and was effective in reducing abdominal fat deposition.Wishing to explain in a scientific way, inconsistent results which they obtained in their studies, majority of authors concluded that the effect of probiotics depend on the combination of bacterial strains contained in the probiotic preparation, level of its inclusion in the mixture, composition of mixture, quality of chickens and conditions of the environment in the production facility (Patterson and Brukholder, 2003).

Prebiotics

Prebiotics are defined as “Nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity ofone or a limited number of bacteria in the colon” (Gibson and Roberfroid, 1995). Prebiotics are non-digestible feed ingredients that beneficially affect the host by selectively altering the composition and metabolism of the gut microbiota (Huyghebaer