**2. Probiotics**

Probiotics are "live organisms and its substances that contribute to gut's microbial equilibrium," as Parker put it in 1974. Definition of probiotics as "a live microbial feed additive that beneficially impacts the host animal by enhancing its gut microbial balance" was established in 1989. More recently, "live microorganisms that when administered in suitable proportions impart a health benefit on the host" has been used to characterize probiotics [10].

Probiotics that are regularly utilized include strains of *Bacillus subtilis, Lactobacillus, Saccharomyces cerevisiae*, *Bifidobacterium, Aspergillus*, and *Streptococcus* all of which are capable of growth promotion and antimicrobial activity against pathogenic bacteria like *Escherichia coli, Salmonella typhimurium, Clostridium perfringens*, *Staphylococcus aureus*, etc. [11]. These strains can be isolated from fermented products or animal body such as breast milk, gut, fecal matter. *Lactobacillus plantarum* and *Leuconostoc mesenteroides* probiotics from some non-conventional sources of vegetables and fruits are also reported [12].

In the past, the probiotic market was predominantly led by the Asia Pacific region, with Europe following suit. The worldwide probiotic market's estimated worth is approximately USD 57.8 billion in 2022, and projections indicate a compound annual growth rate (CAGR) of 7.5% [13]. Apart from being consumed by humans, probiotics are also experiencing a growing application in animals, particularly in the context of poultry farming. The rising demand for poultry probiotic components in poultry diet can be attributed to the growing interest of consumers in eating more protein-rich meals like eggs. Probiotics employed in poultry farming may consist of a solitary strain or a blend of two or more strains, serving the purpose of disease prevention, health enhancement, and the augmentation of poultry growth and efficiency. It is widely recognized that utilizing a combination of multiple strains of probiotics can yield synergistic advantages [14]. Probiotics exist in different forms such as granules,

powder, liquid paste and gel out of which dry forms are better for gastric environment and longer shelf life [11]. Several review articles have reported that probiotic use in poultry has provided various benefits such as improvement in growth performance, along with immune enhancement, and sustainability of gut microbes.
