DEFINITION OF PROBIOTICS

Scientifically, probiotics are defined as live microorganisms that can provide health benefits to the host when consumed in adequate amounts [23]. Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer health benefits on the host, and in poultry, they are primarily used to improve gut microbiota balance and modulate the avian immune system [24]. Thus, probiotics not only act as commensal microbes but are active organisms that have been experimentally shown to produce physiological effects on the host, such as modulating the immune system, improving intestinal function and integrity, and inhibiting the growth of pathogens [25].

Conceptually, the definition of probiotics encompasses several fundamental scientific aspects. First, probiotics must consist of living microorganisms, namely cells that can survive until they reach the digestive tract and continue their biological activity [26]. This viability is assessed by the microbe's ability to withstand extreme conditions, including gastric pH, bile salts, and other gastrointestinal conditions [27]. Second, the effects of probiotics are dose-dependent, so they must be administered in sufficient quantities [28]. Various studies have shown that the effective dose range is between 10⁶ and 10¹⁰ colony-forming units (CFU), depending on the strain and the intended use [29].

The third important aspect is that the benefits of probiotics are strain-specific [30]. This means that each strain of microorganism has different genetic characteristics, colonization abilities, and physiological effects [31]. Therefore, claims of probiotic effectiveness cannot be generalized at the genus or species level but must be proven for each strain [32]. Fourth, probiotics must have a verified safety profile, including not carrying potentially transferable AMR genes (ARGs), not producing toxins, and not exhibiting pathogenic properties [33]. This safety assessment refers to the Generally Recognized as Safe (GRAS) or Qualified Presumption of Safety (QPS) systems used in evaluating microorganisms for feed and food applications [34].

In the context of poultry, probiotics are defined as microorganisms that not only improve gastrointestinal health but also modulate the immune system, enhance vaccine responses, and suppress the colonization of pathogens such as Salmonella, Escherichia coli, and Clostridium spp. [35]. Thus, probiotics function as biological agents that help maintain gastrointestinal balance and enhance the performance of the poultry immune system through mechanisms involving interactions among the gut microbiota, the intestinal epithelium, and the GALT [36].