BACTERIAL STRAINS USED

The selection of bacterial strains is a crucial factor in determining the effectiveness of probiotics in poultry, as each strain has unique capabilities in gut colonization, immune system modulation, and pathogen growth inhibition [255]. Commonly used strains include Lactobacillus spp., Bifidobacterium spp., Bacillus spp., Enterococcus spp., and Streptococcus spp., each with its own distinct metabolic characteristics and immunomo-dulatory effects [28].

Lactobacillus spp. are among the most commonly used probiotic strains in poultry [40]. This bacterium can colonize the intestinal epithelium, producing lactic acid to lower the lumen's pH and inhibiting the growth of enteric pathogens such as Salmonella spp. and E_.__ coli_ [256]. Furthermore, Lactobacillus can stimulate the phagocytic activity of macrophages and heterophils and increase antibody production at both the mucosal and systemic levels [97].

Bifidobacterium spp. functions in the fermentation of oligosaccharides into SCFAs, which help maintain the integrity of the intestinal epithelium and act as immune signals for dendritic cells and lymphocytes [257]. These metabolites promote immune cell proliferation, reduce excessive inflammation, and strengthen the adaptive immune response to vaccines and pathogen infections [258].

Bacillus spp., particularly spore strains, are highly resistant to heat and stomach acid, allowing them to survive in the intestines [259]. Furthermore, Bacillus spp. can produce digestive enzymes, bacteriocins, and antimicrobial metabolites that support the growth of beneficial microbiota while suppressing pathogen colonization [260].

Enterococcus spp. and Streptococcus spp. are used as probiotics for their ability to stimulate both mucosal and systemic immune responses and to compete with pathogens for space and nutrients [261, 262]. These strains are often combined with Lactobacillus or Bacillus to enhance their synergistic effects in modulating the immune system and supporting gastrointestinal health [263].

Beyond biological performance, strain selection should consider regulatory approval status in the target market, documented safety (including absence of virulence factors or transferable resistance genes), and consistency of commercial production. For multinational poultry operations, differences in approval status across regions (e.g., EU vs. various Asian or other developing countries) may influence product availability and formulation strategies [264].

The selection of probiotic strains requires consideration of their resistance to the poultry digestive tract, colonization capacity, immunomodulatory potential, and safety [265]. Ultimately, successful probiotic implementation requires integration of biological efficacy, regulatory compliance, and economic return [15]. A structured evaluation combining scientific evidence, farm level performance data, and cost analysis provides a rational framework for decision-making in modern poultry production systems. A suitable combination of strains can enhance the effectiveness of probiotics in strengthening innate and adaptive immunity, increasing resistance to infection, and supporting sustainable poultry production performance [17].