TYPES AND SOURCES OF PROBIOTICS FOR POULTRY

A variety of microorganisms have been identified and used as probiotics in poultry for their biological properties, ability to colonize the intestinal tract, and physiological effects on the host [37]. Generally, probiotics for poultry are derived from groups such as lactic acid bacteria, spore-forming bacteria, commensal anaerobes, and several types of yeast, which are known for their high stability and safety [38]. Strains are selected through a rigorous process that includes testing for resistance to gastric acid and bile salts, adhesion to the intestinal epithelium, competition with pathogens, and potential modulation of the immune system [39]. However, beyond these biological criteria, commercial applicability in poultry production also depends on processing stability, shelf life, consistency of in vivo performance, and regulatory acceptance, which create important functional distinctions among probiotic groups [35].

The Lactobacillus spp. group is the most commonly used probiotic species in the poultry industry [40]. Species such as Lactobacillus acidophilus, Lactobacillus plantarum, and Lactobacillus reuteri are known to produce lactic acid, bacteriocins, and various bioactive metabolites that inhibit the growth of pathogenic bacteria and help maintain the balance of the gut microbiota [41]. Furthermore, these bacteria can enhance mucosal immune system activity by stimulating IgA production and regulating cytokine responses [42]. Notably, several Lactobacillus strains, particularly L._ acidophilus_ and L_.__ plantarum_, are supported by substantial in vivo poultry studies demonstrating improvements in intestinal morphology and immune parameters. Nevertheless, their relatively lower resistance to high temperature feed pelletization and environmental stress may limit viability during industrial feed processing unless protective technologies (e.g., microencapsulation or post-pellet spraying) are applied [41].

Additionally, Bacillus spp., such as Bacillus subtilis and Bacillus licheniformis, are widely used as probiotics due to their ability to form spores that are highly resistant to high temperatures, environmental stress, and feed pelletization processes [43]. Bacillus strains can produce digestive enzymes, improve nutrient utilization, and provide immunomodulatory effects through macrophage activation and regulation of inflammatory responses [44]. The robustness of their spores makes this group highly superior as commercial probiotics, particularly in intensive poultry production systems [45]. These species are among the most extensively validated probiotics in commercial broiler and layer trials, with strong in vivo evidence supporting their effects on growth performance and feed efficiency. Compared with non-spore-forming bacteria, Bacillus spp. exhibit superior thermal resistance, storage stability, and survivability during feed manufacturing, making them particularly compatible with large-scale commercial poultry systems [44].

The Bifidobacterium spp. group, although more commonly found in mammals, has also been shown to be beneficial for birds, particularly in maintaining intestinal microbial balance and inhibiting the colonization of pathogenic bacteria [46]. Bifidobacterium bifidum and _Bifidobacterium _animalis contribute to the production of acetic acid and various other metabolites that help strengthen the integrity of the intestinal mucosal barrier [47]. However, compared to Lactobacillus and Bacillus, the evidence base in poultry remains more limited, and some functional claims are partially extrapolated from mammalian models. However, compared with Lactobacillus and Bacillus, poultry-specific in vivo data remain more limited, and some functional claims are partially extrapolated from mammalian models, which may reduce translational certainty under commercial poultry conditions [41, 43].

On the other hand, several Enterococcus species, including Enterococcus faecium, are used as probiotics due to their ability to rapidly colonize the digestive tract of poultry and efficiently compete with pathogens such as Salmonella and E_.__ coli_ [48]. However, the use of these bacteria must be accompanied by a thorough safety assessment to ensure that they do not contain potentially transmitted ARGs [49]. This regulatory concern may restrict broader industry adoption despite demonstrated pathogen exclusion capacity.

In addition to bacterial groups, probiotics derived from yeast, such as Saccharomyces boulardii, also have significant potential to support poultry health [50]. This yeast is known to be resistant to various antibiotics and gastrointestinal conditions, and to suppress pathogen colonization through toxin-binding mechanisms, increase digestive enzyme activity, and modulate both inflammatory and anti-inflammatory immune responses [51]. While promising poultry studies exist, certain mechanistic insights, particularly for S. boulardii_, are derived from non-avian models and require further confirmation in controlled in vivo poultry experiments. While promising, certain mechanistic interpretations of S._ boulardii are based on non-avian studies and require further controlled validation in poultry [50].

From a critical comparative perspective, clear functional trade-offs emerge among probiotic groups. Lactobacillus spp. are strongly associated with enhanced mucosal and humoral immunity (particularly IgA stimulation and cytokine modulation), making them suitable for immune-oriented interventions [40]. Bacillus spp. combine moderate but consistent immunomodulatory effects with marked improvements in nutrient digestibility and feed conversion efficiency [43]. Bifidobacterium spp. primarily reinforce barrier integrity and microbiota stability, indirectly supporting immune resilience. Enterococcus spp. and S_._ boulardii emphasize pathogen exclusion but present either regulatory or evidentiary limitations [48].

Importantly, the dominance of Bacillus spp. in commercial poultry systems is largely driven by technological robustness and economic practicality rather than purely superior immunological potency. Spore formation allows survival during feed pelletization temperatures exceeding 80°C, extended storage without refrigeration, and stability under variable farm conditions [43]. This ensures predictable dosing and minimizes viability losses during distribution. Moreover, the enzyme-producing capacity of Bacillus strains directly enhances feed efficiency, an economically critical parameter in intensive broiler and layer production [44]. Combined with strong field validation and regulatory acceptance, these advantages explain why Bacillus-based probiotics currently dominate commercial poultry markets. To enhance comparative clarity, Table 1 summarizes the major probiotic groups by strain type, dominant immune effects, processing stability, and level of industrial application [40–51].

Probiotic group | Representative strains | Dominant immune and functional effects | Processing and gastrointestinal stability | Industry application level | References
Lactobacillusspp. | Lactobacillus acidophilus, Lactobacillus plantarum, Lactobacillusreuteri | Strong mucosal immune stimulation through enhanced IgA production, cytokine modulation, improved intestinal morphology, and pathogen inhibition via lactic acid and bacteriocin production | Moderate heat resistance; relatively sensitive to feed pelletization and environmental stress unless protected by microencapsulation or post-pellet application | Widely used; strong in vivo poultry evidence and extensive experimental support | [40–42]
Bacillus spp. | Bacillus subtilis, Bacillus licheniformis | Moderate but consistent immune modulation, macrophage activation, cytokine balance, digestive enzyme production, improved nutrient utilization and feed efficiency | Very high stability due to spore formation; highly resistant to pelletization, storage, and farm environmental stress | Dominant in commercial poultry systems; extensive field validation and strong economic relevance | [43–45]
Bifidobacterium spp. | Bifidobacterium bifidum, Bifidobacteriumanimalis | Enhancement of intestinal barrier integrity, microbiota stabilization, acetic acid production, and indirect immune resilience support | Moderate gastrointestinal survival; lower thermal and processing stability compared with spore-forming bacteria | Limited to moderate use; fewer poultry-specific in vivo studies | [46, 47]
Enterococcusspp. | Enterococcus faecium | Rapid intestinal colonization, strong pathogen competition, exclusion of Salmonella spp. and Escherichia coli | Good gastrointestinal survival; requires strict safety and antimicrobial resistance screening | Moderate use; restricted by regulatory and biosafety concerns | [48, 49]
Yeast (Saccharomycesboulardii) | S . boulardii | Pathogen toxin binding, modulation of inflammatory and anti-inflammatory immune responses, improved digestive enzyme activity | High gastrointestinal resilience and good feed stability | Emerging use; promising but still variable poultry-specific evidence | [50, 51]