GENERAL MECHANISMS OF PROBIOTICS IN THE DIGESTIVE TRACT

Probiotics act through several interrelated biological mechanisms to maintain digestive health and support the immune system in poultry [52]. However, not all proposed mechanisms are supported by the same level of experimental evidence, and their consistency varies depending on strain, dosage, host age, and production conditions. Therefore, it is important to distinguish between well-established mechanisms and those that remain emerging or strain-dependent [12].

One important mechanism is competitive exclusion, the ability of probiotic microorganisms to compete with pathogens for colonization sites and nutrient sources on the intestinal epithelium [53]. Competitive exclusion against pathogens such as Salmonella spp., E_.__ coli_, and Clostridium perfringens is considered a well-established mechanism, particularly for Lactobacillus and Bacillus strains supported by in vivo challenge studies in broilers [20]. By occupying receptors typically used by pathogens, probiotics can inhibit the adhesion of harmful microorganisms such as Salmonella spp., E_.__ coli_, and C_.__ perfringens_; this mechanism is directly associated with reduced incidence of enteric infections and improved disease resistance in broilers and layers [54]. However, the magnitude of pathogen reduction is highly strain-specific and not uniformly reproducible across all experimental models or farm conditions. Furthermore, biofilm formation has been proposed as an additional protective mechanism, but evidence for stable biofilm-mediated protection in commercial poultry environments remains limited and should be regarded as an emerging hypothesis rather than a universally confirmed mechanism [55].

In addition to direct competition, probiotics also produce a variety of antimicrobial metabolites, including organic acids such as lactic, acetic, and butyric acids, as well as hydrogen peroxide and bacteriocins [56]. Organic acids help lower the pH of the intestinal lumen, creating less than ideal conditions for the growth of pathogenic bacteria [57]. Meanwhile, bacteriocins, antimicrobial peptides produced via ribosomal synthesis, provide an additional layer of protection by inhibiting pathogen cell wall formation or disrupting membrane integrity [58]. Nevertheless, bacteriocin-mediated pathogen inhibition is strongly strain-dependent, and in vivo efficacy does not always correlate with in vitro antimicrobial activity, highlighting inconsistency across studies. Thus, while antimicrobial metabolite production is biologically plausible and experimentally supported, its quantitative contribution to pathogen suppression under field conditions remains variable [59].

Probiotics help strengthen the integrity of the intestinal mucosa by modulating the expression of tight junction proteins, such as occludin, claudin, and zonula occludens [60]. Upregulation of tight junction gene expression has been repeatedly observed in controlled poultry experiments, particularly with selected Lactobacillus and Bacillus strains, suggesting that barrier reinforcement is a moderately well-established mechanism [61]. However, the translation of molecular changes into consistent improvements in growth performance or reduced systemic inflammation is not uniformly observed across all trials, indicating context dependency [62]. Increased mucus production by goblet cells has also been described, but evidence remains inconsistent, with some studies reporting significant effects while others show minimal or no change, suggesting strong strain and environmental interactions [63].

Another important mechanism is the ability of probiotics to alter the composition of the gut microbiota [64]. In commercial poultry production, probiotic supplementation has become increasingly prevalent, particularly following restrictions on AGPs, with industry reports indicating widespread adoption in broiler and layer operations across Europe and parts of Asia [8]. The presence of probiotic microbes helps establish a more stable microbial community, as reflected by an increase in beneficial bacteria such as Lactobacillus and Bifidobacterium and a decrease in the populations of pathogenic and opportunistic bacteria [65]. While shifts in microbial composition are commonly reported, the direction and magnitude of these changes vary substantially across studies, and causality between microbiota alteration and immune enhancement is not always clearly demonstrated. In many cases, microbiota modulation is inferred from correlation rather than mechanistic proof [66]. Moreover, some Bacillus-based products exert beneficial effects without persistent colonization, suggesting that transient metabolic or immunological signaling, rather than stable microbiota restructuring, may underlie observed benefits. This indicates that microbiota modulation is partly strain-dependent and not a universal requirement for probiotic efficacy [67].

Associations between probiotic supplementation and improved antibody titers following vaccination have been documented [21]. However, enhancement of post-vaccination immune responses appears highly strain-specific and influenced by timing, dosage, and baseline immune status, with some studies reporting significant improvements and others observing negligible effects [29]. Therefore, vaccine responsiveness should be interpreted as a promising but not universally guaranteed outcome of probiotic supplementation.

As shown in Figure 1[52–67], probiotics support poultry health through competitive exclusion, antimicrobial production, barrier modulation, and microbiota interaction. Among these, competitive exclusion and organic acid-mediated pathogen suppression are the most consistently supported mechanisms in poultry [53–59], whereas biofilm formation, extensive microbiota restructuring, and universal enhancement of vaccine responses remain emerging or strain-dependent phenomena requiring further controlled validation [55, 66, 67].

Figure 1: General mechanisms of probiotics in the digestive tract. This schematic illustration was conceptually developed based on published evidence [52–67] and generated using artificial intelligence tools (ChatGPT 5.2), then subsequently modified by the authors.

POSTBIOTICS AND PARAPROBIOTICS: NEXT-GENERATION ALTERNATIVES TO LIVE PROBIOTICS

Probiotics are traditionally defined as live microorganisms that confer health benefits to the host when administered in adequate amounts. However, recent advances in microbial biotechnology have expanded this concept beyond viable cells alone [13].

In addition to live probiotics, increasing attention has been directed toward postbiotics and paraprobiotics as next-generation functional feed additives. Paraprobiotics refer to inactivated (e.g., heat-killed) microbial cells that retain structural components capable of interacting with host immune receptors [68]. Postbiotics, by contrast, consist of microbial-derived metabolites or cell-free supernatants, including short-chain fatty acids (SCFAs), bacteriocins, peptides, teichoic acids, exopolysaccharides, and other bioactive compounds [69].

Recent meta-analyses (2024–2025) in broiler chickens indicate that postbiotics can significantly improve growth performance, FCR, villus height-to-crypt depth ratio, and both systemic and mucosal immune parameters. Several studies report enhanced intestinal morphology, increased expression of tight junction proteins, and modulation of cytokine profiles comparable to, or in some cases exceeding, the effects observed with live probiotic supplementation [15, 70, 71].

Mechanistically, live probiotics exert their effects through competitive exclusion, modulation of microbiota composition, metabolite production, and direct interaction with intestinal epithelial and immune cells [14]. Postbiotics and paraprobiotics, however, may act primarily through bioactive molecular signaling rather than colonization. Structural components such as peptidoglycan and lipoteichoic acid can interact with pattern recognition receptors (PRRs; e.g., TLRs), thereby stimulating controlled immune responses [72]. SCFAs and other metabolites contribute to epithelial integrity, anti-inflammatory signaling, and pathogen suppression. Because these preparations do not require microbial viability, their functional effects are less dependent on successful gut colonization [73].

From a commercial perspective, postbiotics and paraprobiotics offer several practical advantages. They demonstrate greater stability during feed pelleting and storage under high temperature and humidity, reducing losses in activity associated with heat-sensitive live cells [74]. Moreover, the absence of viable microorganisms minimizes concerns regarding horizontal gene transfer of ARGs, translocation, or unintended ecological persistence. These characteristics make them particularly attractive for intensive production systems where feed processing conditions may compromise live probiotic viability [75].

Nevertheless, certain limitations remain. Unlike live probiotics, postbiotics do not replicate or dynamically adapt to the gut environment, and their effects may depend on continuous supplementation [76]. Additionally, optimal dosing, standardization of bioactive components, and harmonized regulatory classification (as a feed additive vs. a functional metabolite) require further clarification [77].