PROBIOTIC–MICROBIOTA INTERACTIONS IN SUPPORTING IMMUNITY

The gut microbiota of poultry plays a crucial role in maintaining immune homeostasis and gastrointestinal health. Microbiota imbalance (dysbiosis), which can occur due to stress, infection, or antibiotic use, is often associated with increased pathogen colonization and a decreased immune response [187]. Probiotic supplemen-tation helps modify the microbiota and restore balance, supporting innate and adaptive immune function [188].

It should be emphasized that microbiota modulation does not always translate directly into measurable immune enhancement. Some studies demonstrate significant shifts in microbial composition without parallel increases in antibody titers or cytokine responses, indicating that microbial changes alone are insufficient predictors of functional immunity [189, 190].

Recent advances in multi-omics technologies, including 16S rRNA gene sequencing, shotgun metagenomics, metabolomics, and transcriptomics, have provided deeper mechanistic insights into probiotic–microbiota–host interactions [191]. High-throughput sequencing analyses frequently report increases in alpha diversity indices (e.g., Shannon and Chao1) following probiotic supplementation, although the magnitude and consistency of these changes vary among strains and production environments [192]. Beta-diversity analyses often reveal distinct clustering of treated versus control groups, suggesting measurable shifts in microbial community structure [193].

One of the primary mechanisms of action of probiotics is to restore the composition of the gut microbiota [194]. Probiotics can increase the number of beneficial bacteria, such as Lactobacillus spp. and Bifidobacterium spp., which play a role in producing bioactive metabolites, SCFAs, and antimicrobial peptides. These metabolites not only inhibit the growth of pathogens but also interact with epithelial cells and GALT to strengthen the integrity of the mucosal barrier and stimulate immune cell activity [195].

Metabolomic profiling has further linked probiotic-induced microbiota shifts to increased production of SCFAs, particularly acetate, propionate, and butyrate. SCFAs serve not only as energy substrates for enterocytes but also as signaling molecules that regulate tight junction protein expression, mucin production, and anti-inflammatory pathways [196]. Integrative analyses using multivariable association models (e.g., MaAsLin2) have identified significant associations between specific bacterial taxa and host immune markers, including correlations between butyrate-producing taxa and increased mucosal IgA levels or anti-inflammatory cytokine expression [197].

In addition to increasing the number of beneficial bacteria, probiotics also help reduce the population of pathogens such as Salmonella spp., E_.__ coli_, and Clostridium perfringens. This occurs through competition for space and nutrients (competitive exclusion), the production of antimicrobial compounds such as organic acids and bacteriocins, and an enhanced mucosal immune response that limits pathogen colonization [198]. Thus, probiotics function as mediators of microbial ecology, maintaining a balance between commensal and pathogenic microbiota. However, competitive exclusion effects are strain-specific, and not all probiotic formulations consistently reduce pathogen load in vivo. Variability in colonization ability and farm hygiene conditions further complicates the establishment of a definitive strain hierarchy [199].

At the host molecular level, transcriptomic studies demonstrate that probiotic supplementation can modulate key immune and stress-related pathways, including suppression of NF-κB activation under heat or pathogenic stress conditions, upregulation of tight junction–related genes (e.g., occludin, claudins), and modulation of cytokine gene expression (e.g., IFN-γ, IL-10, and IL-1β) [200]. Emerging evidence also suggests potential involvement in T regulatory (Treg) cell differentiation and epigenetic modulation through histone acetylation pathways influenced by SCFAs, although these mechanisms require further validation in poultry models [201].

The synergistic interaction among probiotics, the microbiota, and the immune system contributes to increased resistance to infection, strengthened mucosal immune response, and enhanced vaccine effectiveness and overall gastrointestinal health. Therefore, modulating the microbiota with probiotics is a crucial strategy in modern poultry production to naturally and sustainably improve performance, health, and pathogen resistance [202]. As illustrated in Figure 3[191–200], the gut–immune axis in poultry highlights the bidirectional interaction between gut microbiota, the intestinal epithelial barrier, and GALT, where microbial metabolites modulate immune responses while immune signaling maintains epithelial integrity and microbial balance.

Figure 3: Gut–immune axis in poultry showing the bidirectional interaction between gut microbiota, intestinal epithelial barrier, and gut-associated lymphoid tissue (GALT). This schematic illustration was conceptually developed based on published evidence [191–200] and generated using artificial intelligence tools (ChatGPT 5.2), then subsequently modified by the authors.