STRAIN-SPECIFIC EFFECTS AND QUANTITATIVE EVIDENCE

Probiotic efficacy in poultry is widely acknowledged to be strain-specific; however, many published studies and narrative summaries tend to generalize effects at the genus level (e.g., Lactobacillus and Bacillus) without direct comparative synthesis.

Recent systematic reviews and meta-analyses (2024–2025) highlight substantial heterogeneity in outcomes, particularly regarding antibody titers, Salmonella reduction, and feed conversion ratio (FCR). Pooled analyses indicate that while overall probiotic supplementation may improve performance and selected immune markers, effect sizes vary considerably depending on strain identity, dosage, bird age, challenge model, and environmental conditions. In several meta-analyses, between-study heterogeneity (I²) was moderate to high, underscoring the inconsistency of responses across trials.

For example, L_.__ acidophilus_ supplementation has frequently been associated with increased mucosal IgA secretion and, in some vaccination trials, elevated serum IgY titers; however, quantitative synthesis reveals that improvements in antibody response are not uniformly significant across all studies. Similarly, reductions in cecal Salmonella counts range from negligible to approximately 1 log₁₀ CFU/g depending on strain and experimental challenge conditions [41].

In contrast, B_.__ subtilis_ strains tend to show more consistent improvements in growth performance parameters, particularly FCR, likely due to increased enzyme production and enhanced nutrient digestibility. Nevertheless, immune endpoints such as cytokine expression (e.g., Interferon-gamma [IFN-γ], interleukin [IL]-1β, IL-6, IL-10) and lymphoid organ indices (bursa, spleen, thymus weights) show variable modulation, suggesting that performance benefits do not always parallel measurable immune enhancement [42].

Multi-strain formulations (e.g., combinations of Lactobacillus spp. and Bacillus spp.) are sometimes reported to exert additive or synergistic effects on mucosal IgA levels and pathogen suppression. However, recent quantitative reviews caution that multi-strain products do not consistently outperform well-characterized single strains, and, in some analyses, the magnitude of the effect is comparable or highly context-dependent [43]. This finding reinforces the importance of precise strain-level evaluation rather than broad taxonomic categorization.

Taken together, these findings indicate that probiotic effects cannot be reliably inferred at the genus level. Table 2 summarizes the comparative effects of selected probiotic strategies in broiler chickens based on strain-specific evidence, highlighting differences in immune responses, lymphoid organ modulation, pathogen reduction, and production performance [41–43].

Probiotic strategy | Immune response (IgA/ IgY , cytokines) | Effects on lymphoid organs | Salmonella reduction | Performance (BWG/FCR) | Overall consistency of evidence | References
Lactobacillus acidophilus (single strain) | Increased mucosal IgA with moderate consistency; serum IgY response variable across vaccination and challenge studies; cytokine modulation highly context-dependent | Mild increase in bursa weight reported in some trials; spleen and thymus responses inconsistent | Variable reduction, ranging from negligible effect to approximately 1 log₁₀ CFU/g in cecal content under challenge conditions | Modest improvement in FCR; BWG response generally variable | Moderate heterogeneity among studies; strain- and challenge-dependent outcomes | [41]
Bacillus subtilis (single strain) | Variable cytokine modulation (e.g., IFN-γ, IL-1β, IL-6, IL-10); limited consistent evidence for IgY enhancement | Inconsistent effects on bursa, spleen, and thymus weights | Strain-dependent and variable across experimental models | More consistent improvement in FCR and sometimes BWG, likely associated with enzyme production and improved nutrient digestibility | Relatively consistent for performance, but heterogeneous for immune endpoints | [42]
Multi-strain formulations | Potential additive or synergistic effects on mucosal IgA and cytokine responses; effect magnitude varies substantially | Variable and highly dependent on strain combination and dosage | Sometimes greater reduction than control groups; not consistently superior to optimized single strains | Variable effects on BWG and FCR; strongly context-dependent | High between-study heterogeneity; inconsistent superiority over single strain probiotics | [43]