Simulated poultry-house PM _2.5 exposure reveals a gut–lung axis mechanism of microbial propionate in protecting against pneumonia

Poultry house fine particulate matter (PM_2.5) poses significant respiratory risks to poultry by penetrating deep into the lung and triggering inflammatory cascades. In this study, 21- to 28-day-old broilers were exposed to total suspended particulates enriched in PM_2.5 (2 mg/m³, 2 h/day) to investigate pulmonary injury and gut-lung axis perturbations. PM_2.5 exposure induced collapse of the hexagonal lobular architecture, elevated pulmonary expression of IL-1β, IL-2, IL-6, IL-8, and IL-10, and activated NF-κB signaling. Concurrently, cecal microbiota α-diversity increased while the community shifted toward pro-inflammatory taxa (<i>Alistipes</i>, <i>Rikenellaceae</i>) and away from SCFA-producing species (<i>Bacteroides uniformis</i>, <i>Parabacteroides</i>). Oral supplementation of <i>B. uniformis</i> restored its abundance, replenished acetate and propionate levels, and attenuated lung injury by reducing APC activation (CD40, CCL4) and Th1 polarization (T-bet, IFN-γ, IL-18), while promoting regulatory T cell markers (FoxP3). Dietary sodium propionate supplementation in feed (0.4%) similarly mitigated pulmonary inflammation and Th1 skewing, albeit without enhancing Treg responses. These findings demonstrate that PM_2.5-induced lung damage is intricately linked to gut dysbiosis and SCFA depletion and that restoration of <i>B. uniformis</i> or its metabolite propionate can recalibrate the gut-lung axis to suppress innate and adaptive inflammatory pathways. This work highlights microbiota- and metabolite-based interventions as promising strategies to protect poultry respiratory health and performance under air-polluted conditions.IMPORTANCEThis study reveals that poultry house-derived PM_2.5 not only causes direct lung inflammation but also perturbs the gut-lung axis by depleting beneficial SCFA-producing bacteria. The resulting gut dysbiosis amplifies respiratory immune injury, highlighting a previously underappreciated systemic effect of airborne pollutants in livestock environments. Our findings suggest that microbiota- and metabolite-targeted dietary strategies can mitigate air pollution-induced health risks in poultry. This work provides new insights into the broader ecological and agricultural consequences of PM_2.5 exposure and supports sustainable, non-antibiotic interventions to enhance animal welfare and productivity under deteriorating air quality conditions.