Rosmarinic Acid Ameliorates PM2.5-Induced Alterations in Gut Microbiota and Intestinal Inflammation in Broilers

(1) Airborne fine particulate matter (PM2.5) poses a growing threat to poultry production by impairing intestinal health, disturbing microbial balance, and reducing growth performance. Rosmarinic acid (RA), a natural polyphenol with antioxidant, anti-inflammatory, and gut microbiota-regulating properties, can effectively maintain intestinal homeostasis. To date, its protective effects against PM2.5-induced intestinal injury in broilers remain largely unclear. This study investigated whether dietary RA supplementation mitigates intestinal damage and microbiota dysbiosis caused by PM2.5 in broilers and explored the related mechanisms. (2) A total of 144 21-day-old broilers were randomly allocated to three groups, control (CON), PM2.5 exposure (PM), and PM2.5 exposure plus rosmarinic acid (RA), with six replicates of eight broilers each. (3) Results indicated that PM2.5 exposure severely impaired growth performance, whereas dietary RA significantly increased average daily feed intake and average daily gain, decreased the feed-to-gain ratio, and elevated final body weight in broilers. RA significantly attenuated PM2.5-induced intestinal inflammation, as evidenced by reduced expression of inflammatory cytokines (IL-6 and IFN-γ) and downregulation of key components in the TLR4 signaling pathway (TLR4, MyD88, and NF-κB). Inhaled PM2.5 exposure impaired the intestinal epithelial barrier, marked by decreased mRNA levels of MUC2 and CLDN1 and increased caspase3 expression. Dietary RA treatment effectively restored these indicators, suggesting its role in maintaining epithelial integrity. Furthermore, RA reshaped the gut microbiota structure, altering both α- and β-diversity. Notably, RA led to a higher proportion of potentially health-promoting bacterial taxa, including Lactobacillus, V9D2013_group, and Oscillospirales, while reducing opportunistic pathogens like Shuttleworthia. (4) In conclusion, RA alleviates PM2.5-induced intestinal inflammation, reinforces the epithelial barrier, and modulates the intestinal microbiota in broilers, likely through inhibition of the TLR4/NF-κB signaling. These findings reveal a novel mechanism by which RA mitigates pollutant-induced intestinal injury via gut microbiota modulation and TLR4/NF-κB suppression, offering new insights into the gut–lung axis in avian species.