Rosmarinic Acid Attenuates Salmonella enteritidis-Induced Inflammation via Regulating TLR9/NF-κB Signaling Pathway and Intestinal Microbiota

<i>Salmonella enteritidis</i> (<i>SE</i>) infection disrupts the homeostasis of the intestinal microbiota, causing an intestinal inflammatory response and posing a great threat to human and animal health. The unreasonable use of antibiotics has led to an increase in the prevalence of drug-resistant <i>SE</i>, increasing the difficulty of controlling <i>SE</i>. Therefore, new drug strategies and research are urgently needed to control <i>SE</i>. Rosmarinic acid (RA) is a natural phenolic acid with various pharmacological activities, including antioxidant, anti-inflammatory and antibacterial properties. However, the protective effects and mechanism of RA on intestinal inflammation and the gut microbial disorders caused by <i>SE</i> have not been fully elucidated. In this study, RAW264.7 cells, MCECs and BALB/c mice were challenged with <i>SE</i> to assess the protective effects and mechanisms of RA. The results showed that RA enhanced the phagocytic ability of RAW264.7 cells, reduced the invasion and adhesion ability of <i>SE</i> in MCECs, and inhibited <i>SE</i>-induced inflammation in cells. Moreover, RA inhibited the activation of the NF-κB signaling pathway by upregulating TLR9 expression. Importantly, we found that RA provided protection against <i>SE</i> and increased the diversity and abundance of the intestinal microbiota in mice. Compared with infection control, RA significantly increased the abundance of <i>Firmicutes</i> and <i>Acidibacteria</i> and decreased the abundance of <i>Proteobacteria</i>, <i>Epsilonbacteraeota</i> and <i>Bacteroidota</i>. However, RA failed to alleviate <i>SE</i>-induced inflammation and lost its regulatory effects on the TLR9/NF-κB signaling pathway after destroying the gut microbiota with broad-spectrum antibiotics. These results indicated that RA attenuated <i>SE</i>-induced inflammation by regulating the TLR9/NF-κB signaling pathway and maintaining the homeostasis of the gut microbiota. Our study provides a new strategy for preventing <i>SE</i>-induced intestinal inflammation.