The procedures for the care and use of animals were approved by the Ethics Committee of Mianyang Normal University, and all relevant institutional and governmental regulations concerning the ethical use of animals were strictly followed. For the present study, SPF rats (Kunming species, both male and female, weighing 180-220 g) were utilized. The animals were obtained from a commercial source (see Table of Materials). All animals were housed in a pathogen-free environment and provided ad libitum access to food.

1. Preparation of reagents

1. Prepare a 5% sodium salicylate solution by weighing 2.5 g of sodium salicylate powder (see Table of Materials) with an electronic scale. Dissolve the powder in a small amount of distilled water and dilute it to a total volume of 50 mL.
2. Prepare the decoction of D. caudatum (12.8 g/kg). Weigh 128 g of D. caudatum (see Table of Materials), place it into a round-bottom flask containing 1000 mL of laboratory distilled water, and concentrate it to a final volume of 100 mL.

2. Grouping and administration

1. Divide the rats into different groups and intragastrically administer the required solutions.
   NOTE: For the present study, 18 rats (9 males, 9 females) were divided into three groups: the control group (Group C), the model group (Group M), and the treatment group (Group T), with 6 rats in each group. The control group received saline solution, and the model group received a 5% sodium salicylate solution for 5 weeks at a dose of 10 mL/kg/day¹⁵. The treatment group was pretreated with a 5% sodium salicylate solution for 5 weeks at a dose of 10 mL/kg/day, followed by administration of the D. caudatum decoction at a dosage of 1 mL/100 g for 10 days^15,16.
2. Collect feces from the rats in dry and sterile microcentrifuge tubes using the tail-carrying method¹⁷ on the last day of the experiment and freeze them in a liquid nitrogen ultra-low-temperature freezer.
3. After the end of sampling, sacrifice the rats by cervical dislocation (following institutionally approved protocols). Open the abdominal cavity with scissors to expose the abdominal aorta.
   1. Insert the needle (23-25 G) almost parallel into the abdominal aorta, collect the serum, and preserve it in an ultra-low-temperature freezer. Take out the stomachs from the abdominal cavity¹⁶ and soak in a 10% formalin solution for preservation.
      NOTE: In Group M, the chronic gastritis model is considered successful if the gastric mucosa is dilated and congested with regular glandular morphology¹⁵, while observing a small amount of inflammatory cell infiltration in the lamina propria.

3. Pathological section of gastric wall

1. Take the gastric wall samples from rats in each group for pathological sections. Subsequently, perform routine hematoxylin-eosin staining to observe and analyze the pathological changes¹⁸.

4. Determination of serum gastrointestinal hormones

1. In the serum sample, detect the contents of gastrin (GAS) and malondialdehyde (MDA)¹⁹ using ELISA following the manufacturer's instructions (see Table of Materials). Utilize a microplate reader for the analysis.

5. Fecal total DNA extraction and PCR amplification and purification

1. Perform microbial DNA extraction utilizing the commercially available DNA isolation kit (see Table of Materials) and determine the concentration and purity using UV-Vis spectrophotometer¹⁶. Subsequently, assess DNA integrity through 1% agarose gel electrophoresis¹⁸.
2. Amplify the bacteria 16S rRNA gene with primers 338F (5′-ACT CCT ACG GGA GGC AGC AG-3′) and 806R (5′-GAC TAC HVG GGG GTW TCT AT-3′) using a thermocycler PCR system¹⁷. The primers are obtained from commercial sources (see Table of Materials).
   NOTE: Follow the PCR program: denaturation (3 min, 95 °C), followed by 27 cycles (30 s, 95 °C; 30 s, 55 °C; 45 s, 72 °C), and a final extension (10 min, 72 °C). Conduct PCR reactions with the following components: 4 µL of 5× DNA polymerase buffer, 2 µL of 2.5 mM dNTPs, 0.8 µL of each primer (5 µM), 0.4 µL of DNA polymerase, and 10 ng of template DNA (see Table of Materials).
3. Extract, purify, and quantify the PCR products sequentially using 2% agarose gel, commercially available DNA gel extraction kit, and fluorometer, respectively¹⁷.

6. Sequencing

NOTE: Step 6 and step 7 are performed following previously published methods^20,21.

1. Utilize the Illumina platform for sequencing.
   NOTE: One end of the DNA fragment complements the base of the connector embedded in the chip, securing it onto the chip. The other end randomly complements the base of another adjacent buried joint, forming a "bridge."
2. Perform PCR amplification to generate DNA clusters. Linearize DNA amplifiers into single strands.
3. Introduce an altered DNA polymerase alongside deoxyribonucleotide triphosphates (dNTPs) featuring four distinct fluorescent markers. Synthesize only one base per cycle.
4. Utilize a laser to scan the surface of the reaction plate and determine the nucleotide types polymerized during the initial reaction of each template sequence.
5. Chemically cleave the "fluorescence group" and "termination group" to reestablish the 3' sticky end. Subsequently, advance to polymerizing the second nucleotide.
6. Obtain the sequence of template DNA fragments by analyzing the fluorescence signal results collected in each round.

7. Processing of sequencing data

1. Initially, trim the reads of raw fastq data at any site with a score < Q20 and eliminate reads with uncertain bases. Additionally, permit two mismatches in precisely matched primers. Subsequently, merge the sequences with an overlap >10 bp.
2. Cluster operational taxonomic units (OTUs) with 97% similarity using UPARSE, and remove chimeric sequences with UCHIME^20,21.
3. Employ the RDP Classifier algorithm to analyze the taxonomy of each 16S rRNA gene sequence against the Silva (SSU123) 16S rRNA database, using a confidence threshold of 70%.
4. Perform Alpha diversity analysis and Beta diversity analysis using Mothur and Qiime, respectively (see Table of Materials).

8. Statistical analysis

1. Analyze the data in this study using statistical analysis software (see Table of Materials). Employ one-way analysis of variance (ANOVA) for comparisons among multiple groups and utilize the least-significant-difference (LSD) test for comparisons between two groups.
2. Consider P < 0.05 as statistically significant in all comparisons. P < 0.01 is regarded as extremely significant.