The purpose of this clinical trial is to investigate the impact of Bifidobacterium longum(BL) on the clinical prognosis of patients with acute pancreatitis(AP), to analyze the correlation between BL and intestinal barrier function, as well as the gut microbiota, and to observe adverse reactions and risks in patients with AP after the use of BL.

Participants will be randomly assigned to two groups: the intervention group and the control group. They will receive:

• Intervention group: Standard clinical treatment + BL capsules (10^10 CFU), twice a day, for a total of 14 days;
• Control group: Standard clinical treatment + placebo capsules, for a total of 14 days.

A total of 60 patients will be included in this study.

Rationale:The impairment of the intestinal mucosal barrier in patients with acute pancreatitis (AP) plays a crucial role in the progression to severe AP(SAP). Our previous research found that the early gut microbiota structure of AP patients is significantly different from that of healthy individuals, characterized by a marked increase in the relative abundance of conditional pathogens such as Escherichia coli and Shigella, while beneficial bacteria that produce short-chain fatty acids, such as Bifidobacterium, are significantly reduced, especially in patients with SAP. Bifidobacterium longum (BL), a well-known probiotic, has been used to treat a variety of diseases. In our previous animal experiments, we found that BL could alleviate pancreatic damage and inflammatory responses in AP mice and regulate the balance of the gut microbiota. Based on these findings, this study aims to assess the impact of BL on the clinical prognosis of AP patients through a randomized controlled trial, in order to provide a scientific basis for the application of BL in the treatment of AP and to further explore its potential clinical value.

Objective: The purpose of this clinical trial is to investigate the impact of BL on the clinical prognosis of patients with AP, to analyze the correlation between BL and intestinal barrier function, as well as the gut microbiota, and to observe adverse reactions and risks in patients with AP after the use of BL.

Study design: Single-center, randomized, double-blind, placebo-controlled study.

Study population:60 adult patients with acute pancreatitis. Intervention: The intervention group receives standard clinical treatment plus BL capsules (10^10 CFU), twice a day, for a total of 14 days; the control group receives standard clinical treatment plus placebo capsules, for a total of 14 days.

Main study parameters/endpoints: The primary endpoint is the number of days without SIRS within 14 days; the secondary endpoints include infectious complications (including fungal infections), parameters related to systemic inflammatory response, intestinal barrier function and gut microbiota composition, indicators related to recovery of intestinal function, antibiotic use, laboratory-related indicators, and clinical outcomes.

Safety: Throughout the study (or afterwards), treatment-emergent adverse events (TEAEs) were recorded, including gastrointestinal adverse reactions (abdominal pain, nausea, vomiting, bloating, or diarrhea) and allergic reactions, and adverse events that led to discontinuation of the study drug were documented.

Inclusion Criteria:

1. Age 18-75 year;
2. The diagnosis of acute pancreatitis according to the revised Atlanta classification;
3. The onset time of acute pancreatitis is within 48 hours;
4. APACHE II score of ≥8, or C-reactive protein > 150 mg/L, or SIRS score of ≥3;
5. Signed the informed consent.

Exclusion Criteria:

1. Within 48 hours of onset, there is multi-organ failure;
2. Use of probiotics within the last month;
3. Pancreatitis following endoscopic retrograde cholangiopancreatography (ERCP);
4. Intra-operative diagnosis;
5. Infection/sepsis caused by a second disease;
6. Malignancy;
7. Immunocompromised patients;
8. Pregnancy and/or lactation;
9. Allergy to Bifidobacterium longum.