The goal of this observational study is to evaluate the orointestinal microbiome and microbial derived metabolome in patients suffering from acute pancreatitis as a biomarker for severity. The main questions it aims to answer are:

• Can the orointestinal microbiome robustly predict the course of acute pancreatitis?
• How does the microbiome impact the severity of an acute pancreatitis?

Buccal/ rectal swabs, plasma and stool is collected from patients with acute pancreatitis within 48h after hospital admission.

Despite intensive research, early prediction of the course of acute pancreatitis (AP) is still not satisfactorily possible. Results of a European multicenter study showed that the intestinal microbiome is superior to established scores as a marker of severity in patients with AP. Hereby, a classifier was established using 16 differentially abundant rectal species and systemic inflammatory response syndrome (SIRS) and achieved an AUROC of 85%. Surprisingly, all species in the severe AP group were members of taxonomic families known for their short-chain fatty acid (SFCA) production. This observation contrasts with translational pancreatitis studies in mice. Based on these publications, a clinical trial is currently being initiated to treat severe AP with SCFA (NCT06147635). However, previous well-designed RCT that analyzed the effects of probiotics in predicted severe AP resulted in a worse outcome for patients in the probiotic arm. Consequently, national and international guidelines recommend against the usage of probiotics in AP.

Collectively, more research is needed to further elucidate the role of the oro-intestinal microbiota in the development of severe AP. To validate the results of previously mentioned multicenter study and to profoundly analyze the role of microbial metabolites and the fungeome, patients with AP will be prospectively recruited.

1. Buccal and rectal swabs, stool and plasma will be obtained to analyze the orointestinal microbiome and microbial derived metabolites.
2. Centers from different continents with different ethical background and dietary habits will enroll patients to gain a more generalizable microbial profile.
3. Microbial shifts were observed between severe AP (RAC 3) and mild/ moderate severe (RAC 1+2).
4. It is expected that the microbial compositions change during the inflammatory process upon early phase of pancreatitis. To minimize this microbial alternating effect a short time frame from hospital admission to recruitment (48h) is set.

Inclusion Criteria:

• Patients with initial diagnosis (< 48h) of acute pancreatitis
• Age ≥ 18 years
• Patients able to understand/ give their written consent

Exclusion Criteria:

• Recurrent acute pancreatitis (>2 previous episodes)
• Clinical or imaging signs of chronic pancreatitis
• Referred patients with length of hospital stay > 48h

Patients with acute pancreatitis within 48h after hospital admission. Acute pancreatitis is defined if 2 of the following criteria are fullfilled.

1. Lipase > 3x the upper limit
2. Typical abdominal pain (belt-like upper abdominal pain)
3. Characteristic CT findings