Background Acute appendicitis is a local intestinal inflammation with unclear origin. The aim was to test whether bacteria in appendicitis differ in composition to bacteria found in caecal biopsies from healthy and disease controls.

Methods and patients We investigated sections of 70 appendices using rRNA-based fluorescence in situ hybridisation. Four hundred caecal biopsies and 400 faecal samples from patients with inflammatory bowel disease and other conditions were used as controls. A set of 73 group-specific bacterial probes was applied for the study.

Results The mucosal surface in catarrhal appendicitis showed characteristic lesions of single epithelial cells filled with a mixed bacterial population (‘pinned cells’) without ulceration of the surroundings. Bacteria deeply infiltrated the tissue in suppurative appendicitis. Fusobacteria (mainly Fusobacterium nucleatum and necrophorum) were a specific component of these epithelial and submucosal infiltrates in 62% of patients with proven appendicitis. The presence of Fusobacteria in mucosal lesions correlated positively with the severity of the appendicitis and was completely absent in caecal biopsies from healthy and disease controls. Main faecal microbiota represented by Bacteroides, Eubacterium rectale (Clostridium group XIVa), Faecalibacterium prausnitzii groups and Akkermansia muciniphila were inversely related to the severity of the disease. The occurrence of other bacterial groups within mucosal lesions of acute appendicitis was not related to the severity of the appendicitis. No Fusobacteria were found in rectal swabs of patients with acute appendicitis.

Conclusions Local infection with Fusobacterium nucleatum/necrophorum is responsible for the majority of cases of acute appendicitis.