Acute appendicitis presents as two different diseases: uncomplicated and complicated acute appendicitis. The majority are uncomplicated, which may be treated with antibiotics instead of surgery and possibly even with symptomatic therapy. Complicated acute appendicitis involves necrosis leading to perforation and either peritonitis or a periappendicular abscess. However, the microbiological differences and the role of appendicoliths, fecal concretions in the appendix, in appendicitis severity remain poorly understood.

The first part of the thesis was designing and conducting the Microbiology APPendicitis Acuta (MAPPAC) trial. The second aim was to identify differences in appendiceal microbiota between uncomplicated and complicated acute appendicitis. Third study aimed to revise an appendicolith classification from 1966 and to assess the structural and elemental composition of the appendicolith. Fourth study explored the microbiome composition of appendicoliths to elucidate their formation mechanisms.

The prospective MAPPAC trial was conducted involving patients with CT confirmed acute appendicitis. The results showed differing appendiceal microbiomes associated with uncomplicated and complicated acute appendicitis. Appendicolith classification showed that even the softest appendicoliths (class 1) are visible by CT. Further results corroborate earlier observations of a concentrically layered inner structure built around distinguishable core, which was detected in most of the harder appendicoliths (class 2 and 3) and was associated with elemental composition. Appendicolith microbiome, enriched with specific Actinobacteria, suggest their potential roles in appendicolith formation.

In conclusion, this thesis bridges some knowledge gaps regarding the role of the appendiceal microbiome and appendicoliths in the severity of acute appendicitis, adding to the understanding of this very common surgical disease.