Klebsiella are non-motile, Gramnegative rods that belong to the Enterobacteriaceae family. Klebsiella bacteria are considered commensal but act as opportunistic bacteria in the GI tract. Klebsiellais a leading cause of hospital-acquired infections.

Klebsiella is part of the normal intestinal flora. The environment likely acts as a reservoir for human acquisition, either as colonization or infection. It is frequently found in water, sewage, soil, and plant surfaces.

Klebsiella possesses virulence factors, such as a capsule, lipopolysaccharides, and pili. Klebsiella translocates across the intestinal epithelium via a transcellular mechanism by active bacterial invasion. This allows it to penetrate the intestinal barrier and enter systemic circulation causing extraintestinal disease. Cytotoxins produced by Klebsiella oxytoca are associated with antibiotic-associated hemorrhagic colitis (AAHC). Ankylosing spondylitis and Crohn’s disease have been shown to be triggered by Klebsiella pneumoniae. Increased starch consumption by genetically susceptible patients (HLA-B27 allelotypes) could trigger disease by enhancing the growth of Klebsiella in the gut. The cross-reactive antibodies between Klebsiella and AS/Crohn’s trigger inflammatory cascades, such as the complement system, as well as producing various cytokines causing pathologic changes.

Klebsiella can asymptomatically colonize the GI tract. However, depending on host factors and immunocompetence, it may cause diarrhea and systemic illnesses.