| Summary: | <p>Abstract</p> <p>Background</p> <p><it>Klebsiella pneumoniae </it>is a capsulated Gram negative bacterial pathogen and a frequent cause of nosocomial infections. Despite its clinical relevance, little is known about the features of the interaction between <it>K. pneumoniae </it>and lung epithelial cells on a cellular level, neither about the role of capsule polysaccharide, one of its best characterised virulence factors, in this interaction.</p> <p>Results</p> <p>The interaction between <it>Klebsiella pneumoniae </it>and cultured airway epithelial cells was analysed. <it>K. pneumoniae </it>infection triggered cytotoxicity, evident by cell rounding and detachment from the substrate. This effect required the presence of live bacteria and of capsule polysaccharide, since it was observed with isolates expressing different amounts of capsule and/or different serotypes but not with non-capsulated bacteria. Cytotoxicity was analysed by lactate dehydrogenase and formazan measurements, ethidium bromide uptake and analysis of DNA integrity, obtaining consistent and complementary results. Moreover, cytotoxicity of non-capsulated strains was restored by addition of purified capsule during infection. While a non-capsulated strain was avirulent in a mouse infection model, capsulated <it>K. pneumoniae </it>isolates displayed different degrees of virulence.</p> <p>Conclusion</p> <p>Our observations allocate a novel role to <it>K. pneumoniae </it>capsule in promotion of cytotoxicity. Although this effect is likely to be associated with virulence, strains expressing different capsule levels were not equally virulent. This fact suggests the existence of other bacterial requirements for virulence, together with capsule polysaccharide.</p>
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