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Phagocytosis is a key process of the immune system. The human pathogen  Klebsiella pneumoniae  is a well known example of a pathogen highly resistant to phagocytosis. A wealth of evidence demonstrates that the capsule polysaccharide (CPS) plays a crucial role in resistance to phagocytosis. The amoeba  Dictyostelium discoideum  shares with mammalian macrophages the ability to phagocytose and kill bacteria. The fact that  K. pneumoniae  is ubiquitous in nature and, therefore, should avoid predation by amoebae, poses the question whether  K. pneumoniae  employs similar means to counteract amoebae and mammalian phagocytes. Here we developed an assay to evaluate  K. pneumoniae - D. discoideum  interaction. The richness of the growth medium affected the threshold at which the  cps  mutant was permissive for  Dictyostelium  and only at lower nutrient concentrations the  cps  mutant was susceptible to predation by amoebae. Given the critical role of bacterial surface elements on host-pathogen interactions, we explored the possible contribution of the lipopolysaccharide (LPS) and outer membrane proteins (OMPs) to combat phagoyctosis by  D. discoideum . We uncover that, in addition to the CPS, the LPS O-polysaccharide and the first core sugar participate in  Klebsiella  resistance to predation by  D. discoideum .  K. pneumoniae  LPS lipid A decorations are also necessary to avoid predation by amoebae although PagP-dependent palmitoylation plays a more important role than the lipid A modification with aminoarabinose. Mutants lacking OMPs OmpA or OmpK36 were also permissive for  D. discoideium  growth. Except the LPS O-polysaccharide mutants, all mutants were more susceptible to phagocytosis by mouse alveolar macrophages. Finally, we found a correlation between virulence, using the pneumonia mouse model, and resistance to phagocytosis. Altogether, this work reveals novel  K. pneumoniae  determinants involved in resistance to phagocytosis and supports the notion that  Dictyostelium  amoebae might be useful as host model to measure  K. pneumoniae  virulence and not only phagocytosis.

Role of Bacterial Surface Structures on the Interaction of Klebsiella pneumoniae with Phagocytes