Influence of the polysaccharide capsule on virulence and fitness of Klebsiella pneumoniae

Lisa Zierke ¹ Rodi Mourad ¹ Thomas Peter Kohler ¹ Mathias Müsken ² Sven Hammerschmidt ^1*

• ¹ Interfaculty Institute of Genetics and Functional Genomics, Department of Molecular Genetics and Infection Biology, University of Greifswald, Greifswald, Germany
• ² Central Facility for Microscopy, Helmholtz Center for Infection Research, Helmholtz Association of German Research Centers (HZ), Braunschweig, Lower Saxony, Germany

The capsular polysaccharide (CPS) of pathogenic bacteria is a critical virulence factor, often avoiding phagocytosis by host immune cells but also interfering with the contact of the pathogen with host cells and contributing to biofilm formation. Klebsiella pneumoniae, a Gram-negative human pathogen associated with high antimicrobial resistances, produces 77 CPS serotypes. The CPS masks proteinaceous factors but also protects K. pneumoniae from uptake by host phagocytic cells and activation of the complement system. In addition to nosocomial, urinary tract and bloodstream infections or pneumonia hypervirulent strains have a highly mucoid phenotype and can cause soft tissue infections, liver abscesses and meningitis as well. The CPS is therefore crucial for both escaping detection by the immune system and enhancing the virulence potential. In this study, we generated a non-encapsulated mutant (Kpn2146∆wza) to observe how the CPS interferes with K. pneumoniae adhesion, survival in blood, and invasiveness in an experimental infection model. Infection of A549 lung epithelial cells showed similar adherence levels for the wild-type and noncapsulated strain, while our data showed a moderately higher internalization of Kpn2146Δwza when compared to the wild-type. In whole blood killing assays, we demonstrate that the K. pneumoniae capsule is essential for survival in human blood, for protecting K. pneumoniae against the recognition and clearing by the human immune system, and complement-mediated opsonization and killing, as the non-encapsulated mutant could not survive in either whole blood or human plasma. Infections of Galleria mellonella larvae showed a significant decreased virulence potential of the CPS deficient mutant. In conclusion, our data indicates a crucial role of CPS in vivo.