Background. Peptidoglycan-associated lipoprotein (Pal), murein lipoprotein (LppA), and outer membrane protein A (OmpA) are dominant outer membrane proteins (OMPs) that are released by gram-negative bacteria during sepsis. OMPs are implicated in the maintenance of cell envelope integrity. Here, we characterize the roles of these OMPs in pathogenesis during bacteremia caused by Klebsiella pneumoniae. Methods. pal-, lppA-, and ompA-deficient K. pneumoniae strains were constructed using an unmarked

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... deletion method. Serum sensitivity, antiphagocytosis activity, outer membrane permeability, and sensitivity to anionic detergents and antimicrobial polypeptides were determined for these OMP gene deletion mutants. The ability of these OMP gene deletion mutants to induce immune responses was compared with that of the wild-type strain in a bacteremic mouse model. Results. Klebsiella pneumoniae strains deleted for pal or lppA exhibited reduced protection from serum killing and phagocytosis; perturbation to the outer membrane permeability barrier and hypersensitivity to bile salts and sodium dodecyl sulfate. The strain mutated for lppA had reduced ability to activate Toll-like receptor 4. Immunization of mice with the pal or lppA mutant provided protection against infection by the wild-type strain. Conclusions. Our findings indicate that K. pneumoniae Pal and LppA proteins are important in the maintenance of cell integrity, contribute to virulence, and could be used as attenuated vaccines. Keywords. Klebsiella pneumoniae; outer membrane proteins; peptidoglycan-associated lipoprotein; murein lipoprotein; outer membrane protein A. Klebsiella pneumoniae is an important pathogen in hospital-acquired and community-acquired infections [1] [2] [3] [4] . Both the capsular polysaccharide (CPS) and the O-antigen portion of the lipopolysaccharide (LPS) components are important pathogenic determinants in K. pneumoniae-caused pneumonia, bacteremia, and community-acquired pyogenic liver abscess (PLA) [5] [6] [7] [8] [9] . The maintenance of cell envelope integrity in gram-