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The core lipopolysaccharide (LPS) of Klebsiella pneumoniae contains two galacturonic acid (GalA) residues, but only one GalA transferase (WabG) has been identified. Data from chemical and structural analysis of LPS isolated from a wabO mutant show the absence of the inner core [beta]-Gala residue linked to e-glycero-D-manno-heptose III (L,D-Hep III). An in vitro assay demonstrates that the purified WabO is able to catalyze the transfer of GalA from UDP-GalA to the acceptor LPS isolated from the wabO mutant, but not to LPS isolated from waaQ mutant (deficient in L,D-Hep III). The absence of this inner core [beta]-GalA residue results in a decrease in virulence in a capsule-dependent experimental mouse pneumonia model. In addition, this mutation leads to a strong reduction in cell-bound capsule. Interestingly, a K66 Klebsiella strain (natural isolate) without a functional wabO gene shows reduced levels of cell-bound capsule in comparison to those of other K66 strains. Thus, the WabO enzyme plays an important role in core LPS biosynthesis and determines the level of cell-bound capsule in Klebsiella pneumoniae.

The complete structures of LPS core types 1 and 2 from Klebsiella pneumoniae have been described by other authors. They are characterized by a lack of phosphoryl residues, but they contain galacturonic acid (GalA) residues, which contribute to the necessary negative charges. The presence of a capsule was determined in core-LPS non-polar mutants from strains 52145 (O1 :K2), DL1 (O1 :K1) and C3 (O8 : K66). O-antigen ligase (waaL) mutants produced a capsule. Core mutants containing the GalA residues were capsulated, while those lacking the residues were non capsulated. Since the proteins involved in the transfer of GalA (WabG) and glucosamine residues (WabH) are known, the chemical basis of the capsular-K2-cell-surface association was studied. Phenol/water extracts from K. pneumoniae 52145[DELTA]wabH waaL and 52145[DELTA]waaL mutants, but not those from from K. pneumoniae 52145[DELTA]wabG waaL mutant, contained both LPS and capsular polysaccharide, even after hydrophobic chromatography. The two polysaccharides were dissociated by gel-filtration chromatography, eluting with detergent and metal-ion chelators. From these results, it is concluded that the K2 capsular polysaccharide is associated by an ionic interaction to the LPS through the negative charge provided by the carboxyl groups of the GalA residues.

Two porins, OmpK36 and OmpK35, have been previously isolated from Klebsiella pneumoniae, and they are homologous to the Escherichia coli porins OmpC and OmpF, respectively. Identification of the two porins revealed that most isolates lacking expression of extended-spectrum beta-lactamases express the two porins, while most isolates producing these beta-lactamases express only porin OmpK36, and the OmpK35 porin is either very low or not expressed.

Results show that eight open reading frames of wb(sub)012 gene cluster code for Klebsiella pneumoniae O12-antigen lipopolysaccharide in Escherichia coli K-12. The wb gene cluster belongs to ABC 2 transporter-dependent pathways mediating O-antigen heteropolysaccharides synthesis.