Consequences of Reduction of Klebsiella pneumoniae Capsule Expression on Interactions of This Bacterium with Epithelial Cells

Abstract

Most Klebsiella pneumoniae clinical isolates are fully encapsulated and adhere in vitro to intestinal cell lines with an aggregative pattern. In this study, the influence of the capsule on interactions with epithelial cells was investigated by creating an isogenic mutant defective in the synthesis of the capsule. Determination of the uronic acid content of bacterial extracts confirmed that the mutant did not produce capsular polysaccharides whereas, with the wild-type strain, the level of encapsulation was growth phase dependent and reached a maximum during the lag and early log phases. Assays performed with different epithelial cell lines, Int-407, A-549, and HEp-2, showed that the capsule-defective mutant demonstrated greater adhesion than did the wild-type strain and that the aggregative pattern was maintained, indicating that the capsule was not related to the adhesion phenotype. In contrast, when the mucus-producing HT-29-MTX cells were used, the encapsulated wild-type strain adhered more strongly than did the capsule-defective mutant. No invasion properties were observed with any of the capsular phenotypes or cell lines used. The K. pneumoniae adhesin CF29K was detected by Western blot analysis and enzyme-linked immunosorbent assay on the surface of transconjugants obtained after transfer of a conjugative plasmid harboring the CF29K-encoding genes into both the wild-type and the capsule-defective strains. The amounts of adhesin detected were greater in the capsule-defective background strain than in the wild-type encapsulated strain and were associated with an increase in the level of adhesion to Caco-2 cells. Moreover, RNA slot blot experiments showed that transcription of the adhesin-encoding gene was markedly increased in the capsule-defective mutant compared to the wild-type encapsulated background. These results suggest (i) that the capsule plays an active role during the initial steps of the pathogenesis by interacting with mucus-producing cells but is subsequently not required for the adhesin-related interaction with the epithelial cell surface and (ii) that the expression of the adhesin is modulated by the presence of a capsule at a transcriptional level.

Inability of encapsulated Klebsiella pneumoniae to assemble functional type 1 fimbriae on their surface

“…Expression of capsule in some way interferes with one of the ¢nal stages of ¢mbrial assembly. A similar inverse relationship has recently been described for CF29K adhesin of K. pneumoniae, whereby capsule formation interfered at the transcriptional level with the adhesin biosynthesis [12]. It is possible that in some cases, represented by a small fraction of the cell population of the K21a/s variant, ¢mbrial assembly is altered by another mechanism because the variant exhibited traces of FimH antigen activity in the PP space only.…”

“…The knowledge on the regulation of capsule formation is rather limited. It involves the products of rmpA and RcsA genes which also regulate the synthesis of polysaccharides in enterobacteria [10,11] and recently was reported to be in£uenced by environmental conditions [12]. In the present study, we sought to determine the relationship between the phenotypic expression of capsular material and type 1 ¢mbriae in K. pneumoniae.…”

Critical determinants of the interactions of capsule-expressing Neisseria meningitidis with host cells: the role of receptor density in increased cellular targeting via the outer membrane Opa proteins

“…In this regard, their inhibitory effect can be compared with that of negatively charged surface sialic acids. Although on rare occasions, capsules are themselves involved in adhesion, in general, capsules inhibit OMP function in a wider setting and can be demonstrated in a range of capsule types and OMP adhesin types extending to distinct species (St Geme and Cutter, 1996;Favre-Bonte et al, 1999;Cywes and Wessels, 2001;Joseph and Wright, 2004). It is therefore likely in general that, when capsule is itself not an adhesin, the basis of its inhibitory action may reside in physical masking and/ or its physicochemical properties which may neutralize the charge of an adhesin.…”

Role of capsule in Klebsiella pneumoniae virulence: lack of correlation between in vitro and in vivo studies

“…Only one study has investigated the role of capsule in K. pneumoniae UTI with inconclusive results [11] and studies on the role of capsule in GI colonization are contradictory [12,13]. Interestingly, recent studies have shown that expression of capsule impedes the ability of K. pneumoniae to adhere to and invade epithelial cells in vitro [14,15]. These studies suggest that expression of capsule may be a disadvantage in serum poor host compartments by reducing the ability of the bacteria to adhere to epithelial surfaces.…”

“…Only the K2-serotype gene cluster has been cloned and the genomic organization determined [9,16]. However, only a few open reading frames have been assigned a function and only some parts of the cluster are conserved among di¡erent capsule serotypes [14,16]. Due to the inability to construct well-de¢ned isogenic mutants, most studies investigating the role of capsule on the virulence of K. pneumoniae are based on the use of non-capsulated variants (NCVs) arisen either spontaneously or after mutagenic treatment of the bacteria.…”