| Summary: | <p>Abstract</p> <p>Background</p> <p>An important virulence factor of <it>Klebsiella pneumoniae</it> is the production of capsular polysaccharide (CPS), a thick mucus layer that allows for evasion of the host's defense and creates a barrier against antibacterial peptides. CPS production is driven mostly by the expression of genes located in a locus called <it>cps</it>, and the resulting structure is used to distinguish between different serotypes (K types). In this study, we report the unique genetic organization of the <it>cps</it> cluster from <it>K. pneumoniae</it> Kp13, a clinical isolate recovered during a large outbreak of nosocomial infections that occurred in a Brazilian teaching hospital.</p> <p>Results</p> <p>A pyrosequencing-based approach showed that the <it>cps</it> region of Kp13 (<it>cps</it><sub>Kp13</sub>) is 26.4 kbp in length and contains genes common, although not universal, to other strains, such as the <it>rml</it>BADC operon that codes for L-rhamnose synthesis. <it>cps</it><sub>Kp13</sub> also presents some unique features, like the inversion of the <it>wzy</it> gene and a unique repertoire of glycosyltransferases. In silico comparison of <it>cps</it><sub>Kp13</sub> RFLP pattern with 102 previously published <it>cps</it> PCR-RFLP patterns showed that <it>cps</it><sub>Kp13</sub> is distinct from the C patterns of all other K serotypes. Furthermore, in vitro serotyping showed only a weak reaction with capsular types K9 and K34. We confirm that K9 <it>cps</it> shares common genes with <it>cps</it><sub>Kp13</sub> such as the <it>rml</it>BADC operon, but lacks features like <it>uge</it> and Kp13-specific glycosyltransferases, while K34 capsules contain three of the five sugars that potentially form the Kp13 CPS.</p> <p>Conclusions</p> <p>We report the first description of a <it>cps</it> cluster from a Brazilian clinical isolate of a KPC-producing <it>K. pneumoniae</it>. The gathered data including K-serotyping support that Kp13’s K-antigen belongs to a novel capsular serotype. The CPS of Kp13 probably includes L-rhamnose and D-galacturonate in its structure, among other residues. Because genes involved in L-rhamnose biosynthesis are absent in humans, this pathway may represent potential targets for the development of antimicrobial agents. Studying the capsular serotypes of clinical isolates is of great importance for further development of vaccines and/or novel therapeutic agents. The distribution of K-types among multidrug-resistant isolates is unknown, but our findings may encourage scientists to perform K-antigen typing of KPC-producing strains worldwide.</p>
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