Molecular characterization of the fibronectin-binding protein BBK32 of Borrelia burgdorferi sensu lato - a podcast by Ludwig-Maximilians-Universität München

BBK32, a fibronectin (Fn)-binding protein of Borrelia (B.) burgdorferi sensu lato (s.l.) which is encoded by the bbk32 gene located on the 36kb linear plasmid (lp36) of isolate B31, is playing an important role in serological diagnosis of Lyme borreliosis. Firstly, we were interested in the genomic localization of bbk32 regarding different B. burgdorferi s.l. species as well as between strains of the same species. Southern blot analyses based on 23 strains of the species B. burgdorferi sensu stricto (s.s.), B. afzelii, B. garinii and B. spielmanii revealed that position of bbk32 is rather variable between the species but also within a given species. bbk32 could be located on different linear plasmids (lp), mainly on lp23kb, lp24kb, lp25kb, lp31kb and lp36kb. The meaning of this finding remains unclear so far. Secondly, a mumber of thirteen chimeric polypeptides representing different parts of the N-terminal regions of BBK32 proteins of both B. burgdorferi s.s. isolate B31 and B. garinii isolate PHei were generated. Fn-binding capabilities of those generated polypeptides were evaluated either by Western-ligand blot-based binding assay or by enzyme-linked immunosorbent assay (ELISA)-based binding assay. Results showed that BBK32 from PHei possesses a higher Fn-binding capability than that from B31. Furthermore, the higher Fn-binding capacity is associated with four amino acids (Lysine131, Lysine145, Threonine147 and Isoleucine155) in the 32-amino acid-long segment (from position 131 to 162). Moreover, both gelatin and collagen could partially inhibit the binding of BBK32 to Fn. This suggests that BBK32 might also bind to the collagen-binding domain of Fn (repeat I6-9 and II1, 2) and partially to its N-terminal fibrin-binding domain (repeat I1-5). Though the meaning of the different Fn-binding capacities remains unclear so far, such studies may provide us with markers to define the different pathogenic potentials of various Borrelia species and strains. Thirdly, eight recombinantly prepared BBK32 homologues (either as partial or as whole) were tested in a line assay to evaluate their contribution for serologic diagnosis of Lyme borreliosis. Though BBK32 homologues could react with sera from Lyme borreliosis patients, compared with other Borrelia-antigens established in the Max von Pettenkofer Institute, these BBK32 homologues could not improve the sensitivity and specificity of the class-specific IgG or IgM antibody tests. Nevertheless, this study underlines the fact that the heterogeneity of Lyme disease Borrelia species must be taken into consideration in the microbiological diagnosis of Lyme borreliosis in European patients.

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