Genomic acquisition of a capsular polysaccharide virulence cluster by non-pathogenic Burkholderia isolates

Abstract

BackgroundBurkholderia thailandensis is a non-pathogenic environmental saprophyte closely related to Burkholderia pseudomallei, the causative agent of the often fatal animal and human disease melioidosis. To study B. thailandensis genomic variation, we profiled 50 isolates using a pan-genome microarray comprising genomic elements from 28 Burkholderia strains and species.ResultsOf 39 genomic regions variably present across the B. thailandensis strains, 13 regions corresponded to known genomic islands, while 26 regions were novel. Variant B. thailandensis isolates exhibited isolated acquisition of a capsular polysaccharide biosynthesis gene cluster (B. pseudomallei-like capsular polysaccharide) closely resembling a similar cluster in B. pseudomallei that is essential for virulence in mammals; presence of this cluster was confirmed by whole genome sequencing of a representative variant strain (B. thailandensis E555). Both whole-genome microarray and multi-locus sequence typing analysis revealed that the variant strains formed part of a phylogenetic subgroup distinct from the ancestral B. thailandensis population and were associated with atypical isolation sources when compared to the majority of previously described B. thailandensis strains. In functional assays, B. thailandensis E555 exhibited several B. pseudomallei-like phenotypes, including colony wrinkling, resistance to human complement binding, and intracellular macrophage survival. However, in murine infection assays, B. thailandensis E555 did not exhibit enhanced virulence relative to other B. thailandensis strains, suggesting that additional factors are required to successfully colonize and infect mammals.ConclusionsThe discovery of such novel variant strains demonstrates how unbiased genomic surveys of non-pathogenic isolates can reveal insights into the development and emergence of new pathogenic species.

Protection against Experimental Melioidosis following Immunization with Live Burkholderia thailandensis Expressing amanno-Heptose Capsule

“…The bacteria used in this study were B. thailandensis strains E555 (21) Genetic comparison. The genetic relatedness of B. thailandensis strains (E264, E555, and CDC2721121) to B. pseudomallei K96243 was estimated based on the multilocus sequence typing (MLST) loci (http: //bpseudomallei.mlst.net).…”

“…thailandensis E555 has previously been shown to express a capsular antigen which is similar to the manno-heptose capsule of B. pseudomallei (21). To confirm in vitro expression of this capsule prior to work involving animals, samples of heat-inactivated bacteria were examined using the capsule-specific monoclonal antibody 4VIH12 (27).…”

“…Significant protection was induced by immunization with both of the B. thailandensis strains used. However, immunizations with the recently isolated B. thailandensis strain E555 (21), which has been shown by monoclonal antibody reactivity to express an antigenic capsule similar to the manno-heptose capsule of B. pseudomallei, induced significantly better protection than immunization with a noncapsulated B. thailandensis.…”

Immunisation with proteins expressed during chronic murine melioidosis provides enhanced protection against disease

“…This strain was generated through disruption of the O-antigen synthesis cluster in B. thailandensis E555 [22] by insertional inactivation of the wbiI gene (gene BTH I1483 in B. thailandensis E264) using the pKnock-Km suicide vector [23]. Briefly, an internal fragment from the wbiI gene in B. thailandensis E555 was amplified by PCR using primers 306 (5 -CTGCAGTCTATCGGGCGACGAAGGAG-3 ) and 307 (5 -CTGCAGGCCTTGTCGGTCGAGATCAG-3 ), inserted into pCR ® -blunt II-TOPO ® and cloned into Escherichia coli TOP10 cells.…”

Using multispectral imaging flow cytometry to assess an in vitro intracellular Burkholderia thailandensis infection model

“…Burkholderia thailandensis E555 is an avirulent bacterium related to B. pseudomallei. It is a fully capsulated non-virulent strain of Burkholderia that has been shown to be genetically very similar to fully virulent B. pseudomallei (3). It has also been used as an effective vaccine in a murine model of infection (4).…”