Project description:The bacterium Burkholderia ubonensis is commonly co-isolated from environmental specimens harbouring the melioidosis pathogen, Burkholderia pseudomallei. B. ubonensis has been reported in northern Australia and Thailand but not North America, suggesting similar geographic distribution to B. pseudomallei. Unlike most other Burkholderia cepacia complex (Bcc) species, B. ubonensis is considered non-pathogenic, although its virulence potential has not been tested. Antibiotic resistance in B. ubonensis, particularly towards drugs used to treat the most severe B. pseudomallei infections, has also been poorly characterised. This study examined the population biology of B. ubonensis, and includes the first reported isolates from the Caribbean. Phylogenomic analysis of 264 B. ubonensis genomes identified distinct clades that corresponded with geographic origin, similar to B. pseudomallei. A small proportion (4%) of strains lacked the 920kb chromosome III replicon, with discordance of presence/absence amongst genetically highly related strains, demonstrating that the third chromosome of B. ubonensis, like other Bcc species, probably encodes for a nonessential pC3 megaplasmid. Multilocus sequence typing using the B. pseudomallei scheme revealed that one-third of strains lack the "housekeeping" narK locus. In comparison, all strains could be genotyped using the Bcc scheme. Several strains possessed high-level meropenem resistance (?32 ?g/mL), a concern due to potential transmission of this phenotype to B. pseudomallei. In silico analysis uncovered a high degree of heterogeneity among the lipopolysaccharide O-antigen cluster loci, with at least 35 different variants identified. Finally, we show that Asian B. ubonensis isolate RF23-BP41 is avirulent in the BALB/c mouse model via a subcutaneous route of infection. Our results provide several new insights into the biology of this understudied species.
Project description:Burkholderia pseudomallei, the founding member of the B. pseudomallei complex (Bpc), is a biothreat agent and causes melioidosis, a disease whose treatment mainly relies on ceftazidime and meropenem. The concern is that B. pseudomallei could enhance its drug resistance repertoire by the acquisition of DNA from resistant near-neighbor species. Burkholderia ubonensis, a member of the B. cepacia complex (Bcc), is commonly coisolated from environments where B. pseudomallei is present. Unlike B. pseudomallei, in which significant primary carbapenem resistance is rare, it is not uncommon in B. ubonensis, but the underlying mechanisms are unknown. We established that carbapenem resistance in B. ubonensis is due to an inducible class A PenB ?-lactamase, as has been shown for other Bcc bacteria. Inducibility is not sufficient for high-level resistance but also requires other determinants, such as a PenB that is more robust than that present in susceptible isolates, as well as other resistance factors. Curiously and diagnostic for the two complexes, both Bpc and Bcc bacteria contain distinct annotated PenA class A ?-lactamases. However, the protein from Bcc bacteria is missing its essential active-site serine and, therefore, is not a ?-lactamase. Regulated expression of a transcriptional penB'-lacZ (?-galactosidase) fusion in the B. pseudomallei surrogate B. thailandensis confirms that although Bpc bacteria lack an inducible ?-lactamase, they contain the components required for responding to aberrant peptidoglycan synthesis resulting from ?-lactam challenge. Understanding the diversity of antimicrobial resistance in Burkholderia species is informative about how the challenges arising from potential resistance transfer between them can be met.IMPORTANCE Burkholderia pseudomallei causes melioidosis, a tropical disease that is highly fatal if not properly treated. Our data show that, in contrast to B. pseudomallei, B. ubonensis ?-lactam resistance is fundamentally different because intrinsic resistance is mediated by an inducible class A ?-lactamase. This includes resistance to carbapenems. Our work demonstrates that studies with near-neighbor species are informative about the diversity of antimicrobial resistance in Burkholderia and can also provide clues about the potential of resistance transfer between bacteria inhabiting the same environment. Knowledge about potential adverse challenges resulting from the horizontal transfer of resistance genes between members of the two complexes enables the design of effective countermeasures.
