Burkholderia thailandensis is a gram-negative bacterium that naturally inhabits soil and water environments, primarily in tropical and subtropical regions like Southeast Asia and northern Australia. It can be found in various environmental samples, including water from Texas and Puerto Rico, and soil from Mississippi. While generally considered non-pathogenic to humans, it can occasionally act as an opportunistic pathogen.
A Model for Understanding Pathogens
Burkholderia thailandensis serves as a research model due to its close genetic and physiological resemblance to Burkholderia pseudomallei, a human pathogen responsible for melioidosis. These two species diverged approximately 47 million years ago, yet they share over 85% of their genes, and their genomes exhibit significant synteny. This genetic similarity allows researchers to study many aspects of B. pseudomallei in a safer laboratory setting.
The shared genetic makeup includes crucial virulence factors, such as the Type III Secretion System (TTSS), which is highly conserved between them. By studying B. thailandensis, scientists can investigate the mechanisms by which B. pseudomallei causes disease, including its ability to invade and replicate within mammalian cells, escape from host immune responses, and spread. This research contributes to developing therapeutic strategies for melioidosis without requiring the stringent Biosafety Level 3 (BSL-3) containment facilities needed for B. pseudomallei. As a Biosafety Level 2 organism, B. thailandensis is a suitable surrogate.
Unique Molecular Mechanisms
Beyond its role as a model organism, B. thailandensis possesses a complex quorum sensing circuitry. Quorum sensing is a communication system used by bacteria to coordinate group behaviors based on population density. Bacteria release chemical signals called autoinducers, and as their concentration increases, they trigger a collective response.
Burkholderia thailandensis contains three complete acyl-homoserine lactone (AHL) quorum sensing circuits, along with LuxR homologs for signal detection. This system allows B. thailandensis to regulate processes including biofilm formation, gene expression, and the production of specialized metabolites. For instance, the BtaR2-BtaI2 system produces an antibiotic active against gram-positive bacteria, demonstrating the influence of these communication networks on its chemical output.
B. thailandensis also produces a diverse array of specialized metabolites. These unique chemical compounds have applications such as antimicrobial activity against other soil bacteria like Bacillus subtilis. Some of these metabolites include methylated 4-hydroxyalkenylquinolines, which are structurally similar to compounds produced by Pseudomonas aeruginosa. These compounds contribute to the bacterium’s ecological interactions and serve as a source for novel natural products.
Distinguishing It From Its Harmful Relative
Despite its close genetic ties and shared biological processes with Burkholderia pseudomallei, Burkholderia thailandensis is generally considered harmless to immunocompetent humans. A key difference lies in B. thailandensis’s ability to assimilate L-arabinose, a sugar that B. pseudomallei cannot metabolize due to lacking the entire arabinose-assimilation operon. This metabolic distinction is often used in laboratory settings to differentiate the two species.
Beyond metabolic differences, genetic variations contribute to the reduced virulence of B. thailandensis. While the virulence-associated Type III Secretion System (TTSS) is conserved in both species, the B. thailandensis TTSS is negatively regulated by the presence of L-arabinose, which may partly explain its lower pathogenicity. B. thailandensis also lacks bacterial factors, such as a gene cluster involved in the production of capsular polysaccharides, known to contribute to the full virulence of B. pseudomallei.
The infectious dose of B. thailandensis in animal models is significantly higher, approximately 1,000 times greater than that of B. pseudomallei, illustrating its attenuated pathogenicity. While rare human infections with B. thailandensis have been reported, primarily in individuals with underlying health conditions, its environmental niche and host interactions differ from its pathogenic counterpart.