Background Burkholderia pseudomallei is the causative agent of melioidosis, a disease of significant morbidity and mortality in both human and animals in endemic areas. Much remains to be known about the contributions of genotypic variations within the bacteria and the host, and environmental factors that lead to the manifestation of the clinical symptoms of melioidosis.Methodology/Principal FindingsIn this study, we showed that different isolates of B. pseudomallei have divergent ability to kill the soil nematode Caenorhabditis elegans. The rate of nematode killing was also dependent on growth media: B. pseudomallei grown on peptone-glucose media killed C. elegans more rapidly than bacteria grown on the nematode growth media. Filter and bacteria cell-free culture filtrate assays demonstrated that the extent of killing observed is significantly less than that observed in the direct killing assay. Additionally, we showed that B. pseudomallei does not persistently accumulate within the C. elegans gut as brief exposure to B. pseudomallei is not sufficient for C. elegans infection.Conclusions/SignificanceA combination of genetic and environmental factors affects virulence. In addition, we have also demonstrated that a Burkholderia-specific mechanism mediating the pathogenic effect in C. elegans requires proliferating B. pseudomallei to continuously produce toxins to mediate complete killing.

The nematode Caenorhabditis elegans is hypersusceptible to Burkholderia pseudomallei infection. However, the virulence mechanisms underlying rapid lethality of C. elegans upon B. pseudomallei infection remain poorly defined. To probe the host-pathogen interaction, we constructed GFP-tagged B. pseudomallei and followed bacterial accumulation within the C. elegans intestinal lumen. Contrary to slow-killing by most bacterial pathogens, B. pseudomallei caused fairly limited intestinal lumen colonization throughout the period of observation. Using grinder-defective mutant worms that allow the entry of intact bacteria also did not result in full intestinal lumen colonization. In addition, we observed a significant decline in C. elegans defecation and pharyngeal pumping rates upon B. pseudomallei infection. The decline in defecation rates ruled out the contribution of defecation to the limited B. pseudomallei colonization. We also demonstrated that the limited intestinal lumen colonization was not attributed to slowed host feeding as bacterial loads did not change significantly when feeding was stimulated by exogenous serotonin. Both these observations confirm that B. pseudomallei is a poor colonizer of the C. elegans intestine. To explore the possibility of toxin-mediated killing, we examined the transcription of the C. elegans ABC transporter gene, pgp-5, upon B. pseudomallei infection of the ppgp-5::gfp reporter strain. Expression of pgp-5 was highly induced, notably in the pharynx and intestine, compared with Escherichia coli-fed worms, suggesting that the host actively thwarted the pathogenic assaults during infection. Collectively, our findings propose that B. pseudomallei specifically and continuously secretes toxins to overcome C. elegans immune responses.