Lab Research Focus
Bubonic plague is transmitted by fleas whose digestive tracts are blocked by a bacterial mass of Yersinia pestis. A visible extracellular biofilm also is formed on Caenorhabditis elegans anterior cuticle, especially the head, to block feeding when the nematode is exposed to Y. pestis or the closely related bacterium Y. pseudotuberculosis. The biofilm shares similar physical characteristics with the Y. pestis mass in the blocked flea. Moreover, the hmsHFRS operon of Y. pestis, which contains predicted polysaccharide-biosynthetic genes, is required for both flea transmission of plague and biofilm formation on C. elegans. These suggest that flea blockage by Y. pestis is a biofilm-mediated process and establish C. elegans as an experimentally tractable surrogate for fleas.
In current laboratory models for biofilm formation, bacteria move and attach to a stationary surface such as glass and plastic, finally developing into a mature biofilm. However, the biofilm formation on C. elegans appears to be special, as both C. elegans and Yersinia bacteria are alive and the surface that the biofilm adheres to is movable. It therefore is not obvious whether the biofilm matrix on C. elegans is produced by the bacteria or the nematode, and whether nematode locomotion plays a role on the biofilm accumulation. In the current work, we first selected the lectin wheat germ agglutinin (WGA), which specifically binds N-acetyl-D-glucosamine and sialic acid, as a strong probe for biofilm detection on C. elegans. With this probe, we then produced multiple lines of evidence that the biofilm matrix on C. elegans is produced by Yersinia and picked up by the nematode as the animal moves through the bacterial lawn. We further conclude that Yersinia biofilm accumulation on C. elegans depends on continuous nematode exposure to a lawn of Yersinia bacteria. Biofilm carbohydrate is present in bacterial lawns prior to addition of nematodes thus suggesting that biofilm formation does not involve signaling between the two organisms. In addition, the biofilm on C. elegan was found to be dissolved in alkaline solutions with a high pH. Similar to immuno-precipitation, the dissolved biofilm sugars specifically bound alkaline phosphatase labeled WGA to form aggregates that can be easily precipitated and detected by a color reaction. Yersinia biofilm sugars were purified by WGA affinity chromatography. The preliminary data indicates that the biofilm contains glucose, mannose, xylose, fucose, and sialic acid. The complete glycosyl composition of the purified biofilm sugars and potential genes related to sialic acid biosynthesis in Yersinia bacteria are being pursued.
The long-term goal of this research is to understand the mechanism by which Y. pestis establishes and maintains colonization of its vector, the flea. This information is important to develop reagents that could reduce or eliminate plague and be used in response to bioweapon attacks with plague-carrying fleas. More generally, the Yersinia-C. elegans system also serves as a model for biofilm formation on a living surface. Because of the great versatility of C. elegans as an experimental animal, the nematode surface can be experimentally altered, allowing investigation of the host factors involved in biofilm attachment and growth. Characterization of these factors could produce insights into the means by which bacterial pathogens adhere to host tissues in biofilm-mediated infectious disease.
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