Sun Nyunt Wai
My research can be divided in to two main research areas of interest. 1. Molecular mechanisms controlling biofilm formation, virulence factor expression, rugose switching. 2. The relationship between the release of outer membrane vesicles and secretion of virulence factors in Vibrio cholerae and enterobacteria.
Project 1.Cholera is a serious epidemic disease that has
killed millions of people and continues to be a major health
problem worldwide. The bacterium Vibrio cholerae is a motile, gram-negative curved rod with a single polar flagellum. It was discovered that V. cholerae O1 from the Peru epidemic was able shift to a phenotype having wrinkled or rugose colony morphology. It is suggested that the V. cholerae rugose phenotype represent a fully virulent survival form of the organism that can persist in the presence of free chlorine. However, the relationships between the phenotypic and genetic characteristics associated with rugose morphology, and their relative importance in pathogenicity still remained to be identified.
We have isolated the rugose variants of V. cholerae O1 strain TSI-4 and V. cholerae O139 MO10 from starvation medium and determined EPS expression on the surface of the rugose strains by polycationic ferritin-labeled thin-section electron microscopy. We found that they produced a continuous biofilm on the colonized surface and that the polysaccharide may have a role in resistance to osmotic and oxidative stress. While there seem to be a role for quorum sensing in regulation of virulence properties in such pathogens the actual controlling mechanisms are far from understood.
Project 2. Outer membrane vesicles have been broadly defined as spherical fragments of the bacterial membrane and are produce by a wide variety of Gram-negative bacteria during normal growth. Vesicles have been proposed to play a role in several virulence mechanisms: periplasmic enzyme delivery DNA transport and evasion of the immune system. Our work also showed that the cholera toxin-like toxin LT (heat labile toxin) from enterotoxigenic Escherichia coli (ETEC) is excreted via outer membrane vesicles.
We have recently studied a cytotoxic protein (ClyA) in E. coli that we found was exported through outer membrane vesicles. Furthermore, the cytotoxin was acting much more potent on human target cells when present in vesicles in com-parison with the purified cytotoxin (Wai et al. 2003).We investigated the vesicles by electron microscopy and atomic force microscopy and by biochemical approaches. According to our present results, vesicles can be proposed to play a role in export and activation of the virulence factors to the external environment.