<P> Electroporation is another method of promoting competence . In this method the cells are briefly shocked with an electric field of 10 - 20 kV / cm, which is thought to create holes in the cell membrane through which the plasmid DNA may enter . After the electric shock, the holes are rapidly closed by the cell's membrane - repair mechanisms . </P> <P> Most species of yeast, including Saccharomyces cerevisiae, may be transformed by exogenous DNA in the environment . Several methods have been developed to facilitate this transformation at high frequency in the lab . </P> <Ul> <Li> Yeast cells may be treated with enzymes to degrade their cell walls, yielding spheroplasts . These cells are very fragile but take up foreign DNA at a high rate . </Li> <Li> Exposing intact yeast cells to alkali cations such as those of cesium or lithium allows the cells to take up plasmid DNA . Later protocols adapted this transformation method, using lithium acetate, polyethylene glycol, and single - stranded DNA . In these protocols, the single - stranded DNA preferentially binds to the yeast cell wall, preventing plasmid DNA from doing so and leaving it available for transformation . </Li> <Li> Electroporation: Formation of transient holes in the cell membranes using electric shock; this allows DNA to enter as described above for bacteria . </Li> <Li> Enzymatic digestion or agitation with glass beads may also be used to transform yeast cells . </Li> </Ul> <Li> Yeast cells may be treated with enzymes to degrade their cell walls, yielding spheroplasts . These cells are very fragile but take up foreign DNA at a high rate . </Li>

What characteristic of the s strain allows it to be pathogenic