A new method of plasmid DNA preparation by sucrose-mediated detergent lysis from Escherichia coli (gram-negative) and Staphylococcus aureus (gram-positive)

A simple and cheap method of plasmid DNA preparation from both gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli) organism is presented here. In this method, in place of the high-priced chemicals lysostaphin and lysozyme which are commonly used for removal of cell-wall during plasmid DNA preparation from gram-positive and gram-negative bacteria, respectively, only sucrose has been used. Firstly, bacteria is treated with Trizma (pH 8.0) containing 100% sucrose (hypertonic solution). Due to this osmotic shock, protoplasm covered by the plasma membrane of bacteria possibly shrinks and becomes detached from the cell-wall. Osmotically sensitive cells thus formed, from gram-positive (S. aureus) and gram-negative (E. coli) bacteria, are finally lysed by the lysis mixture, containing brij 58 and sodium deoxycholate. The lysate is centrifuged at 15,000 rpm for 30 min to pellet the cell debris. The supernatant containing plasmid DNA is treated with either polyethylene glycol or isopropanol. The precipitate which contains plasmid DNA is dissolved in a buffer containing Tris, EDTA, NaCl, and sodium dodecyl sulfate (pH 8.0); thus protein is denatured and removed. Finally, RNA is removed by RNase treatment. The average yield of staphylococcal plasmid DNA as well as plasmid pBR322 from E. coli HB101 in 100% sucrose-treated preparations is greater than that of lysostaphin- and lysozyme-treated preparations. This method is applicable for both large-scale and small-scale preparations. The substrate activity for restriction enzyme, cloning, transforming ability, and electron microscopic profile of the plasmid DNA prepared by this method remains unaltered.