Simple method for demonstrating small plasmid deoxyribonucleic acid molecules in oral streptococci

A simple procedure for rapidly demonstrating small plasmids (less than 10 megadaltons) in oral streptococci is described. Logarithmic-phase, glycine-treated cells from 1.5-ml broth cultures were converted to osmotically fragile forms and lysed with sodium dodecyl sulfate. After the hydrodynamic shearing of host chromosomal deoxyribonucleic acid, such lysates were analyzed by low-voltage agarose gel electrophoresis. Small plasmids, migrating significantly faster than chromosomal deoxyribonucleic acid under such conditions, were readily visualized by ethidium bromide staining.