Influence of bacterial age and pH of lysis buffer on type of nucleic acid isolated

This study deals with the determination of the factors which influence isolation of RNA using a silica-based nucleic acid isolation protocol. Four bacteria were included in the investigation: Campylobacter jejuni, Escherichia coli, Listeria monocytogenes and Pseudomonas fluorescens. An increase of RNA yield could be obtained with all four strains at the exponential growth phase, reaching a maximum at the beginning of the stationary phase. Later the amount of RNA isolated gradually decreased. At the late stationary phase DNA was also isolated in case of Escherichia coli and Pseudomonas a fluorescens. DNA was only obtained at this stage of the growth curve, probably because RNA contents of the cell decreases in the late stationary phase thus enabling DNA only now to bind to the silica used in the isolation procedure. In the exponential and stationary phase, when there is a competition between RNA and DNA to bind to the silica, there is a preferential binding of RNA over DNA to the silica. It was shown that a pH of 6.0 and 6.5 promoted isolation of RNA. At pH 8.0 and 8.5 large amounts of DNA were obtained from the Gram-negative bacteria. Thus, the silica based nucleic acid isolation method enables isolation of either RNA or DNA when taking into account the appropriate conditions of pH of the buffers and applying the corresponding incubation time of the bacterial culture. A preceding treatment of lysozyme and proteinase K was sufficient to accomplish lysis of the Gram-positive cell.