Cell volume changes correlate with efficiency of transformation of Escherichia coli with plasmid DNA

The transformability of batch cultures of Escherichia coli with pBR322 plasmid DNA was found to be dependent on the inoculum size used as starter. Highest rates of competence were achieved in cultures from the early exponential phase (OD660= 0.15) following > 250-fold dilution of an overnight (37°C) culture into prewarmed broth. Modal cell volume and transformation frequency changed with similar periodicity throughout growth and the highest transformability occurred when recipient cells had the greatest volumes. The range over which transformability fluctuated (ca. 100-fold) vastly exceeded the range (ca. 2-fold) in cell volume observed. This suggests that simple increase in surface area available for DNA uptake alone cannot explain these findings. It is more likely that stresses caused by cell division make the cell envelope expose more binding sites for DNA uptake or otherwise make the surface more permeable to DNA during transformation.