Pure plasmid DNA is a critical component to success in any molecular biology lab as the majority of follow-up applications, from sequencing to transfection, relies on uncontaminated DNA. The Qiagen plasmid purification kits are designed to isolate and purify plasmid DNA with specific consideration given to size and copy number. The kit is based on specific binding of plasmid DNA to a silica-gel membrane in high salt buffer followed by use of a low salt buffer (or water) to facilitate elution. To obtain purified DNA from a bacterial suspension one only has to follow three steps: 1) bacterial cell lysis, 2) binding of the plasmid DNA to the silica-gel, and 3) washing/elution of the purified DNA. The kit comes with all components necessary for cell lysis, silica-gel spin columns, high and low salt buffers and wash buffers to perform plasmid DNA purification. The binding capacity of the columns within the miniprep kit is 20 ug of plasmid DNA. This kit can be used in three different formats: single spin columns, 8-well strips and 96-well plates.
I have used this kit for quite a number of years. I frequently use the spin columns and occasionally, a vacuum manifold, when I have more than 10 samples. The easy three-step system and fast, reliable results within an hour, make this system very attractive. As for yield, I frequently recover up to 17 ug of total plasmid DNA from a single 5 ml high-copy plasmid prep. Spectrophotometric analysis, enzymatic digests and follow-up agarose gels show that the plasmid DNA is pure and suitable for downstream reactions. The same is true for sequencing, library screens and bacterial transformation. In my personal experience, to elute the maximum amount of DNA from the columns, I use hot water (70C) in two 25–30 ul portions (raises yield as much as 25%). This is especially important for precious low copy plasmids. If you have a vacuum manifold that is compatible with the Qiagen system, I recommend using it only when you have more than 10 samples since the device will be used only for the binding and washing steps. Bacterial lysis and elution must be performed manually using a microcentrifuge. Two positive qualities of the Qiagen kits I have used include helpful handbooks (make sure you follow their suggestions) and thorough online support. Single buffers and solutions can be re-ordered to fill existing kits. In the latest version of the plasmid prep kits, Qiagen even includes a color system for detection of completed cell lysis.
Although the price/sample (around one US dollar) is on the higher end compared to some competitors, the ease of use, and time, labor and equipment savings combined with pure, high yield, make this kit and its Midi and Maxi brothers highly recommendable. In fact, most sequencing core facilities I am familiar with suggest the use of the Qiagen plasmid purification kit.
Stefan M. Kanzok, PhD
Malaria Research Insitute
Johns Hopkins Bloomberg School of Public Health
Baltimore, USA