Simplifying Strain Selection

CONSIDERATIONS FOR CHOOSING THE BEST E. COLI STRAIN FOR ANY APPLICATION

They’re everywhere: our bodies, our environments, and of course, our laboratories. Escherichia coli is the most well-known bacterial species and has become an irreplaceable tool in the molecular biology field. There are hundreds, if not thousands, of different E. coli strains used in scientific research, each with their own unique modifications. This can make choosing the correct strain for your research a daunting task. The guidelines below can simplify this process and help you choose the best strain for your application.

Laboratory E. coli strains used in molecular cloning can typically be classified as either expression strains or propagation strains. If you need to amplify and extract a particular plasmid or construct, it is best to use a propagation strain. On the other hand, if your goal is to express a protein encoded within a plasmid, an expression strain is the best choice. While there are many details involved in choosing a strain, the table below provides an overview of the most common laboratory strains and their key features.

Strain	Propagation or Expression?	Antibiotic Resistance	When to Use?
DH5α	Propagation	N/A	General Cloning and Blue/White Screening
JM109	Propagation	N/A	General Cloning and Blue/White Screening
HB101	Propagation	Streptomycin	Good for pBR322 Plasmids
Zymo 10B	Propagation	Streptomycin	General Cloning and Blue/White Screening
TG1	Propagation	N/A	General Cloning and Blue/White Screening
NEB Stable	Propagation	N/A	Good for Lentivirus Cloning Experiments
XL1 Blue	Propagation	Tetracycline	Blue/White Screening
XL10 Gold	Propagation	Tetracycline and Chloramphenicol	Large Plasmid Propagation
BL21 (DE3)	Expression	N/A	General Protein Expression
BL21 (DE3) pLysS	Expression	Chloramphenicol	Toxic Protein Expression
Lemo21 (DE3)	Expression	Chloramphenicol	Insoluble/Membrane Protein Expression
T7 Express	Expression	N/A	General Protein Expression
BLR (DE3)	Expression	Tetracycline	Unstable Protein Expression
XJa (DE3) Autolysis	Expression	Chloramphenicol	Recombinant Protein Expression
XJb (DE3) Autolysis	Expression	Chloramphenicol	Recombinant Protein Expression

It is important to have a defined research goal when choosing an E. coli strain. Are you aiming to clone highly repetitive or large plasmids? If so, use a specialized strain that can maintain high transformation efficiency. Are you aiming to express a toxic protein? If so, choose a strain that has been designed to withstand toxic environments.

Both propagation and expression strains carry specialized mutations that allow them to thrive under specific conditions and it is important to keep these features in mind when choosing the right strain for your project. Mix & Go! Competent Cells are a great choice for propagation, as they enable high transformation efficiencies in just 20 seconds. If you are in search of an expression strain, XJ Autolysis Competent Cells can simplify the process with an efficient, gentle lysis system. No matter the application, Zymo Research scientists are available to assist you with your strain selection process.