Competent E.coli: To buy or not to buy? |
Buying competent cells from commercial suppliers is convenient, provides a guarantee of quality and gives access to strains with a variety of in-built traits that assist with things like maintenance of plasmid integrity (more on these traits later). However, this can be an expensive business. Alternatively, competent cells of any strain, including the specially-constructed commercial strains, can be prepared easily in the lab (for protocols see here and here). The critical factor is whether you can generate cells of high enough competence yourself.
Competence is a measure of how efficiently the cells can be transformed and is defined as the number of colonies formed per µg of supercoiled DNA. It can be easily calculated transforming the cells with a known amount of supercoiled DNA then dividing the number of colonies formed by the amount of DNA (in µg) used. The level of competency that is required depends on the application. Here are a few examples:
| Competence (cfu/µg) | Application |
| >1×109 | Genomic library construction, Quikchange |
| 1×106 | Routine cloning/sub-cloning |
| 1×103 | Transformation of circular plasmid DNA |
If you can generate cells with these sorts of competencies, then there is no need to buy in competent cells, unless you are from a super-rich lab, or use competent cells very infrequently. However, although the protocols are simple, obtaining home made cells with a high competence is a bit of a knack. Here are some tips that should help:
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Photo: Jessica Shannon
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