Why choose cells at exponential growth for transformation?

G'Day Steffi:

For making competent bacteria (e.g., E coli) exponentially growing cells give the greatest number of viable cells (and therefore a larger total number of competent cells), as well as an increase in the amount of DNA that can be taken up/cell for some bacterial strains. This will vary depending on the strain of bacteria used. Sometimes if you grow bacteria overnight in liquid culture you may see a browny-black pellet after centrifuging the cells - these are mainly dead bacteria!). The induced competency depends on many factors (e.g., whether CaCl2, polyethylene glycol or some other chemical was used; the concentration of these chemicals; treatment of cells during the procedure; whether freshly prepared or frozen bacteria are used; etc). For the transformation itself the maximum number of viable cells is also desired.

For other bacteria (e.g., some gram positive strains) natural competency requires the presence of a DNA-binding protein on the surface of the cell. The presence of this protein is correlated with nutritional depletion - i.e. when the cells reach a high density and start to run out of nutrients they become more competent!

Last edited by Steve Lolait (20th Apr 2011 13:19:36)