Influences of growth temperature and preparation of competent cells on efficiency of chemically-induced transformation in Escherichia coli DH5α

It is well known that cellular cultures grown at different temperatures will display altered membrane lipid composition to maintain a relatively constant membrane fluidity across a range of temperatures. One of the proposed theories on the mechanism of chemically-induced transformation is that a higher temperature of the heat-shock step increases the fluidity of the host cell membrane, allowing plasmid DNA to insert into the cell through transient gaps in the membrane. We sought evidence for this model by growing Escherichia coli DH5α at two different temperatures (37 oC and 42 oC) and transforming cells from each culture with pUC19 plasmid DNA. At a constant heat-shock temperature of 42 oC, the cells grown at 42 oC were expected to have a higher survival rate, but a lower degree of membrane fluidity and thus lower transformation efficiency than those cells grown at 37 oC. Our results showed no evidence that the differences in fluidity between cells grown at these two experimental temperatures were sufficient to affect any kind of change in transformation efficiency. However, our results did show a benefit to using cells grown at 42 oC for chemically-induced transformations as a result of their higher survival of heat-shock. _______________________________________________________________