|
Plate | cells that grow on these plate |
LB plate | all viable cells |
LB plate + antibiotic | cells that still carry the plasmid |
LB plate + IPTG (1 mM) | cells that have lost the plasmid or mutants that have lost the ability to express the target gene |
LB plate + antibiotic + IPTG (1 mM) | only mutants that retain the plasmid but have lost the ability to express the target gene |
In a typical culture useful for producing target protein, almost all cells will form colonies both on the LB plate and on the LB plate + antibiotic; less than 2% of the cells will form a colony on the LB plate + IPTG; and less than 0.01% will form a colony on the LB plate + antibiotic + IPTG.
With unstable target plasmids, the fraction of cells that have lost the plasmid will be reflected by an increase in colonies on the LB plate + IPTG and a decrease on ther LB plate + antibiotic.
Protocol
1. | Immediately before induction with IPTG (at OD600 is approx. 0.6), take a 100-ml aliquot of the cell culture. |
2. | Make a serial dilution of the cell suspension, including a 105 and 106 dilution. |
3. | Plate cells at a dilution of 105 on the LB plate + IPTG and on the LB plate + IPTG + antibiotic. |
Plate cells at a dilution of 106 on the LB plate and on the LB plate + antibiotic. | |
4. | Incubate the plates overnight a 37°C. |
5. | Count the number of colonies on each plate. |
Last updated: 14 Nov 2000