For routine plasmid transformations, where high efficiency is not required, the following method which is a modification of that described by Sambrook and Russell (2001) was used. An overnight culture of the recipient strain was subcultured in fresh LB and grown till mid-exponential phase. The culture was chilled on ice for 15 min, and the steps hereafter were done on ice or at 4°C. The culture was centrifuged, and the pellet was resuspended in one third volume of cold 0.1 M CaCl2. After 15 min incubation on ice, the cells were again recovered by centrifugation, and resuspended in one tenth volume of cold 0.1 M CaCl2. The suspension (0.1 ml) was incubated on ice for 1 h after which DNA was added (~10-100 ng of DNA in less than 10 μl volume). The mixture was again incubated on ice for 30 min, and then heat shocked for 90 seconds at 42°C. Immediately 0.9 ml of LB broth was added to the tube and incubated at 37°C for 45 min for phenotypic expression of the antibiotic marker before being plated on selective medium at various dilutions. A negative control tube (with no plasmid DNA addition) was also routinely included in each of the experiments

Calcium chloride method