Aims: The calcium chloride chemical transformation of Escherichia coli is still the most widly used cloning method in small laboratories. Therefore, any practicable improvement in its transformation efficiency seems to be of general interest.
Methods and results: We found that giving calcium chloride competent cells a 1 min microwave pulse at the lowest power setting (180 W), instead of the classic 1-2 min 42 degrees C heat-shock step, increases the transformation efficiency around threefold (3.3 +/- 0.5). Moreover, when both treatments were given in a 2-min 42 degrees C - 5 min on ice -1 min microwave pulse sequence, an additional improvement of 1.6 was obtained, resulting in an overall increase in efficiency of approximately 5.3-fold compared to classical heat shock.
Conclusions: This transformation method significantly improves the classical heat shock treatment.
Significance and impact of the study: This method might be useful to those laboratories that cannot afford an electroporation apparatus.