Fifty microliters of different concentrations of competent cells (6.0 × 107–1.2 × 109 CFU/ml) were mixed with 2 μl pBBR1MCS (50 ng/μl) and electroporated at 18 kV/cm, 25 μF and 200 Ω. The results aremeans±SD (error bars) of three independent triplicate experiments