Fifty microliters of competent cells (2.4 × 108 CFU/ml) prepared at OD600 1.0 were electroporated with 2 μl pBBR1MCS (50 ng/μl) at 18 kV/cm, 25 μF and 200 Ω and incubated for various length of time (1 to 5 h) before spreading on plates containing chloramphenicol