As shown in Fig. 5, transformation efficiency was closely related
to the concentration of competent cells. At the concentration of
2.4 × 108 CFU/ml, 8.6 × 107 transformants per μg of pBBR1MCS DNA
was obtained, resulting in 1.5–3.8 times higher yields than other cell
concentrations. Apparently, the optimal competent cell concentration
of S. maltophilia S21 was 2.4 × 108 CFU/ml.