Bacillus subtilis develops competence for uptake of plasmid DNA when growing in milk products.

Transformation with plasmid DNA of naturally competent cells of Bacillus subtilis 168 in milk products was studied. Plasmid pMG36enpr, a broad host-range lactococcal vector carrying an erythromycin resistance and the B. subtilis npr gene encoding neutral protease, was taken up by B. subtilis cells grown in UHT chocolate milk. Under these conditions competence was optimal during transition from exponential to stationary growth phase, resulting in 9 x 10(1) transformants per 0.01 microgram DNA. No manipulation of the cells was necessary for competence to develop. When cells were pregrown in synthetic medium, higher transformation rates were obtained in assays, where the subsequent transformation experiments were either done in chocolate milk diluted 1:1 (v/v) with synthetic growth medium (up to 8 x 10(2) transformants) or in undiluted chocolate milk (1 x 10(2) transformants). The number of transformants was reduced to 4 x 10 (1), when diluted milk or flavored milks were used. No transformants were obtained in diluted yoghurt. Controls, in which both the preculturing and the transformation assays were done in synthetic medium, gave the maximum number of transformants (4 x 10(3) transformants per 0.01 microgram DNA).