BACILLUS-SUBTILIS DEVELOPS COMPETENCE FOR UPTAKE OF PLASMID DNA WHEN GROWING IN MILK-PRODUCTS

Transformation with plasmid DNA of naturally competent cells of Bacill us subtilis 168 in milk products was studied. Plasmid pMG36enpr, a bro ad host-range lactococcal vector carrying an erythromycin resistance a nd the B. subtilis npr gene encoding neutral protease, was taken up by B. subtilis veils grown in UHT chocolate milk. Under these conditions competence was optimal during transition from exponential to stationa ry growth phase, resulting in 9x10(1) transformants per 0.01 mu g DNA. No manipulation of the cells was necessary for competence to develop. When cells were pregrown in synthetic medium, higher transformation r ates were obtained in assays, where the subsequent transformation expe riments were either done in chocolate milk diluted 1:1 (v/v) with synt hetic growth medium (up to 8x10(2) transformants) or in undiluted choc olate milk (1x10(2) transformants). The number of transformants was re duced to 4x10(1), when diluted milk or flavored milks were used. No tr ansformants 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 (4x10(3) transformant s per 0.01 mu g DNA).