Effect of medium composition on the maintenance of a recombinant plasmid in Bacillus subtilis.

Recombinant plasmid pCED3 [confers beta-galactosidase production (LacZ+) and kanamycin resistance (Kmr)] in Bacillus subtilis was found to be both segregationally and structurally unstable. Since many solutions to segregational instability are already available, the problem of structural instability was specifically addressed by inclusion of kanamycin in the growth media. Culture instability was found to be highest in complex and defined media supporting high growth rates. Stabilization over the duration of the experiment (40 generations) was achieved by use of a recently developed chemically defined medium supporting a lower growth rate. Slowing down growth by decreasing temperature was much less effective. A major effect of the growth medium appears to be that of decreasing the growth rate advantage held by cells with plasmid deletions over parental cells containing the intact plasmid.