PLASMID INSTABILITY IN AN INDUSTRIAL STRAIN OF BACILLUS-SUBTILIS GROWN IN CHEMOSTAT CULTURE

A pUB110-derived plasmid/Bacillus subtillis host combination was segre gationally unstable when grown in chemostat culture with complex or mi nimal medium and under starch, glucose or magnesium limitation. The ki netics of plasmid loss were described in terms of the difference in gr owth rates between plasmid-containing and plasmid-free cells (dmu) and the rate at which plasmid-free cells were generated from plasmid-cont aining cells (R). Loss of plasmid-containing cells from the population was dmu dominated. Changes in medium composition and the nature of gr owth limitation caused variations in both dmu and R. The plasmid was m ost stable in glucose-limited chemostat cultures with minimal medium a nd least stable under starch limitation with complex medium. R and dmu were smaller for cultures in complex media than those in minimal medi a. Limitation by strach induced expression of the plasmid-encoded HT a lpha amylase gene and was associated with increased values of R and dm u. Magnesium limitation in minimal medium caused a significant increas e in dmu and a decrease in R.