Stability of single-stranded DNA plasmids during continuous culture of Bacillus subtilis, and the effects of host chemostat-experience

Abstract The stability of a selection of single-stranded DNA plasmids was compared in continuous culture of Bacillus subtilis 168-CU267. Plasmid pUB110 and its derivatives were found to be 100% stable in long-term cultures with carbon, nitrogen, potassium, sulfur or magnesium as limiting nutrients. In phosphate-limited culture, pUB110 showed only slight instability, but its derivatives pPL603, pPL608 and pSM112 were rapidly lost from the cultures after a lag of variable length during which no plasmid-free bacteria were detected. The proportion of plasmid-bearing bacteria in the culture sometimes showed temporary recovery prior to ultimate loss. Plasmids pHV33 and pC194 were lost rapidly without a preceding lag phase. Chemostat experience (800 generations in phosphate-limited continuous culture) of the host bacterium greatly enhanced the retention of pC194-carrying bacteria in phosphate-limited cultures, but effects on retention of the other plasmids were not significant.