STABILITY OF SINGLE-STRANDED-DNA PLASMIDS DURING CONTINUOUS-CULTURE OF BACILLUS-SUBTILIS, AND THE EFFECTS OF HOST CHEMOSTAT-EXPERIENCE

The stability of a selection of single-stranded DNA plasmids was compa red in continuous culture of Bacillus subtilis 168-CU267. Plasmid pUB1 10 and its derivatives were found to be 100% stable in long-term cultu res with carbon, nitrogen, potassium, sulfur or magnesium as limiting nutrients. In phosphate-limited culture, pUB110 showed only slight ins tability, but its derivatives pPL603, pPL608 and pSM112 were rapidly l ost from the cultures after a lag of variable length during which no p lasmid-free bacteria were detected. The proportion of plasmid-bearing bacteria in the culture sometimes showed temporary recovery prior to u ltimate loss. Plasmids pHV33 and pC194 were lost rapidly without a pre ceding lag phase. Chemostat experience (800 generations in phosphate-l imited continuous culture) of the host bacterium greatly enhanced the retention of pC194-carrying bacteria in phosphate-limited cultures, bu t effects on retention of the other plasmids were not significant.