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Title: Segregational stability of plasmids in Bacillus subtilis
Author: Sanders, Rhiannon
ISNI:       0000 0001 3550 6886
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1986
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The stability of plasmids in Bacillus subtilis in the absence of selective pressure under long term chemostat culture and under different conditions of nutrient limitation was investigated. Culture and sampling conditions were established to allow the detection of plasmid-free cells and samples were routinely screened in order to monitor the structural integrity of the resident plasmid. Initially the stability of the plasmid pHV14-F (carrying the repressor gene of phage 0105) in the Bacillus subtilis strain 3G18 was investigated. This plasmid was found to be segregationally and structurally highly unstable. The stability of the Staphylococcus aureus plasmids pC194 and pUB110 along with the Bacillus cereus plasmid pBC16 was examined in the Bacillus subtilis strain 168 trp in chemostat culture under carbon, magnesium and phosphate limitation. All three plasmids were found to be remarkably stably maintained through up to 100 culture generations of their host strain under all three of the nutrient limitations. Under enforced competition from plasmid-free cells at 1% of the chemostat inoculum all three plasmids were retained in the culture but in the presence of 50% plasmid-free cells in the inoculum plasmid-carrying cells were rapidly displaced from the culture. Phosphate limitation was found to exert a slightly greater stringency of selection for plasmid-free cells than either carbon or magnesium limitations (which had similar effects). A derivative of pC194 was isolated, pC194-Ki which was segregationally unstable, the Bacillus subtilis culture losing its plasmid within 30 generations. On restriction endonuclease digestion analysis the plasmid was revealed to be physically very closely related to pHV14 (pC194 and pBR322 ligated at their HindIII sites). Under magnesium limitation all Bacillus subtilis cultures after 2-3 days were predominantly composed of cells with an unusual helical morphology. The development of this morphology under the conditions employed and the actual form of the helical cells themselves was found to be different from previously reported instances of the appearance of helical cells in cultures of Bacillus subtilis.
Supervisor: Not available Sponsor: Science and Engineering Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QR Microbiology