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Title: Plasmid spread and chromosome mobilisation in Escherichia coli K12 populations
Author: Cullum, John Anthony
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1978
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The phenomenon of "epidemic spread" of a transmissible plasmid through a recipient population was re-examined. The phenomenon was studied quantitatively in the model system of F'lac in Escherichia coli K12. The rate of plasmid spread by retransfer was rather low because of the low frequency of continuing transfer by recipients that had received F'lac. The F'lac mating system was studied to answer the following questions: (i) What is the effect of parental concentration on mating efficiency? (ii) What proportion of parents are competent to mate? (iii) How many cells can competent parents mate with? (iv) How efficient is plasmid establishment in recipient cells after transfer? (v) How long are the lags between rounds of donor transfer and between a recipient receiving F'lac and becoming a competent donor? The variation in mating efficiency with parental concentration was incompatible with mating following bimolecular reaction kinetics and collision rate was not a limiting factor at concentrations above 5 x 107 cells/ml. The majority of both donors and recipients were competent to mate in 30', but whilst donors were fairly monogamous, recipients could mate with an average of 2-3 donors. Plasmid establishment after transfer was efficient and segregation had little effect on progeny growth rate. There was a lag of 30'-40' between rounds of donor competence and a lag of 90' before new progeny became competent donors. The rates of segregation due to plasmid incompatibility were calculated under several simple models. Experiments with two colEl derivatives were in reasonable agreement with the predictions of a model based on random pool replication. We studied the dependence on the recA function of the recombination needed for Hfr formation. We obtained Hfr clones from a recA F+ strain, but the rate of Hfr formation was only about 1% that in an isogenic rec+ strain, so Hfr formation in rec+ strains is probably mostly mediated by the host's normal recombination system. Our experiments suggested that "Type II" F+ strains were not defective in Hfr formation as had been previously reported, but had some secondary defect. The reA mutation reduced chromosome transfer in the same proportion as Hfr formation so the relationship between the two processes remains unclear.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available