Graduate Thesis Or Dissertation
 

Conjugal transfer of plasmid genes between lactic Streptococcus and Leuconostoc starter bacteria

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  • Mutants of Streptococcus lactis 7962 with various combinations of plasmid molecular masses were screened for nisin production. Nisin was produced by both wild type and mutants that contained a 17.5 Md plasmid, which was usually obscured by the chromosomal fragments. No nisin was produced by plasmid-free mutants. Sucrose fermentation and nisin production were related. A transconjugant obtained from nisin-producing donor S. lactis 7962 and recipient Leuconostoc dextranicum 181 was a "super-nisin" producer. Like the recipient, the transconjugant was resistant to both 7962 phage and to vancomycin (>1,000 ug/ml); but the plasmid profile showed an extra 17.5 Md plasmid. Conjugation between Lac⁺ S. lactis C2 and Lac⁻ L. cremoris CAF7 was also performed. It was a high-frequency conjugation type. Transconjugants fermented lactose significantly faster than did the wild type. When grown in litmus milk fortified with 0.2% yeast extract Lac⁺ transconjugants reached pH 4.68 within 24 h at 30 C: they were also positive for diacetyl production. The Lac⁺ Leuconostoc were resistant to c2 phage and also to >500 ug/ml vancomycin. Plasmid profiles of 10 Lac⁺ transconjugants showed two unique patterns. A novel enlarged plasmid was found. Application of Southern blot hybridization technique revealed some homology with the 30 Md plasmid of donor, recipient and the transconjugants as well as with some of the remaining plasmids of the donor.
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