Antimicrobial resistance and plasmid properties of Leuconostoc and group N Streptococcus Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wm117r49k

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  • A method for rapid isolation of plasmid DNA from group N streptococci was developed. Chief advantages of the method were simplicity, the utilization of microliter quantities of reagents, and the obtainment of preparations highly enriched for covalently closed circular plasmid DNA. The method was also applied to over 15 Leuconostoc strains, and recovery of plasmids in the 1-to-76 megadalton mass range was demonstrated. Similarly to lactic streptococci, leuconostocs (other than L. oenos) contained at least one, and usually more, plasmid species. Plasmid DNA could not be demonstrated in four L. oenos strains examined. Thirty-eight strains of lactic streptococci were challenged with all major classes of antimicrobials, in the Bauer-Kirby disc test. Minimal inhibitory concentrations of some antimicrobials were determined for the less susceptible strains. Presence of high-level resistance factors could not be detected, except in the case of nisin. Streptococcus lactis ATCC 7962 was resistant to >40-fold higher concentrations of this bacteriocin (>64 μg/ml) than most other strains tested. This strain was a potent nisin producer. Accidental contamination of one S. cremoris culture with Leuconostoc led to the discovery that most leuconostocs are insensitive to the antibiotic vancomycin. Tests performed with one such strain showed that resistance did not depend on drug inactivation. Derivative strains of S. lactis NCDO 1404 (a reference nisin producer which contained seven plasmid species) were obtained by protoplast- or temperature-induced plasmid curing. Curing also occurred spontaneously, during growth in broth. Comparison of derivatives and the parental strain provided evidence for plasmid linkage of proteinase and lactose fermentation in NCDO 1404. The four possible combinations of proteinase (+/-) and lactose fermentation (+/-) phenotypes could be differentiated on buffered, milk-based agar media containing pH indicators.
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