Studies on plasmids of some lactic acid bacteria.
thesisposted on 19.11.2015 by Kiswar. Yeasmin
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A survey was conducted of a number of lactic acid bacteria of the genera Erysipelothrix, Listeria and Streptococcus to investigate the presence of detectable plasmids. When plasmids were detected an attempt was made to correlate their presence with a phenotypic trait. In addition some plasmids were investigated to determine certain physical characteristics, e.g. molecular weight and also the degree of base pair homology between plasmids isolated from different bacteria. The results indicate that of the strains of Erysipelothrix investigated none appeared to contain extrachromosomal DNA. Of the strains of the genus Listeria, plasmids were detected only in one representative of Listeria monocytogenes serovar 5. This particular serovar is characterized by enhanced haemolytic activity when compared with strains of the various other species in the genus Listeria. However, no evidence was obtained to indicate that the haemolysis was plasmid encoded. Plasmid deoxyribonucleic acid was detected in a number of strains representing a variety of species in the genus Streptococcus. The results obtained confirmed earlier reports that the haemolytic activity of "Streptococcus faecalis var. zymogenes" is plasmid encoded. The results also indicate that the haemolytic activity of haemolytic strains of bacteria previously named "Streptococcus durans" was also plasmid encoded. Investigation of the properties of the plasmid DNA from both these sources however did not show a high degree of homology, thus indicating that the plasmids had not been derived from the same source. Plasmids conferring drug resistance were isolated from representatives of the species Streptococcus faecium. Pigmentation in some strains of "Streptococcus faecium var. casseliflavus" appeared to correlate with the presence of plasmid DNA. Strong evidence was also obtained which indicated that lactose fermentation in Streptococcus bovis was at least, in part, due to the presence of plasmids. Although plasmids were detected in representatives of the species Streptococcus agalactiae, Streptococcus equinus, ''Streptococcus faecalis var. liquefaciens", "Streptococcus faecium var. mobilis", their presence could not be correlated with any particular phenotypic trait. No plasmids were detected in those strains of Str. faecalis and "Str. faecalis var. malodoratus" examined.