Evidence for the presence of plasmids in four therapeutically important strains of Lactobacillus acidophilus

Four therapeutically important strains of Lactobacillus acidophilus designated as R, 301, 1899 and NCFM were screened for the presence of plasmids. Two lysis methods were used for the isolation of plasmid DNA: an alkaline method and a more gentle technique. It was found that the gentle lysis method yielded better plasmid DNA both quantitatively and qualitatively. All four strains studied apparently possess plasmids. The strains 301 and NCFM possessed one plasmid each, with a size of 4.2 kb, whereas R possessed three plasmids (3.5, 2.4 and 2.1 kb) and 1899 possessed two plasmids (4.1 and 4.2 kb). Restriction analysis revealed that the plasmid DNA from strain R was cleaved by Bam HI but not by Hind III and Eco RI. The plasmid DNA from the remaining three strains was cleaved by all three restriction enzymes used.