Isolation and characterization of plasmids from strains of streptococcus thermophilus bulgarian yogurt starters

Abstract

Twenty Bulgarian Streptococcus thermophilus strains used in industrial milk fermentation were analyzed for their plasmid content. Six of them were found to contain one or two plasmids. The newly isolated plasmids were divided into three groups based on DNA homology. Six plasmids, sharing homology with pt38 and p2992, were classified within group I, two plasmids were classified into group II and one plasmid, p2991, possessing homology only with itself, was classified in Group III. Hybridization experiments revealed low homology between streptococcal and Lactobacillus plasmids. Using plus-origin- specific (dso) DIG-labeled probes we determined that two of the plasmids (pt38 and p2992) belong to the pC194 plasmid family, that uses the rolling circle mode of replication. One plasmid from group I, pt38 was further characterized. We constructed a restriction map and identified the restriction fragment bearing the dso of replication. pt38 appears to be an excellent candidate for creation of a shuttle vector for gene expression in Escherichia coli and the Streptococcus thermophilus because it is small in size, has five unique restriction sites and known location of the plus-origin of replication. © 2003 Taylor and Francis Group, LLC.