DNA-DNA homology between plasmids from Streptococcus thermophilus

Fift Y strains of Streptococcus thermophilus isolated from raw milk, industrial starters or issued from collections were studied for their plasmid complement. Most of them did not contain plasmid DNA. Six strains only harboured one plasmid and three strains two plasmids. Ali these plasmids were small sized: 2.9 to 7.6 kilobases. Restriction analysis and DNA-DNA hybridization were used to demonstrate similarity between plasmids of these strains and to distinguish several homology groups among them.