DNA-DNA hybridization analysis ofStreptococcus thermophilusplasmids

Five of the nine known plasmid DNAs of Streptococcus thermophilus ranging from 2.2 kbp to 14.7 kbp in molecular size, were found to share homologous sequences as evidenced by DNA-DNA hybridization analyses. The four other S. thermophilus plasmids also lacked homology with one another implying that they may represent unique DNA homology groups. The lack of homology between the S. thermophilus probe (pER8, 2.2 kbp) and plasmids of Lactococcus lactis, L. cremoris and lactobacilli also implied a fundamental difference in the origins of plasmids of thermophilic and mesophilic lactic acid bacteria.