A chromosomal repeated sequence from Streptococcus thermophilus was identified as a new insertion sequence (IS), IS1191. This is the first IS element characterized in this species. This 1313 bp element has 28 bp imperfect terminal inverted repeats and is flanked by short direct repeats of 8 bp. The single large open reading frame of IS1191 encodes a 391-amino-acid protein which displays homologies with transposases encodes by IS1201 from Lactobacillus helveticus (44.5% amino-acid sequence identity) and by the other ISs of the IS256 family. One of the copies of IS1191 is inserted into a truncated iso-IS981 element. The nucleotide sequences of two truncated iso-IS981s from S. thermophilus and the sequence of IS981 element from Lactococcus lactis share more than 99% identity. The distribution of these insertion sequences in L. lactis and S. thermophilus strains suggests that intergeneric transfers occur during cocultures used in the manufacture of cheese.

Summary -Nine bacteriophages of Streptococcus salivarius subsp thermophilus isolated from different fermentation accidents, with different geographic origins, were compared according to their genomic structures and growth characteristics. Their genomes are linear, double-stranded DNA, with complementary cohesive ends. They belong to one group and are c1assed into 2 closely related strains. The phages of the same strain show the same restriction pattern with BamHI, EcoRI, Hindlll and Pvu II. The bacteriophages <1>81.2 and A 1.1 representative of the strain 1 and strain 2 respectively were characterized and compared according to their morphology, growth characteristics, restriction maps and DNA homology. They belong to Bradley's group B. The optimum concentration of 25 mM was determined for phage infections requiring calcium ions. One-step growth studies show that the latent period is 25 min for both A1.1and B1.2;their burst size is 88 and 56 particles per infected cell, respectively. Their genomes are 36.2 ± 0.5 and 35.6 ± 0.5 kbp in length, respectively.The restriction maps of these genomes are constructed. Cross-hybridization experiments showed that the 2 bacteriophages are clossly related. Using Southern blot hybridization, no homology was detected between the DNA of various bacterial strains, te, strains sensitive or resistant to phage infections, and the B1.2DNA, indicating that these bacteria are not Iysogenic for these phages.