Analysis of the genetic polymorphism between three Streptococcus thermophilus strains by comparing their physical and genetic organization.

The physical maps of Streptococcus thermophilus CNRZ368 and NST2280 strains were constructed by analysing PFGE patterns obtained with the low-frequency-cutting enzymes SmaI, BssHII and SfiI. Their chromosomes are 1864 and 1840 kb circular molecules, respectively. Comparison of their physical maps with that of the reference A054 strain revealed a relatively conserved organization of the restriction sites. Three variable regions were detected with the map of CNRZ368 whereas 15 were found with the map of NST2280. To construct the genetic maps, probes corresponding to 10 single-copy genes, the rrn genes and the insertion sequences IS1191, IS981 and ISS1 were hybridized to Southern blots of chromosomal DNA digested with the different mapping enzymes. Comparison of the genetic maps of the three strains showed a conserved location of the mapped single-copy genes. However, six rrn loci were present in the chromosome of A054 and CNRZ368 whereas five were present in the NST2280 chromosome. A polymorphism was also found in the copy number of the insertion sequences between the three strains.