We describe a novel insertion sequence (IS) element, IS, which was found to be closely related to IS, which is found only in ; we found that these two IS eleemnts have a level of sequence identity of 61.5% and also have almost identical terminal inverted repeats. IS was present both and strains isolated from humans. In contrast, IS was absent from strains isolated from sheep. A DNA fingerprint analysis performed with anothe IS element, IS, which is present in and , revealed that isolates obtained from sheep are distinct from human isolates. Thus, human and ovine strains comprise two distinct populations, indicatng that little or no transmission occurs between sheep and humans. An IS-associated restriction frragment length polymorphism analysis revealed that strains isolated from sheep are genetically more polymorphic that the human population, which is genetically very homogeneous. This suggests that human strains diverged from a single clone only recently. IS is present in a subset of strains that were derived mainly from pigs and rabbits, suggesting that these strains and to have adapted to different hosts (sheep and humans). Once in the human host, probably acquired IS from In contrast to human isolates, strains produced polymorphic IS-related DNA fingerprint patterns.


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