Graduation date: 2007Tetracycline (Tet)-resistant strains of Chlamydia suis were isolated from swine farms in the Midwest. The isolation of the resistant strains was significant because Tet is an antibiotic used to treat infections by veterinarians and doctors. One of the tetracycline (Tet)-resistant strains, R19, was able to survive in Tet up to 4 μg/ml. This is in contrast to two Tet-sensitive strains C. suis S45 and C. trachomatis L2, which were sensitive to 0.1 μg/ml. Using sequencing and Southern blot analysis, we were able to identify a set of related genomic islands present in the Tet-resistant strains but not in the Tetsensitive strain. The genomic islands inserted at the same position, interrupting the invasin gene within the chlamydial chromosome. The islands contained the tet(C) resistance determinant, the Tet repressor, and genes found in plasmids in other bacterial species. In addition, the islands also contained an insertion element, termed IScs605. The IScs605 insertion element was similar to IS605 insertion elements found in Helicobacter pylori and contains 2 phylogenically distinct transposases. This was the first identification of a resistance island and an insertion element in any chlamydiae. The insertion element was further characterized by using a mating assay in E. coli. The IS element was placed on a high copy plasmid, pUC18, and a medium copy vector, pBBR1MCS, and transposition activity studied. IScs605 was shown to have high transposition activity compared to the negative control, and had a strong preference to insert adjacent to the pentanucleotide 5’-TTCAA. In addition, the orfA gene was found to be essential for transposition activity while orB was not essential. The transposition activity in E. coli provides strong evidence that this IS element was responsible for the integration of the tet(C) genomic islands into the chlamydial chromosome. Collectively, this work identifies the first genomic island in the chlamydiae and is the first investigation of horizontal resistance in any obligate intracellular bacterium. In addition, it is anticipated that the C. suis Tet-resistance islands may be useful in the development of a transformation system for the chlamydiae.