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Tetracycline (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. |
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