| Summary: | Plasmid-mediated multidrug resistance in <i>E. coli</i> is becoming increasingly prevalent. Considering this global threat to human health, it is important to understand how plasmid-mediated resistance spreads. From a cohort of 123 patients with recurrent urinary tract infections (RUTI) due to extended spectrum beta-lactamase (ESBL)-producing <i>Escherichia coli</i> (ESBL <i>E. coli</i>), only five events with a change of ESBL <i>E. coli</i> strain between RUTI episodes were identified. Their <i>bla</i><sub>CTX-M</sub> encoding plasmids were compared within each pair of isolates using optical DNA mapping (ODM) and PCR-based replicon typing. Despite similar <i>bla</i><sub>CTX-M</sub> genes and replicon types, ODM detected only one case with identical plasmids in the sequential ESBL <i>E. coli</i> strains, indicating that plasmid transfer could have occurred. For comparison, plasmids from seven patients with the same ESBL <i>E. coli</i> strain reoccurring in both episodes were analyzed. These plasmids (encoding <i>bla</i><sub>CTX-M-3</sub>, <i>bla</i><sub>CTX-M-14</sub>, and <i>bla</i><sub>CTX-M-15</sub>) were unaltered for up to six months between recurrent infections. Thus, transmission of <i>bla</i><sub>CTX-M</sub> plasmids appears to be a rare event during the course of RUTI. Despite the limited number (<i>n</i> = 23) of plasmids investigated, similar <i>bla</i><sub>CTX-M</sub>-<sub>15</sub> plasmids in unrelated isolates from different patients were detected, suggesting that some successful plasmids could be associated with specific strains, or are more easily transmitted.
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