Antibiotic Resistance and Population Dynamics of Escherichia coli in Relation to a Large Scale Antibiotic Consumption Intervention

Abstract: Antibiotic resistance challenges the practice and development of modern medicine. The aim of this thesis was to test the hypothesis that antibiotic resistance is reversible once the selection pressure of an antibiotic is removed. A decisive reduction (85%) in trimethoprim and trimethoprim-sulfamethoxazole over 24 months in Kronoberg County, Sweden, is described. The resistance baseline prior to the intervention and the effects of the intervention on resistance levels, trimethoprim resistance genes (dfr-genes) and population structure in Escherichia coli were studied.The effects of different algorithms for excluding patient duplicate isolates were small but systematic. An identical algorithm was used throughout.The drastic decrease in the use of trimethoprim containing drugs did not result in a corresponding decrease in trimethoprim resistance. This was true both for total trimethoprim resistance and for trimethoprim mono-resistance. The distributions of E. coli phenotypes, dfr-genes and E. coli sequence types were stable. The marginal effect on resistance rates was explained by a low fitness cost of trimethoprim resistance observed in vitro and the high levels of associated resistance in trimethoprim resistant isolates.Trimethoprim resistance was, although widespread in the E. coli population, more common in certain E. coli sequence types. The distributions of dfr-genes were different in E. coli and K. pneumoniae and between different E. coli sequence types. These results indicate mechanisms related to the genetic back-bone of E coli to be important for the acquisition and persistence of antibiotic resistance.The findings of this thesis indicates that, at least for some classes of antibiotics, we may have overestimated the usefulness of a strategy for reversing antimicrobial resistance based on the fitness cost of resistance. We have equally underestimated the conserving effects of associated resistance. The stability of the dfr-genes and E. coli sequence types underlines the importance of associated resistance and successful lineages in the spread and maintenance of antibiotic resistance in E. coli.