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Extended-spectrum beta-lactamase (ESBL)-producing Escherichia (E.) coli occur frequently all over the world, not only in human and veterinary medicine clinical-associated contexts but also in wildlife and the environment, which are considered to be less affected by antimicrobial compounds. Specifically, international high-risk multi-drug resistant clones and lineages of ESBL-associated sequence type (ST) 131 and, to a lesser extent, 648 seem to be of importance. Their possession of ESBL- and non-resistance-factor-carrying plasmids should be more closely examined; the success of certain bacterial clones and lineages might not depend on antimicrobial resistance (AMR) alone. My thesis aimed at investigating the interaction of ESBL-plasmids with chromosomally-encoded, non-resistance features of ST131 and ST648 E. coli. This interaction is important as it might result in benefits for the bacterial host beyond those of AMR such as virulence, fitness and metabolism, yet presents no parallel fitness costs.
ESBL-plasmid-carrying wild-type (WT) E. coli strains were compared to a constructed, corresponding ESBL-plasmid “cured” variant (PCV) and to a complementary ESBL-carrying transformant (T) in fitness/metabolic assays, biofilm and motility assays, and RNA sequence analysis. No differences were observed in the strains’ growth or metabolic behaviors. Some differed in biofilm and motility assays, however, exemplified by an enhanced curli and/or cellulose production and a reduced swimming capacity of WTs/Ts compared to the corresponding PCV. RNA sequencing mostly confirmed the phenotypic results on a transcriptomic level, revealing the chromosomally-encoded csgD-pathway to be a key factor.
The results clearly indicate that ESBL-plasmid carriage does not necessarily lead to a fitness/metabolic disadvantage for the bacterial host. On the contrary, the results suggest that an interaction of ESBL-plasmids with the bacterial host’s chromosome in some strains, especially in terms of non-resistance-associated features, presumably contributes to the pandemic success of some isolates of ESBL-producing E. coli clones and lineages in various hosts and habitats, also beyond high antimicrobial selection pressures.