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Extended-spectrum beta-lactamase (ESBL) producing  Escherichia coli  have become prevalent worldwide, with  E. coli  of sequence type 131 (ST131) as the dominant genotype.  E. coli  ST131 predominantly exhibits the serotype O25, is associated with the ESBL CTX-M-15 and belongs to a well-defined subclade within the Fim H 30-R clade, Fim H 30-Rx/C2. Multidrug resistance may have fitness costs for the bacteria. The aim of the current study was to investigate the fitness burden compared to a susceptible ST131 isolate without resistance genes  in vitro  and  in vivo  and describe genetic differences between fit and less fit isolates.    Materials and methods  From a collection of clinical ESBL and non-ESBL  E. coli  isolates from urinary tract infection, we selected 16  bla  CTX–M–15 -positive isolates of ST131. The  in vitro  fitness was examined, and relative bacterial fitness (fit  t  ) was determined by direct competition with a fully susceptible ST131 isolate and illustrated in percent, with &amp;lt;100% resulting in a lower fitness, compared to the susceptible reference isolate. The isolates were subjected to whole-genome sequencing and analyzed for resistance markers, plasmids, phage content, and serotype.  In vivo  competition was tested in a mouse colonization model.    Results  The majority (12 out of 16) of the CTX-M-15-producing isolates had a slightly lower relative fitness compared to the susceptible ST131 isolate (mean, 97.6%; range, 82.6–108%)  in vitro . Three isolates had a better fitness than the susceptible ST131 isolate, and one isolate had an identical fitness to the susceptible ST131 isolate. The  in vitro  fitness showed no correlation to the number of plasmids, number of phages, number of resistances, or genome size. For the  in vivo  competition assays, all three ESBL-producing isolates showed better colonization of the ESBL-resistant ST131 isolates compared to the susceptible ST131 isolate.    Conclusion  This study shows that ESBL-producing ST131/ H 30-Rx are not necessarily burdened by multidrug resistance, however, have a better  in vitro  fitness than the susceptible isolate. These data contribute to the understanding of the success of ST131/ H 30-Rx, although they do not indicate ways to overcome this highly fit, virulent, and antimicrobial-resistant clone.

In vitro Relative Fitness, in vivo Intestinal Colonization and Genomic Differences of Escherichia coli of ST131 Carrying blaCTX–M–15