Escherichia coli of sequence type (ST) 131 resistant to fluoroquinolones and producer of CTX-M-15 is globally one of the major extraintestinal pathogenic E. coli (ExPEC). ST131 phylogenesis showed that multidrug-resistant ST131 strains belong to a clade called C, descending from an ancestral clade called B, comprising mostly antibiotic-susceptible strains. Antibiotic resistance could appear as one of the keys of the clade C global success. We hypothesized that other features of ST131 clade C could contribute to this success since other major global ExPEC clones (ST73, ST95) are mostly antibiotic-susceptible. To test this hypothesis, we measured the growth abilities, early biofilm formation and virulence-factor content of a collection of clade B and clade C strains. Moreover, using competition assays, we measured the capacity of selected representative strains of clades B and C to colonize the mouse intestine and urinary tract, and to kill mice in a septicemia model. Clade B and C strains had similar growth ability. However, clade B strains were more frequently early biofilm producers, expressed mostly faster their type 1 fimbriae and displayed more virulence factor-encoding genes than clade C strains. Clade B outcompeted clade C in the gut and/or urinary tract colonization models and in the septicemia model. These results strongly suggest that clade C strain evolution includes a loss of virulence, i.e . a process that could enhance micro-organism persistence in a given host and thus optimize transmission. This process, associated with acquired antibiotic-resistance, could ensure clade C strain survival in environments under antibiotic pressure. Importance Extraintestinal pathogen Escherichia coli (ExPEC) are virulent but mostly antibiotic-susceptible. One worrying exception is ST131, a major multidrug resistant ExPEC clone that has spread worldwide since the 2000s. To contain the emergence of this threatening clone, we need to understand what factors favored its emergence and dissemination. Here, we investigated whether multidrug-resistant ST131 had advantageous phenotypic properties beyond multidrug resistance. To this end, we competed the emergent multidrug-resistant ST131 with its antibiotic-susceptible ancestor in different conditions: biofilm production, in vivo colonization and virulence experiments. In all in vivo competitions, we found that multidrug-resistant ST131 was losing to its ancestor, suggesting a lesser virulence of multidrug-resistant ST131. It was previously described that losing virulence can increase micro-organism persistence in some populations and subsequently its level of transmissibility. Thus, a decreased level of virulence, associated with multidrug resistance, could explain the global success of ST131.
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