Correlation between <em>In Vivo</em> Biofilm Formation and Virulence Gene Expression in <em>Escherichia coli</em> O104:H4

<div><p>The emergence of novel pathogens poses a major public health threat causing widespread epidemics in susceptible populations. The <em>Escherichia coli</em> O104:H4 strain implicated in a 2011 outbreak in northern Germany caused the highest frequency of hemolytic uremic syndrome (HUS) and death ever recorded in a single <em>E. coli</em> outbreak. Therefore, it has been suggested that this strain is more virulent than other pathogenic <em>E. coli</em> (e.g., <em>E. coli</em> O157:H7). The <em>E. coli</em> O104:H4 outbreak strain possesses multiple virulence factors from both Shiga toxin (Stx)-producing <em>E. coli</em> (STEC) and enteroaggregative <em>E. coli</em> (EAEC), though the mechanism of pathogenesis is not known. Here, we demonstrate that <em>E. coli</em> O104:H4 produces a stable biofilm <em>in vitro</em> and that <em>in vivo</em> virulence gene expression is highest when <em>E. coli</em> O104:H4 overexpresses genes required for aggregation and exopolysaccharide production, a characteristic of bacterial cells residing within an established biofilm. Interrupting exopolysaccharide production and biofilm formation may therefore represent effective strategies for combating future <em>E. coli</em> O104:H4 infections.</p> </div>