<i>Escherichia coli</i> O157:H7 senses microbiota-produced riboflavin to increase its virulence in the gut

Abstract

Riboflavin is produced by most commensal bacteria in the human colon, where enterohemorrhagic <i>Escherichia coli</i> (EHEC) colonizes and causes diseases. Sensing environmental signals to site-specifically express the type-III secretion system (T3SS), which injects effectors into host cells leading to intestinal colonization and disease, is key to the pathogenesis of EHEC. Here, we reveal that EHEC O157:H7, a dominant EHEC serotype frequently associated with severe diseases, acquired a previously uncharacterized two-component regulatory system <i>rbfSR</i>, which senses microbiota-produced riboflavin to directly activate the expression of LEE genes encoding the T3SS in the colon. <i>rbfSR</i> is present in O157:H7 and O145:H28 but absent from other EHEC serotypes. The binding site of RbfR through which it regulates LEE gene expression was identified and is conserved in all EHEC serotypes and <i>Citrobacter rodentium</i>, a surrogate for EHEC in mice. Introducing <i>rbfSR</i> into <i>C. rodentium</i> enabled bacteria to sense microbiota-produced riboflavin in the mouse colon to increase the expression of LEE genes, causing increased disease severity in mice. Phylogenic analysis showed that the O55:H7 ancestor of O157:H7 obtained <i>rbfSR</i> which has been kept in O157:H7 since then. Thus, acquiring <i>rbfSR</i> represents an essential step in the evolution of the highly pathogenic O157:H7. The expression of LEE genes and cell attachment ability of other EHEC serotypes in the presence of riboflavin significantly increased when <i>rbfSR</i> was introduced into them, indicating that those serotypes are ready to use RbfSR to increase their pathogenicity. This may present a potential public health issue as horizontal gene transfer is frequent in enteric bacteria.