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Enterohemorrhagic  Escherichia coli  (EHEC) is a significant cause of serious human gastrointestinal disease worldwide. EHEC strains contain a pathogenicity island called the locus of enterocyte effacement (LEE), which encodes virulence factors responsible for damaging the gut mucosa. The Cpx envelope stress response of  E. coli  is controlled by a two-component system (TCS) consisting of a sensor histidine kinase (CpxA) and a cytoplasmic response regulator (CpxR). In this study, we investigated the role of CpxRA in the expression of LEE-encoded virulence factors of EHEC. We found that a mutation in  cpxA  significantly affected adherence of EHEC to human epithelial cells. Analysis of this mutant revealed the presence of high levels of CpxR which repressed transcription of  grlA  and  ler , the main positive virulence regulators of the LEE, and influenced negatively the production of the type 3 secretion system–associated EspABD translocator proteins. It is known that CpxR activates  rpoH  (Sigma factor 32), which in turns activates transcription of the  lon  protease gene. We found that transcription levels of  ler  and  grlA  were significantly increased in the  lon  and  cpxA lon  mutants suggesting that  lon  is involved in down-regulating LEE genes. In addition, the  Galleria mellonella  model of infection was used to analyze the effect of the loss of the  cpx  and  lon  genes in EHEC's ability to kill the larvae. We found that the  cpxA  mutant was significantly deficient at killing the larvae however, the  cpxA lon  mutant which overexpresses LEE genes  in vitro , was unable to kill the larvae, suggesting that virulence in the  G. mellonella  model is T3SS independent and that CpxA modulates virulence through a yet unknown EHEC-specific factor. Our data provides new insights and broadens our scope into the complex regulatory network of the LEE in which the CpxA sensor kinase plays an important role in a cascade involving both global and virulence regulators.

frontiers in cellular and infection microbiology

The Two-Component System CpxRA Negatively Regulates the Locus of Enterocyte Effacement of Enterohemorrhagic Escherichia coli Involving σ32 and Lon protease