The EspD Protein of Enterohemorrhagic Escherichia coli Is Required for the Formation of Bacterial Surface Appendages and Is Incorporated in the Cytoplasmic Membranes of Target Cells

The formation of EspA-containing surface appendages in pathogenic Escherichia coli strains, both enteropathogenic E. coli (EPEC) and Shiga toxin-producing E. coli strains, is essential for critical events in the infective process, e.g., localized bacterial adherence to host cells with formation of microcolonies and induction of attaching and effacing lesions. It has been reported that EPEC mutants deficient in the production of EspD, which is encoded by the esp operon, are unable to accumulate actin underneath adherent bacteria but exhibit an attachment similar to that of the wild type. Here, we report the construction and characterization of an in-frame espD deletion mutant of the enterohemorrhagic E. coli (EHEC) strain EDL933. In contrast to what was observed in EPEC mutants, the EDL933 espD mutant not only lacked the capacity to accumulate actin but also exhibited an impaired attachment to HeLa cells. The synthesis of the EspD protein was also essential for the formation of EspA-containing filaments. Finally, localization studies demonstrated that the EspD protein is transferred to the cytoplasm and integrated into the cytoplasmic membranes of infected cells. These results help to elucidate the underlying molecular events in infections caused by EHEC.