Maturation of functional type III secretion machinery by activation of anaerobic respiration in enterohaemorrhagic Escherichia coli

Enterohaemorrhagic Escherichia coli (EHEC) is a gastrointestinal pathogen that causes diarrhoea and more severe diseases in humans. A key feature of EHEC is the type III secretion system (TTSS), which translocates virulence factors (effectors) directly into host cells. In this study, the expression and secretion of effectors in EHEC grown under anaerobic conditions were examined. The secretion of effectors was greatly enhanced, without an increase in their expression levels, when EHEC was grown in the presence of specific electron acceptors, such as trimethylamine N-oxide (TMAO) and nitrate, for anaerobic respiration. The activation of the TTSS was dependent on the activity of respiratory systems, including electron-acceptor-specific signalling systems and reductases. Although de novo protein synthesis was not required for TTSS activation, the inhibition of respiratory activity abolished secretion. EHEC grown with either TMAO or nitrate possessed a more intact type III secretion (TTS) apparatus, including the needle protein EscF and the translocator protein EspA, than EHEC grown without an electron acceptor. These observations suggest that activation of either the TMAO- or the nitrate-specific respiratory system accelerates the maturation of functional TTS apparatus under anaerobic growth conditions.