Figure 8. Model for crosstalk between the std operon and SPI-1.

Lack of std expression (e. g., under laboratory conditions and in the ileum) permits HilD synthesis and subsequent expression of SPI-1. Transcription of std (e. g., in Dam⁻ mutants and in the caecum) yields StdE and StdF products, resulting in postranscriptional inhibition of HilD synthesis, and subsequent SPI-1 repression.