To evaluate if hilD_a resulted in the reduced expression of SPI-1 genes observed in broth culture conditions also during infection of human cells, bacterial RNA from ER1175, ER1175::hilD_a and ER1175ΔhilD was extracted at 0, 30 and 60 min post infection (t0, t30, t60) and qRT-PCR was performed for hilD and six other SPI-1 genes, namely the genes encoding the four transcriptional regulators HilC, RtsA, HilA, and InvF, the T3SS-1 structural component InvA, and the effector protein SipB (Supplementary Methods). The gene discussed is SPI1; the disease is infection.