SPI1 and infection: Intravital imaging of iv injected embryos, which express mCherry in macrophages (Tg(mpeg1:mCherry-F), referred to as mpeg1:mCherry, or Tg(mpeg:Gal4/uas-mCherry-NTR), referred to as mpeg1/umn) also showed that pu.1 knockdown embryos contained fewer bacteria at 24 hours post infection (hpi) than control embryos (Fig 1C and 1D and S2 Fig), demonstrating that macrophages are critical for bacterial replication.