This is also the first study in spaceflight and in simulated microgravity to evaluate microgravity-induced changes of the pathogen S. marcescens in a live host, as previous studies in the field of host-pathogen interactions have focused primarily on Salmonella15 and other microbes.48–50 Utilizing common immune pathway mutants, we determined that regardless of whether the Imd or Toll pathways were altered, infection with spaceflight sample decreases survival of flies compared to infection with ground sample of S. marcescens (Fig. 4). The gene discussed is TLR4; the disease is infection.