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Effect of simulated microgravity on the resistance of bacterial planktonic and biofilm cells and its molecular physiology.  A.  Matin and S. Lynch, Department of Microbiology & Immunology, Stanford University School of Medicine.

Human immune response is compromised in microgravity of space making bacterial infections potentially more dangerous during space travel and residence. Planktonic (free-living) Escherichia coli, when cultivated under simulated microgravity (SMG) conditions on Earth, exhibited strong increase in resistance especially in the stationary growth phase, a phase commonly involved in initiation of infection. A particularly threatening life-phase of bacteria on Earth is their biofilm, which is a bacterial community surrounded by a slime matrix: diseases in which biofilms play a prominent role tend to be chronic and hard to eradicate. To examine the effect of SMG on biofilm resistance, we devised a modified high aspect to ratio vessel bioreactor that permitted their cultivation under these conditions on Earth. E. coli formed more copious biofilms under SMG conditions, which exhibited greater resistance to antibiotics than their Earth- grown counterparts. A sigma factor, termed ss, controlled E. coli resistance differently in SMG compared to normal gravity conditions. Under the latter conditions, this sigma factor orchestrates resistance comprehensively and in all growth phases. However, under SMG conditions, it played a resistance-determining role only in the stationary growth phase of planktonic cells. Moreover, in the biofilms, while ss continued to have a role in determining resistance to “general” stressors (e.g., salt and acid), a mutant missing this sigma factor was as resistant as the wild type to penicillin and chloramphenicol at the concentrations of these antibiotics that kill Earth-grown biofilms. Molecular regulatory aspect of ss that depend on the folding pattern of its messenger RNA and protein were profoundly altered under SMG. Thus, SMG makes both planktonic as well as biofilm cells of E. coli more dangerous to astronauts and the underlying basis of this phenomenon may include altered folding pattern of macromolecules under SMG.