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Mitigating the Effects of Clinorotation on Escherichia coli through Induced Cellular Neutral Buoyancy.  M.R. Benoit and D.M. Klaus.  Dept. of Aerospace Engineering Sciences, BioServe Space Technologies, Univ. of Colorado, Boulder. 

   Differences in many bacterial growth parameters have been observed during space flight experiments. Weightlessness is believed to cause these affects by altering the distribution of cells and the chemical composition of their surrounding fluid, therefore indirectly changing their physiology. Clinostats have been used in ground-based studies to partially simulate the quiescent fluid environment attained in microgravity.  Results from these studies have, in general, produced bacterial growth characteristics similar to responses reported under space conditions.  A novel approach for evaluating the effects of reduced cell sedimentation is presented here through use of Escherichia coli (E. coli) cultures genetically modified to be neutrally buoyant.  It was hypothesized that the effects of clinorotation on final population density would be eliminated relative to static controls, since the cell distribution would already be near-colloidal.  As a control, E. coli that do not produce gas vesicles, but were otherwise identical to the experimental strain, were grown under clinostat and static conditions, resulting in a 10.5% average difference in cell culture density (p = 0.001).  Gas vesicle producing E. coli cultures were also grown under clinostat and static conditions, and as hypothesized, no significant difference was observed in cell culture density.  In addition, it was estimated that at the sample time of 60 hours, 100% of the control E. coli cells grown under static conditions had sedimented to the container bottom.  However, for the gas vesicle producing E. coli, only 46% of the cells were estimated to reach the container bottom.  An apparent correlation was shown between percent sedimentation and the relative difference of clinostat and static cell culture densities.  These results corroborate the concept that the lack of cumulative cell sedimentation is the dominant effect of space flight on final cell population in non-stirred E. coli cultures. 

   (Supported by NASA: NGT3-52386.)