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Background  Bacterial phenotypes result from responses to environmental conditions under which these organisms grow; reduced gravity has been demonstrated in many studies as an environmental condition that profoundly influences microorganisms. In this study, we focused on low-shear stress, modeled reduced gravity (MRG) conditions and examined, for  Escherichia coli and Staphlyococcus aureus , a suite of bacterial responses (including total protein concentrations, biovolume, membrane potential and membrane integrity) in rich and dilute media and at exponential and stationary phases for growth. The parameters selected have not been studied in  E. coli  and  S. aureus  under MRG conditions and provide critical information about bacterial viability and potential for population growth.    Results  With the exception of  S. aureus  in dilute Luria Bertani (LB) broth, specific growth rates (based on optical density) of the bacteria were not significantly different between normal gravity (NG) and MRG conditions. However, significantly higher bacterial yields were observed for both bacteria under MRG than NG, irrespective of the medium with the exception of  E. coli  grown in LB. Also, enumeration of cells after staining with 4',6-diamidino-2-phenylindole showed that significantly higher numbers were achieved under MRG conditions during stationary phase for  E. coli  and  S. aureus  grown in M9 and dilute LB, respectively. In addition, with the exception of smaller  S. aureus  volume under MRG conditions at exponential phase in dilute LB, biovolume and protein concentrations per cell did not significantly differ between MRG and NG treatments. Both  E. coli  and  S. aureus  had higher average membrane potential and integrity under MRG than NG conditions; however, these responses varied with growth medium and growth phase.    Conclusions  Overall, our data provides novel information about  E. coli  and  S. aureus  membrane potential and integrity and suggest that bacteria are physiologically more active and a larger percentage are viable under MRG as compared to NG conditions. In addition, these results demonstrate that bacterial physiological responses to MRG conditions vary with growth medium and growth phase demonstrating that nutrient resources are a modulator of response.

Effect of modeled reduced gravity conditions on bacterial morphology and physiology