Cellular response of Escherichia coli to microgravity and microgravity analogue culture

Abstract

Spaceflight and spaceflight-analogue studies reveal that microgravity or aspects of the microgravity environment impact the physiology, stress resistance, and molecular biology of microbes. Escherichia coli is an ideal model bacterium for microgravity and microgravity analogue investigations, as it is the most well-characterized organism, and has pathogenic strains that could cause infections during spaceflight. It is also a key player in ecosystems critical for sustaining life in space, such as renewal of resources like oxygen and water, as well as waste recycling. This chapter reviews the current literature on the effect of microgravity and modeled microgravity systems on E. coli physiology and molecular biology, with implications for risk assessment and astronaut health as well as beneficial applications for the general public. Spaceflight experiments suggest that culture in the microgravity environment alters E. coli growth parameters and antibiotic resistance. Studies on Earth under simulated microgravity, referred to as low shear modeled microgravity (LSMMG), indicate that LSMMG affects gene expression, protein synthesis, secondary metabolite production, cellular protein composition, and biofilm formation in this bacterium. An important outcome is that E. coli grown under LSMMG becomes markedly more resistant to antimicrobials like ethanol, salt, and low pH, as well as to some antibiotics, like gentamicin. The increased resistance is dependent on the elevated levels of the general stress response regulator, Sigma S (RpoS), under LSMMG conditions. Significantly, the mechanism underlying this increase in Sigma S implies that LSMMG may alter the folding pattern of macromolecules. The increased general resistance under LSMMG and its dependence on Sigma S indicate that E. coli perceives LSMMG as a stress, and responds to it in a manner similar to that observed for stresses experienced under normal gravity conditions. However, whether these responses are due to a direct effect of microgravity remains to be determined. Contradictory findings with respect to several of the characteristics mentioned above for E. coli cultured under microgravity and LSMMG have been reported. These findings emphasize the need for a thorough examination of the effect of microgravity on the virulence and stress resistance of bacteria in various stages of growth and development, so as to counter the potential health hazards of space travel that bacteria may pose, for better exploitation in establishing sustainable space habitats, and to gain knowledge in how bacteria cause disease on Earth.