High density of intracellular macromolecules creates a special condition known as macromolecular crowding (MC). One well-established consequence of MC is that only a slight change in the concentration of macromolecules (e.g., proteins) results in a shift of chemical equilibria towards the formation of macromolecular complexes and oligomers. This suggests a physiological mechanism of converting cell density changes into cellular responses. In this review, we start by providing a general overview of MC; then we examine the available experimental evidence that MC may act as a direct signaling factor in several types of cellular activities: mechano- and osmosensing, cell volume recovery in anisosmotic solutions, and apoptotic shrinkage. The latter phenomenon is analyzed in particular detail, as persistent shrinkage is known both to cause apoptosis and to occur during apoptosis resulting from other stimuli. We point to specific apoptotic reactions that involve formation of macromolecular complexes and, therefore, may provide a link between shrinkage and downstream responses.

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.