Quantitative High-Resolution Imaging of Live Microbial Cells at High Hydrostatic Pressure

The majority of the Earth's microbial biomass exists in the deep biosphere, in the deep ocean, and within the Earth's crust. Although other physical parameters in these environments, such as temperature or pH, can differ substantially, they are all under high pressures. Beyond emerging genomic information, little is known about the molecular mechanisms underlying the ability of these organisms to survive and grow at pressures that can reach over 1000-fold the pressure on the Earth's surface. The mechanisms of pressure adaptation are also important in food safety, with the increasing use of high-pressure food processing. Advanced imaging represents an important tool for exploring microbial adaptation and response to environmental changes. Here, we describe implementation of a high-pressure sample chamber with a two-photon scanning microscope system, allowing for the first time, to our knowledge, quantitative high-resolution two-photon imaging at 100 MPa of living microbes from all three kingdoms of life. We adapted this setup for fluorescence lifetime imaging microscopy with phasor analysis (FLIM/Phasor) and investigated metabolic responses to pressure of live cells from mesophilic yeast and bacterial strains, as well as the piezophilic archaeon Archaeoglobus fulgidus. We also monitored by fluorescence intensity fluctuation-based methods (scanning number and brightness and raster scanning imaging correlation spectroscopy) the effect of pressure on the chromosome-associated protein HU and on the ParB partition protein in Escherichia coli, revealing partially reversible dissociation of ParB foci and concomitant nucleoid condensation. These results provide a proof of principle that quantitative, high-resolution imaging of live microbial cells can be carried out at pressures equivalent to those in the deepest ocean trenches.