Figure 2. An osmotic force extension assay measures total cell-envelope stiffness.
A) (top) Diagram of a Gram-negative bacterial cell inflated with turgor pressure, P. OM: outer membrane, CW: cell wall, PM: plasma membrane, Cin: cytosolic osmolarity, Cout: osmolarity of the growth medium. (bottom) Turgor pressure is proportional to the difference between the cytosolic and growth medium osmolarities, where RT is the gas constant and T is the temperature. B) Osmolarity of growth medium versus time during an osmotic-force extension experiment. C) Cell-envelope length during an osmotic-force extension experiment. D) Mechanical strain in cell length versus shock magnitude. The dotted line is the best fit using linear regression. Cell envelope stiffness, kenv, is calculated as the inverse of the slope of the regression. E) A pool of color-coded E. coli cells.