Directly measuring turgor pressure in bacterial cells: effects of osmolality of growth and nutrient conditions*

Bacteria generally maintain high turgor pressures (~1-20 atm). Gram positive bacteria typically have higher turgor pressures than gram negative bacteria. Maintaining turgor requires specialized machineries that balance the cytoplasmic osmolyte concentration against the embedding medium. Details of this machinery as well as the roles of the components of the multilayer cell wall are not well understood. Directly measuring turgor has been an experimental challenge, and a rapid, precise method has been lacking. We here present an experimental method based on force spectroscopy using an atomic force microscope to quantify turgor and its variations at the single cell level in vivo. We measured turgor of different gram-negative and gram-positive bacteria as well as the effect of different growth conditions and nutrient concentrations on turgor pressure.