Direct measurement of turgor pressure in E. coli and B. subtilis

The ability to maintain turgor pressure, i.e. osmotic imbalance, across the cell envelope, is a requirement for

proper metabolism and growth in walled microbial cells. To date, turgor pressure has only been estimated or

indirectly measured, because direct access has been difficult due to small cell sizes. How turgor pressure

depends on external conditions such as osmolarity and nutrient content remains largely unknown. Here we

present a micromanipulation technique using an atomic force microscope (AFM) that allows us to directly

measure and track turgor pressure on a living bacterium. We compress single cells between a large bead and a

flat surface using an AFM cantilever. Measured forces and indentation depths are then used to determine turgor

pressure with the help of a mechanical model that describes the deformation of the cell. We report dependencies

on external osmolarities for E. coli and B. subtilis.