Shrinkage of growing Escherichia coli cells by osmotic challenge

The immediate response of growing Escherichia coli to changing external osmotic pressure was studied with stopped-flow turbidimetric measurements with a narrow-beam spectrophotometer. It is shown theoretically that in such a photometer rod-shaped bacteria have an apparent absorbance which is proportional to the inverse of the surface area. The apparent optical density, corrected for effects of alteration of the index of refraction of the medium, increased continuously as the external osmotic pressure was raised. Because of the short time scale of the measurements, the turbidity increases could result either from shrinkage of the cells or from plasmolysis, or both, but not from growth or metabolic adaptation. With low concentrations of pentose such that the external osmotic pressure was not greater than that inside the cells, plasmolysis would not occur and, consequently, only shrinkage of the previously stretched sacculus remains to account for the observed optical effects. Taking the osmotic pressure of the growing cells as 5 atmospheres (506 kPa), the turbidity changes correspond to the murein fabric having been stretched 20% beyond its unstressed equilibrium area during growth under the conditions used.