Preparations of purified peptidoglycan ofEscherichia coli (i.e., sacculi) were studied by low-angle laser light scattering. Control experiments and theoretical calculations based on the Rayleigh-Gans theory showed that the mean sacculus surface area could be accurately inferred from measurements with our apparatus by using computer routines developed previously. Large changes in the mean saccular surface area resulted from alterations in the stress caused by varying the net charge on the sacculi. The net charge was affected by altering the suspending medium pH, causing carboxyl and amino groups in the peptidoglycan to gain or lose protons, or by acetylation or succinylation of the amino groups. A preponderance of either plus or minus charges caused an expansion of the mean sacculus surface area. The largest increase in area probably represents the elastic limit of the peptidoglycan and was 300%o above the area of isoionic sacculi. This degree of expansion is consistent with possible conformations of the intact peptidoglycan structure without necessitating rupture of the wall fabric. Our findings concerning saccular elasticity provide support for the surface stress theory. It provides a mechanism so that bacteria can grow and divide while maintaining turgor pressure, without the necessity of having and using proteins to do the mechanical work.It is now well established that the peptidoglycan sacculus is the stress-resistant, shape-determining structure covering the procaryotic cell (4, 23, 27-29, 34, 37, 42, 45, 46, 49, 63, 72). Its covalent structure makes the sacculus the largest macromolecule in nature and imparts strength to the bacterial wall. Additionally, the ability of the sacculus to stretch is of fundamental importance, both for giving flexibility to the organism and providing a mechanism for its enlargement during growth. The elasticity, as measured by Young's modulus, is the inverse of the ratio that a body is stretched (strain) by a given force (stress) (71). At high-enough stress, a body's elastic limit is reached, and it cannot be stretched further without the rupture of integral covalent bonds. Elastic enlargement of a single cell's wall fabric, composed of cross-linked peptidoglycan, depends on conformational changes that involve only rotations and easily made rearrangements of hydrogen bonds. Enlargement exceeding the elasticity must involve the rupture (hydrolysis in an aqueous environment) of covalent bonds (37,40).Sacculi are too small to have either their elasticity or their rupture limit (extensibility) measured by engineering materials testing methods (71). Although the sacculus cannot be affixed at two ends and mechanically stretched, the relationship between the sacculus net charge and surface area can be determined. We studied sacculi of Escherichia coli from cultures in balanced exponential growth. They were prepared free of almost all nonpeptidoglycan materials. The mean surface area of the sacculi under different conditions was measured by a low-angle laser-light-scattering method. ...