Atomic force microscopy was used to measure the thickness of air-dried, collapsed murein sacculi from Escherichia coliK-12 and Pseudomonas aeruginosa PAO1. Air-dried sacculi from E. coli had a thickness of 3.0 nm, whereas those fromP. aeruginosa were 1.5 nm thick. When rehydrated, the sacculi of both bacteria swelled to double their anhydrous thickness. Computer simulation of a section of a model single-layer peptidoglycan network in an aqueous solution with a Debye shielding length of 0.3 nm gave a mass distribution full width at half height of 2.4 nm, in essential agreement with these results. When E. colisacculi were suspended over a narrow groove that had been etched into a silicon surface and the tip of the atomic force microscope used to depress and stretch the peptidoglycan, an elastic modulus of 2.5 × 107 N/m2 was determined for hydrated sacculi; they were perfectly elastic, springing back to their original position when the tip was removed. Dried sacculi were more rigid with a modulus of 3 × 108 to 4 × 108N/m2 and at times could be broken by the atomic force microscope tip. Sacculi aligned over the groove with their long axis at right angles to the channel axis were more deformable than those with their long axis parallel to the groove axis, as would be expected if the peptidoglycan strands in the sacculus were oriented at right angles to the long cell axis of this gram-negative rod. Polar caps were not found to be more rigid structures but collapsed to the same thickness as the cylindrical portions of the sacculi. The elasticity of intactE. coli sacculi is such that, if the peptidoglycan strands are aligned in unison, the interstrand spacing should increase by 12% with every 1 atm increase in (turgor) pressure. Assuming an unstressed hydrated interstrand spacing of 1.3 nm (R. E. Burge, A. G. Fowler, and D. A. Reaveley, J. Mol. Biol. 117:927–953, 1977) and an internal turgor pressure of 3 to 5 atm (or 304 to 507 kPa) (A. L. Koch, Adv. Microbial Physiol. 24:301–366, 1983), the natural interstrand spacing in cells would be 1.6 to 2.0 nm. Clearly, if large macromolecules of a diameter greater than these spacings are secreted through this layer, the local ordering of the peptidoglycan must somehow be disrupted.

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The topography of lipopolysaccharide (LPS) bilayers as well as the topography and elasticity of membrane vesicles (MV) of Pseudomonas aeruginosa Gram-negative bacteria have been investigated with the atomic force microscope. The LPS bilayer thickness was consistent with stereochemical estimates of the lipid A and core regions of the LPS layer. From measurements on MVs, a thickness of 7.5 nm for the outer membrane was determined. Thickness measurements on dried and hydrated MVs further showed that varying amounts of periplasmic molecular components are encapsulated by MVs. When spherical MVs in liquid attach to a surface, they distort into the shape of a lens. Force spectroscopy on such MVs showed a springlike elastic response. This suggests that the MV envelope is under tension. Analysis of the spring constants in terms of a simple model gave MV osmotic pressures on the order of atmospheres.