Yao X, Jericho M, Pink D, Beveridge T
Department of Physics, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1.
J Bacteriol. 1999 Nov;181(22):6865-75. doi: 10.1128/JB.181.22.6865-6875.1999.
Atomic force microscopy was used to measure the thickness of air-dried, collapsed murein sacculi from Escherichia coli K-12 and Pseudomonas aeruginosa PAO1. Air-dried sacculi from E. coli had a thickness of 3.0 nm, whereas those from P. 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. coli sacculi 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 x 10(7) N/m(2) 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 x 10(8) to 4 x 10(8) N/m(2) 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 intact E. 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.
利用原子力显微镜测量了来自大肠杆菌K - 12和铜绿假单胞菌PAO1的空气干燥、塌陷的胞壁质囊泡的厚度。大肠杆菌的空气干燥囊泡厚度为3.0纳米，而铜绿假单胞菌的囊泡厚度为1.5纳米。重新水化后，两种细菌的囊泡都膨胀到其无水厚度的两倍。对德拜屏蔽长度为0.3纳米的水溶液中模型单层肽聚糖网络的一部分进行计算机模拟，得到半高全宽的质量分布为2.4纳米，与这些结果基本一致。当将大肠杆菌囊泡悬浮在蚀刻到硅表面的窄槽上方，并使用原子力显微镜的尖端下压和拉伸肽聚糖时，测得水合囊泡的弹性模量为2.5×10⁷ N/m²；它们具有完美的弹性，当尖端移除时会弹回到原始位置。干燥的囊泡更坚硬，模量为3×10⁸至4×10⁸ N/m²，有时会被原子力显微镜尖端破坏。长轴与通道轴成直角排列在槽上的囊泡比长轴与槽轴平行的囊泡更易变形，这与如果囊泡中的肽聚糖链与这种革兰氏阴性杆菌的长细胞轴成直角排列的预期相符。未发现极性帽是更坚硬的结构，而是塌陷到与囊泡圆柱形部分相同的厚度。完整大肠杆菌囊泡的弹性使得，如果肽聚糖链一致排列，随着（膨压）压力每增加1个大气压，链间间距应增加12%。假设无应力水合链间间距为1.3纳米（R. E. 伯奇、A. G. 福勒和D. A. 里夫利，《分子生物学杂志》117:927 - 953，1977），内部膨压为3至5个大气压（或304至507千帕）（A. L. 科赫，《微生物生理学进展》24:301 - 366，1983），细胞中的自然链间间距将为1.6至2.0纳米。显然，如果直径大于这些间距的大分子通过该层分泌，肽聚糖的局部有序性必须以某种方式被破坏。
