Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, Nancy Université, CNRS UMR7564, Villers-lès-Nancy, France.
PLoS One. 2011;6(5):e20066. doi: 10.1371/journal.pone.0020066. Epub 2011 May 31.
The physicochemical properties and dynamics of bacterial envelope, play a major role in bacterial activity. In this study, the morphological, nanomechanical and electrohydrodynamic properties of Escherichia coli K-12 mutant cells were thoroughly investigated as a function of bulk medium ionic strength using atomic force microscopy (AFM) and electrokinetics (electrophoresis). Bacteria were differing according to genetic alterations controlling the production of different surface appendages (short and rigid Ag43 adhesins, longer and more flexible type 1 fimbriae and F pilus). From the analysis of the spatially resolved force curves, it is shown that cells elasticity and turgor pressure are not only depending on bulk salt concentration but also on the presence/absence and nature of surface appendage. In 1 mM KNO(3), cells without appendages or cells surrounded by Ag43 exhibit large Young moduli and turgor pressures (∼700-900 kPa and ∼100-300 kPa respectively). Under similar ionic strength condition, a dramatic ∼50% to ∼70% decrease of these nanomechanical parameters was evidenced for cells with appendages. Qualitatively, such dependence of nanomechanical behavior on surface organization remains when increasing medium salt content to 100 mM, even though, quantitatively, differences are marked to a much smaller extent. Additionally, for a given surface appendage, the magnitude of the nanomechanical parameters decreases significantly when increasing bulk salt concentration. This effect is ascribed to a bacterial exoosmotic water loss resulting in a combined contraction of bacterial cytoplasm together with an electrostatically-driven shrinkage of the surface appendages. The former process is demonstrated upon AFM analysis, while the latter, inaccessible upon AFM imaging, is inferred from electrophoretic data interpreted according to advanced soft particle electrokinetic theory. Altogether, AFM and electrokinetic results clearly demonstrate the intimate relationship between structure/flexibility and charge of bacterial envelope and propensity of bacterium and surface appendages to contract under hypertonic conditions.
细菌包膜的物理化学性质和动力学特性对细菌活性起着重要作用。在这项研究中，我们使用原子力显微镜（AFM）和电动（电泳）技术，深入研究了大肠杆菌 K-12 突变细胞的形态、纳米力学和电动特性，作为体相介质离子强度的函数。细菌根据控制不同表面附属物（短而刚性的 Ag43 黏附素、更长而更灵活的 I 型菌毛和 F 菌毛）产生的遗传改变而有所不同。从空间分辨力曲线的分析可以看出，细胞弹性和膨压不仅取决于体相盐浓度，还取决于表面附属物的存在/不存在和性质。在 1mM KNO3 中，没有附属物的细胞或被 Ag43 包围的细胞表现出较大的杨氏模量和膨压（分别约为 700-900kPa 和 100-300kPa）。在类似的离子强度条件下，具有附属物的细胞的这些纳米力学参数显著降低了约 50%至约 70%。定性地说，这种纳米力学行为对表面组织的依赖性在将介质盐含量增加到 100mM 时仍然存在，尽管在定量方面，差异要小得多。此外，对于给定的表面附属物，当增加体相盐浓度时，纳米力学参数的幅度显著降低。这种效应归因于细菌的外向渗透失水，导致细菌细胞质的共同收缩以及表面附属物的静电驱动收缩。前者过程通过 AFM 分析得到证明，而后者由于 AFM 成像不可访问，根据先进的软粒子电动理论解释的电泳数据推断。总之，AFM 和电动结果清楚地表明了细菌包膜的结构/灵活性和电荷与细菌和表面附属物在高渗条件下收缩的倾向之间的密切关系。
