Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
Biochemistry. 2012 Oct 16;51(41):8143-53. doi: 10.1021/bi3012115. Epub 2012 Oct 2.
The bacterial cell wall is essential to cell survival and is a major target of antibiotics. The main component of the bacterial cell wall is peptidoglycan, a cage-like macromolecule that preserves cellular integrity and maintains cell shape. The insolubility and heterogeneity of peptidoglycan pose a challenge to conventional structural analyses. Here we use solid-state NMR combined with specific isotopic labeling to probe a key structural feature of the Staphylococcus aureus peptidoglycan quantitatively and nondestructively. We observed that both the cell-wall morphology and the peptidoglycan structure are functions of growth stage in S. aureus synthetic medium (SASM). Specifically, S. aureus cells at stationary phase have thicker cell walls with nonuniformly thickened septa compared to cells in exponential phase, and remarkably, 12% (±2%) of the stems in their peptidoglycan do not have pentaglycine bridges attached. Mechanistically, we determined that these observations are triggered by the depletion of glycine in the nutrient medium, which is coincident with the start of the stationary phase, and that the production of the structurally altered peptidoglycan can be prevented by the addition of excess glycine. We also demonstrated that the structural changes primarily arise within newly synthesized peptidoglycan rather than through the modification of previously synthesized peptidoglycan. Collectively, our observations emphasize the plasticity in bacterial cell-wall assembly and the possibility to manipulate peptidoglycan structure with external stimuli.
细菌细胞壁对于细胞存活至关重要，也是抗生素的主要靶标。细菌细胞壁的主要成分是肽聚糖，它是一种笼状的大分子，能保持细胞的完整性并维持细胞的形状。肽聚糖的不溶性和异质性对传统的结构分析构成了挑战。在这里，我们使用固态 NMR 结合特定的同位素标记，对金黄色葡萄球菌肽聚糖的一个关键结构特征进行了定量和非破坏性探测。我们观察到，在金黄色葡萄球菌合成培养基（SASM）中，细胞壁形态和肽聚糖结构都是生长阶段的函数。具体来说，与处于指数生长期的细胞相比，处于静止期的金黄色葡萄球菌细胞的细胞壁更厚，隔膜不均匀增厚，而且令人惊讶的是，它们的肽聚糖中有 12%（±2%）的茎没有连接五肽桥。从机制上讲，我们确定这些观察结果是由营养培养基中甘氨酸的耗尽引发的，这与静止期的开始时间一致，而且通过添加过量的甘氨酸可以防止结构改变的肽聚糖的产生。我们还证明，这些结构变化主要发生在新合成的肽聚糖中，而不是通过对先前合成的肽聚糖进行修饰。总的来说，我们的观察结果强调了细菌细胞壁组装的可塑性，以及通过外部刺激来操纵肽聚糖结构的可能性。
