Motion to form a quorum
Motion to form a quorum
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DOI：
10.1126/science.1079805
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发表时间：
2003-07-11
期刊：
影响因子：
56.9
通讯作者：
Austin, RH
中科院分区：
文献类型：
--
作者：
Park, S;Wolanin, PM;Austin, RH
细菌基因的表达经常受到分泌到周围介质中的小分子的调节。这些自诱导剂随着细胞数量的增加而增加，直到达到临界种群密度或“法定数量”，此时细胞产生反应，如毒力或生物膜的形成，需要大量个体的协调活动。一般认为，群体的形成主要是由于有利于生长到高细胞密度的条件(1)。大量研究表明，运动促进了生物膜的形成，但这种依赖性归因于细胞从散装介质到表面的随机运输(2)。这样的实验是在光滑的表面上进行的，这些表面没有首选的表面位置或流动室，在那里引诱剂化学物质的梯度将被分散。在这里，我们表明，给定适当的表面拓扑结构，细菌可以使用趋化性结合并形成quorum。因此，趋化性提供了建立群体依赖相互作用所需的高局部细胞密度的重要机制。在富培养基或最低培养基中培养至中等密度（约2 108个细胞/ml）的大肠杆菌培养物，均匀地填充由硅弹性体制成的小中心外壳（250 m × 250 m）的微流体室（7 mm × 3 mm × 30 m）。在1至3小时的过程中（取决于培养基），细胞通过狭窄的（40米）通道从腔室迁移到中央外壳（图1A）。这种行为在具有运动性但缺乏趋化性的突变株中没有观察到。细胞聚集在笼子里，是因为它们分泌的氨基酸（如甘氨酸）是化学引诱剂，它们相互吸引。随着时间的推移，我们通过分析生长培养基的游离氨基酸含量来检测这种分泌物。丝氨酸受体Tsr是大肠杆菌中最丰富的趋化受体，而天冬氨酸受体Tar的含量也相对较高(3)。氧化还原感应的趋化性受体，Aer，存在于低水平，但仍然有效地调节趋化性。Tsr结合l -丝氨酸的亲和力最高，但它也结合l -丙氨酸、l -半胱氨酸和甘氨酸(4)。而缺失tsr的菌株无法在封闭体中积累，而tar或aer缺失的影响很小或没有影响。此外，加入饱和水平的l -丝氨酸（0.5 mM），它有效地与甘氨酸竞争，完全阻止了野生型细胞的积累。l -天冬氨酸饱和浓度没有影响。在调节自我吸引中最重要的特定氨基酸似乎取决于营养耗尽前的生长条件。先前有研究表明，在琥珀酸中生长的大肠杆菌分泌天冬氨酸，天冬氨酸通过焦油作用，使细胞在软琼脂中聚集成密集的菌落(5)。趋化性通常被认为是细胞扩散的一种机制。然而，在营养匮乏的环境中，细胞本身成为引诱剂分子的来源。趋化系统调节其敏感性，使其能够对极低浓度的引诱性化学物质作出反应，从而增强了细胞分泌的氨基酸的运动(6)。高局部细胞密度的积累可能提供诸如增强遗传交换或抗生素的共同降解以及群体依赖行为的优势。细胞聚集的位置取决于周围环境的几何形状。在有机硅弹性体形成的渗透晶格中的细胞结果表明，大肠杆菌倾向于在…
Bacterial gene expression is frequently regulated by small molecules secreted into the surrounding medium. These autoinducers build up with increasing cell number until a critical population density or “quorum” is achieved, at which point the cells produce a response such as virulence or biofilm formation that requires the coordinated activity of large numbers of individuals. It has generally been assumed that quorum formation derives primarily from conditions favorable for growth to high cell density (1). Numerous studies have shown that motility promotes biofilm formation, but this dependence has been attributed to random transport of cells from the bulk medium to a surface (2). Such experiments have been conducted with smooth surfaces that have no preferred surface sites or flow chambers where gradients of attractant chemicals will be dispersed. Here we show that, given appropriate surface topologies, bacteria can use chemotaxis to associate and form a quorum. Thus, chemotaxis provides an important mechanism for establishing the high local cell densities required for quorum-dependent interactions. A culture of Escherichia coli grown to moderate density (approximately 2 108 cells/ml) in either rich media or in minimal media was used to uniformly fill a microfluidic chamber (7 mm by 3 mm by 30 m) with a small central enclosure (250 m by 250 m) constructed from silicone elastomer. Over the course of 1 to 3 hours (depending on the media), the cells migrate from the chamber into the central enclosure through a narrow (40 m) channel (Fig. 1A). This behavior is not observed with a mutant strain that is motile but deficient in chemotaxis. Cells accumulate in the enclosure because they are attracted to each other due to their secretion of amino acids, such as glycine, that are chemoattractants. We detected this secretion by analysis of the free amino acid content of the growth media over time. The serine receptor, Tsr, is the most abundant chemotaxis receptor in E. coli, whereas the aspartate receptor, Tar, is also present at relatively high levels (3). The redox-sensing aerotaxis receptor, Aer, is present at low levels but still effectively modulates chemotaxis. Tsr binds L-serine with the highest affinity, but it also binds L-alanine, L-cysteine, and glycine (4). Whereas strains with tsr deleted were unable to accumulate in the enclosure, tar or aer deletion had little or no effect. Moreover, addition of saturating levels of L-serine (0.5 mM), which effectively competes with glycine, completely blocked accumulation of wild-type cells. Saturating concentrations of L-aspartate had no effect. The particular amino acid that is most important in mediating self attraction seems to depend on the conditions of growth before nutrient depletion. It has previously been shown that E. coli grown in succinate secrete aspartate, which acts through Tar to cause cells to associate into dense colonies in soft agar (5). Chemotaxis has generally been considered as a mechanism for cell dispersal. In nutrient-depleted environments, however, the cells themselves become sources of attractant molecules. Movement toward the amino acids secreted by the cells is enhanced by the ability of the chemotaxis system to adjust its sensitivity so that it can respond to very low concentrations of attractant chemicals (6). Accumulation of a high local density of cells may offer advantages such as enhanced genetic exchange or communal degradation of antibiotics, as well as the enabling of quorum-dependent behaviors. The sites at which cells accumulate depend on the geometry of their surroundings. Results with cells in percolated lattices formed from silicone elastomer indicate that E. coli tend to accumulate in …