Dusenbery D B
School of Biology, Georgia Institute of Technology, Atlanta, Georgia 30332-0230, USA.
J Bacteriol. 1998 Nov;180(22):5978-83. doi: 10.1128/JB.180.22.5978-5983.1998.
Data on the shapes of 218 genera of free-floating or free-swimming bacteria reveal groupings around spherical shapes and around rod-like shapes of axial ratio about 3. Motile genera are less likely to be spherical and have larger axial ratios than nonmotile genera. The effects of shape on seven possible components of biological fitness were determined, and actual fitness landscapes in phenotype space are presented. Ellipsoidal shapes were used as models, since their hydrodynamic drag coefficients can be rigorously calculated in the world of low Reynolds number, where bacteria live. Comparing various shapes of the same volume, and assuming that departures from spherical have a cost that varies with the minimum radius of curvature, led to the following conclusions. Spherical shapes have the largest random dispersal by Brownian motion. Increased surface area occurs in oblate ellipsoids (disk-like), which rarely occur. Elongation into prolate ellipsoids (rod-like) reduces sinking speed, and this may explain why some nonmotile genera are rod-like. Elongation also favors swimming efficiency (to a limited extent) and the ability to detect stimulus gradients by any of three mechanisms. By far the largest effect (several hundred-fold) is on temporal detection of stimulus gradients, and this explains why rod-like shapes and this mechanism of chemotaxis are common.
关于218个自由漂浮或自由游动细菌属形状的数据显示，这些细菌围绕球形以及轴比约为3的杆状形成了不同的分组。与非运动型属相比，运动型属呈球形的可能性较小，且轴比更大。研究确定了形状对生物适应性七个可能组成部分的影响，并展示了表型空间中的实际适应性景观。由于椭球体的流体动力学阻力系数可以在细菌所处的低雷诺数环境中进行精确计算，因此使用椭球体形状作为模型。比较相同体积的各种形状，并假设偏离球形会带来随最小曲率半径变化的代价，得出了以下结论。球形通过布朗运动具有最大的随机扩散。扁椭球体（盘状）表面积增加，但这种形状很少出现。伸长为长椭球体（杆状）会降低下沉速度，这可能解释了为什么一些非运动型属是杆状的。伸长还（在一定程度上）有利于游泳效率以及通过三种机制中的任何一种检测刺激梯度的能力。到目前为止，最大的影响（数百倍）体现在对刺激梯度的时间检测上，这就解释了为什么杆状形状和这种趋化机制很常见。
