During growth, cells must expand their cell volumes in coordination with biomass to control the level of cytoplasmic macromolecular crowding. Dry-mass density, the average ratio of dry mass to volume, is roughly constant between different nutrient conditions in bacteria, but it remains unknown whether cells maintain dry-mass density constant at the single-cell level and during nonsteady conditions. Furthermore, the regulation of dry-mass density is fundamentally not understood in any organism. Using quantitative phase microscopy and an advanced image-analysis pipeline, we measured absolute single-cell mass and shape of the model organisms Escherichia coli and Caulobacter crescentus with improved precision and accuracy. We found that cells control dry-mass density indirectly by expanding their surface, rather than volume, in direct proportion to biomass growth—according to an empirical surface growth law. At the same time, cell width is controlled independently. Therefore, cellular dry-mass density varies systematically with cell shape, both during the cell cycle or after nutrient shifts, while the surface-to-mass ratio remains nearly constant on the generation time scale. Transient deviations from constancy during nutrient shifts can be reconciled with turgor-pressure variations and the resulting elastic changes in surface area. Finally, we find that plastic changes of cell width after nutrient shifts are likely driven by turgor variations, demonstrating an important regulatory role of mechanical forces for width regulation. In conclusion, turgor-dependent cell width and a slowly varying surface-to-mass coupling constant are the independent variables that determine dry-mass density.

在生长过程中，细胞必须与生物量协调扩大其细胞体积，以控制细胞质大分子拥挤的水平。干重密度，即干重与体积的平均比值，在细菌的不同营养条​​件下大致恒定，但细胞是否在单细胞水平和非稳态条件下保持干重密度恒定仍然未知。此外，在任何生物体中根本不了解干物质密度的调节。使用定量相位显微镜和先进的图像分析管道，我们测量了模式生物大肠杆菌和新月柄杆菌的绝对单细胞质量和形状提高精度和准确度。我们发现，根据经验表面生长定律，细胞通过与生物量增长成正比扩大其表面而不是体积来间接控制干重密度。同时，单元宽度是独立控制的。因此，细胞干质量密度随细胞形状系统地变化，无论是在细胞周期期间还是在营养变化之后，而表面质量比在世代时间尺度上几乎保持不变。养分变化过程中与稳定性的瞬时偏差可以与膨胀压力变化和由此产生的表面积弹性变化相协调。最后，我们发现营养变化后细胞宽度的可塑性变化可能是由膨胀变化驱动的，这表明机械力对宽度调节的重要调节作用。