Robust surface-to-mass coupling and turgor-dependent cell width determine bacterial dry-mass density

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 non-steady conditions. Furthermore, the regulation of dry-mass density is fundamentally not understood in any organism. Using quantitative phase microscopy and a new 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 a new surface growth law. At the same time, cell width is controlled independently. Therefore, cellular dry-mass densi...