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

Cells must balance volume expansion with biomass accumulation to manage macromolecular crowding. Dry-mass density remains roughly constant across nutrient conditions in bacteria, but whether this holds at the single-cell level is unclear. Prior research has shown that macromolecular crowding affects cellular function, but the mechanisms regulating dry-mass density are unknown. No prior work had resolved how cells maintain this property during nonsteady growth. This gap motivated the use of new imaging tools to measure mass and shape at the single-cell level. Existing studies lack precision in tracking surface and volume changes simultaneously. This gap motivated the development of an advanced image-analysis pipeline. The uncertainty around how surface and volume scale with biomass led to the need for empirical growth laws. This gap motivated the investigation of surface-to-mass coupling and turgor effects.

The aim was to determine how bacteria regulate dry-mass density at the single-cell level. This paper tests whether dry-mass density remains constant during growth and nutrient shifts. The specific problem is understanding how cells maintain this property despite changing conditions. The motivation comes from the lack of known regulatory mechanisms in any organism. The study focuses on surface-to-mass coupling as a potential mechanism. The goal is to measure mass and shape with high precision. The purpose is to identify independent variables that determine dry-mass density. The study seeks to clarify how surface and width changes affect macromolecular crowding.

Quantitative phase microscopy was used to measure absolute mass and shape in single cells. An advanced image-analysis pipeline increased precision and accuracy of measurements. The model organisms Escherichia coli and Caulobacter crescentus were studied. Surface area and biomass growth were tracked during the cell cycle. Nutrient shifts were induced to observe changes in dry-mass density. Turgor-pressure variations were analyzed alongside elastic surface changes. Cell width was measured independently of surface expansion. The empirical surface growth law was derived from these data.

Cells control dry-mass density by expanding surface area proportionally to biomass growth. Surface-to-mass ratio remains nearly constant across generation times. Cell width is regulated independently of surface expansion. Dry-mass density varies systematically with cell shape changes. Transient deviations occur during nutrient shifts due to turgor-pressure changes. Elastic surface area changes correlate with these deviations. Plastic changes in cell width after nutrient shifts suggest turgor-driven regulation. These findings support a model where surface and width determine dry-mass density.

Dry-mass density is determined by turgor-dependent cell width and surface-to-mass coupling. Surface-to-mass ratio remains constant over time despite shape changes. Cell width is regulated independently, affecting dry-mass density. Turgor variations drive plastic changes in width after nutrient shifts. These findings suggest mechanical forces play a regulatory role. The empirical surface growth law explains how cells maintain density. This model applies during the cell cycle and after nutrient shifts. The authors propose that surface and width are independent variables in density regulation.