Cell-Size Control and Homeostasis in Bacteria

Abstract

SUMMARY How cells control their size and maintain size homeostasis is a fundamental open question. Cell-size homeostasis has been discussed in the context of two major paradigms: sizer, in which the cell actively monitors its size and triggers the cell cycle once it reaches a critical size, and timer, in which the cell attempts to grow for a specific amount of time before division. These paradigms, in conjunction with the “growth law” [1] and the quantitative bacterial cell cycle model [2], inspired numerous theoretical models [3-9] and experimental investigations from growth [10,11] to cell cycle and size control [12–15]. However, experimental evidence involved difficult-to-verify assumptions or population-averaged data, which allowed different interpretations [1–5,16–20] or limited conclusions [4–9]. In particular, population-averaged data and correlations are inconclusive as the averaging process masks causal effects at the cellular level. In this work, we extended a microfluidic “mother machine” [21] and monitored hundreds of thousands of Gram-negative Escherichia coli and Gram-positive Bacillus subtilis cells under a wide range of steady-state growth conditions. Our combined experimental results and quantitative analysis demonstrate that cells add a constant volume each generation irrespective of their newborn sizes, conclusively supporting the so-called constant Δ model. This model was introduced for E. coli [6,7] and recently revisited [9], but experimental evidence was limited to correlations. This “adder” principle quantitatively explains experimental data at both the population and single-cell levels, including the origin and the hierarchy of variability in the size-control mechanisms, and how cells maintain size homeostasis.

Characterization of dependencies between growth and division in budding yeast

“…An important distinction between the current study and previous analyses is the measurement of cell size. In our datasets, cell size was measured via a fluorescent protein-based proxy for cell mass, whereas recent work in bacteria and budding yeast has focused on cell volume [6,[11][12][13]. Elements of cell volume in budding yeast, particularly the vacuoles, are known to undergo dynamic, regulated changes over the course of the cell cycle [39].…”

“…Assuming a consistent single-cell growth rate across cells, these tenets are sufficient to maintain a consistent size distribution in the population. Recent studies in bacteria have revealed an alternative size control model by which cells add a relatively constant amount of volume over the cell cycle, regardless of their birth size [11][12][13]. Here we use time-lapse microscopy datasets tracking characteristics of individual cells to test these models and further characterize coordination between growth and division in budding yeast.…”

Deletion of a mycobacterial divisome factor collapses single-cell phenotypic heterogeneity

“…We found that the amount of variation in wild type Msm cells (19%) is much higher than Δ lamA Msm cells (12%) (Fig. 4e) and other rod-shaped bacteria, 16, 17 including Corynebacterium glutamicum (Extended Data Fig. 8), a close relative of mycobacteria with similar cell wall architecture and polar elongation but that does not encode LamA.…”

Mitochondrial heterogeneity, metabolic scaling and cell death

“…We used a stochastic hybrid systems approach [52] to simulate a population of exponentially growing cells undergoing 'adder' dynamics [50,51], whereby cells add a constant amount of volume (K v ) to their initial volume (v 0 ) each cell cycle, which has found recent support in bacteria [53]. Cells are modelled to then divide by stochastically partitioning their volume, with the first daughter inheriting volume and Bj€ orklund as a non-heritable parabolic relationship between mitochondrial functionality (which we interpret as rDC) and cell volume v…”