Biphasic growth dynamics duringCaulobacter crescentusdivision

Abstract

1 Cell size is specific to each species and impacts their ability to function. While 1 various phenomenological models for cell size regulation have been proposed, recent 2 work in bacteria have demonstrated an adder mechanism, in which a cell increments 3 its size by a constant amount between each division. However, the coupling between 4 cell size, shape and constriction, remain poorly understood. Here, we investigate size 5 control and the cell cycle dependence of bacterial growth, using multigenerational cell 6 growth and shape data for single Caulobacter crescentus cells. Our analysis reveals 7 a biphasic growth mechanism: a relative timer phase before constriction where cell 8 growth is correlated to its initial size, followed by a pure adder phase during constric-9 tion. Cell wall labeling measurements reinforce this biphasic behavior: a crossover 10 from uniform lateral growth to localized septal growth is observed. We develop a 11 mathematical model that quantitatively explains this mixer mechanism for size con-12 trol. 13 14 We recently introduced a technology that enables obtaining unprecedented amounts of precise 15 quantitative information about the shapes of single bacteria as they grow and divide under non-16 crowding and controllable environmental conditions [1, 2]. Others have developed complementary 17 methods [3-6]. These single-cell studies are generating great interest because they reveal unantic-18 ipated relationships between cell size and division control [5]. Recent work in bacteria that utilize 19 these technologies revealed that a constant size increment between successive generations [7] quan-20 titatively describes the strategy for bacterial size maintenance in E. coli [3-5], B. subtilis [5], C. 21 crescentus [4], P. aeruginosa [8] and even in the yeast S. cerevisiae [9]. This phenomenological ob-22 servation has been termed an adder model for cell size control. Competing models for size control 23 include cell division at a critical size (sizer model) [10] or at a constant interdivision time (timer 24 model) [1]. Analysis of single-cell data show that cell size at division is positively correlated with 25the cell size at birth [1, 4, 5, 11, 12], thus precluding a sizer model. In addition, a negative corre-26 lation between initial cell size and interdivision times, as reported here and in refs [1, 4, 5, 12, 13], 27 is inconsistent with the timer model. However, other studies have suggested mixed models of 28 size control, with diverse combinations of sizer, timer and adder mechanisms [14][15][16][17]. The spatial 29 resolution and statistically large size of our data now allow us to revisit these issues with greater 30 precision. 31While cell size serves as an important determinant of growth, the bacterial cell cycle is composed 32 of various coupled processes including DNA replication and cell wall constriction that have to be 33 2 faithfully coordinated for cells to successfully divide [18]. This raises the question of what other 34 cell cycle variables regulate growth and how the ...

Analysis of Noise Mechanisms in Cell-Size Control

“…In many organisms like Caulobacter crescentus, size control is exerted in only a part of the cell cycle (36,37). More specifically, the cell cycle can be divided into two phases.…”

“…It is worth pointing out that the motivation behind separating the cell-size control into size-based (a) and timebased control ðf n Þ is to study the mixer model (such as that observed in C. crescentus) (36). Although there is some equivalence between the mixer model and the linear map considering a and D n as random variables, the latter is unable to distinguish the biphasic control (Supporting Material, Section S3).…”

Conditions for Cell Size Homeostasis: A Stochastic Hybrid System Approach

“…A key question is how cells regulate their growth and timing of division to ensure that they do not get abnormally large (or small). This problem has ben referred to literature as size homeostasis and is a vigorous area of current experimental research in diverse organisms [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. We investigate if phenomenological models of cell size dynamics based on SHS can provide insights into the control mechanisms needed for size homeostasis.…”

Shapeshifting to Survive: Shape Determination and Regulation in Caulobacter crescentus

“…Two concepts have gained experimental support recently for regulating Caulobacter cell size and shape: (i) length homeostasis through “adder” or “mixer” mechanisms [101,102] and (ii) maintenance of a constant surface area to volume ratio under a given physiological condition [103,104]. The adder principle states that bacterial cells, including Caulobacter , achieve cell length homeostasis by growing a relatively constant amount prior to each cell division, regardless of length at birth [101].…”

“…This idea is supported by time-lapse observation of the growth of thousands of cells, and contradicts earlier models such as the “sizer” model in which cells grow to a particular cell length threshold before dividing. A related model has been recently proposed, positing that Caulobacter cells grow according to a “mixer” mechanism (pre-print, [102]). Cells grow for a fixed fraction of their total cell cycle prior to initiating constriction (i.e.…”