Bacterial Cell Size: Multifactorial and Multifaceted

Abstract

How cells establish, maintain, and modulate size has always been an area of great interest and fascination. Until recently, technical limitations curtailed our ability to understand the molecular basis of bacterial cell size control. In the past decade, advances in microfluidics, imaging, and high-throughput single-cell analysis, however, have led to a flurry of work revealing size to be a highly complex trait involving the integration of three core aspects of bacterial physiology: metabolism, growth, and cell cycle progression.

Keywords: cell cycle; cell size; homeostatic control; sizer; timer.

Figure 1. The addition of a constant volume of material in each generation ensures bacterial cell size homeostasis under steady state conditions

nstead of doubling in size each generation, bacteria add a constant amount of volume regardless of their size at birth. Cells that are “born” too small for stochastic reasons (red) add the same volume of material as “normal” cells (purple) and cells that are born too large (green). Over several generations, the so-called adder mechanism dampens size variants within a population. Δ refers to the amount of material added in each generation under a specific condition. The volume of Δ increases with nutrient availability. Model based on: (3, 16, 52, 92).

Figure 2. Cell size is mediated in part through changes in cell cycle progression

(A) The initiation of replication is dependent on achievement of a critical size E. coli and a critical age in B. subtilis. (Left) Wild type E. coli and B. subtilis cells initiate new rounds of DNA replication upon achievement of a critical mass. (Middle) A “sizer” mechanism governs replication initiation in E. coli. Short E. coli mutants delay initiation (halo) until they reach the same size as their wild type counterparts. The delay is compensated for by an up to 30% increase in the rate of replication fork elongation, permitting cells to divide “on time” (50, 101). (Right) A “timer” mediates replication initiation in B. subtilis. Short B. subtilis mutants initiate DNA replication at the same time post-birth as their wild type counterparts (50). (B) A time line of the bacterial cell cycle. Information about size or time can be integrated at any one of three major events: the initiation of DNA replication, assembly of the tubulin-like cell division protein FtsZ and other “early” cell division proteins at the nascent division site, and recruitment of the “late” cell division proteins and the initiation of cytokinesis.

Figure 3. Cell size is a product of biosynthetic capacity and cell cycle progression

(A) Cell size and composition at steady-state increase linearly with increases in nutrient availability and biosynthetic capacity. In phase contrast images on the bottom of the graph, E. coli cultured in nutrient rich LB + 0.2% glucose (left) are approximately 3-fold larger than the same strain cultured in nutrient poor AB minimal salts + 0.2% succinate. Importantly, the ratio of cell mass to DNA, RNA, and protein remains constant under all conditions [Modified from Fig. 1 in (87)]. (B) Cell size is a function of cell cycle progression and growth. Nutrient flux through central metabolism releases energy and generates building blocks for biosynthesis. On the left, metabolic products serve as signals activating modulators of cell cycle progression to tune cell size in response to changes in nutrient availability. On the right, biosynthetic capacity dictates growth rate, impacting cell size at division. Changes in mass impact the timing of certain cell cycle events via a “sizer” mechanism (Green arrow). Cell cycle dependent signals impacting growth are inferred in the absence of experimental data (Red arrow).