Cell size control in bacteria

Abstract

Like eukaryotes, bacteria must coordinate division with growth to ensure cells are the appropriate size for a given environmental condition or developmental fate. As single-celled organisms, nutrient availability is one of the strongest influences on bacterial cell size. Classic physiological experiments conducted over four decades ago first demonstrated that cell size is directly correlated with nutrient source and growth rate in the Gram-negative bacterium Salmonella typhimurium. This observation subsequently served as the basis for studies revealing a role for cell size in cell cycle progression in a closely related organism, Escherichia coli. More recently, the development of powerful genetic, molecular, and imaging tools has allowed us to identify and characterize the nutrient-dependent pathway responsible for coordinating cell division and cell size with growth rate in the Gram-positive model organism Bacillus subtilis. Here, we discuss the role of cell size in bacterial growth and development and propose a broadly applicable model for cell size control in this important and highly divergent domain of life.

Figure 1. The bacterial division cycle

FtsZ assembly is coordinated with DNA replication and segregation. A. (1) In newborn cells, FtsZ (red) is unassembled. A circular chromosome (blue) with a single origin of replication (green) is located at mid-cell. (2–4) After chromosome replication initiates, the origins of replication separate and move to opposite poles of the cell. Once replication is complete, the condensed chromosomes separate, leaving a nucleoid free space. (3) FtsZ ring formation coincides with chromosome segregation. Assembly starts from a single point at mid-cell and extends bidirectionally. (5) During cytokinesis, the FtsZ ring constricts at the leading edge of the invaginating membrane. B. Immunofluorescence micrograph of exponentially growing B. subtilis cells with FtsZ rings. Arrows indicate examples of cells with rings. Bar = 5μm.

Figure 2. Transient changes in the length of the cell cycle are required for cell size homeostasis under steady state conditions

B. subtilis and E. coli cells exhibit little variation in cell size beyond the requirements of binary fission during steady state growth. Individual cells that are born too short transiently increase the length of their cell cycle to increase size while cells that are too long experience a transient reduction in the length of their cell cycle to reduce the daughter cell size. At the time of division (red rings), the size of all three cells is the same resulting in the production of appropriately sized daughter cells.

Figure 3. A carbon-dependent inhibitor of cell division

A. Carbon source has the largest impact on the size of E. coli and B. subtilis cells at division. A rich carbon source, depicted here as an ice cream sundae, ensures cells are large enough to accommodate extra DNA generated by multifork replication via carbon-dependent inhibition of cell division (red rings). Conversely, a poor carbon source, depicted here as carrots, has little effect on cell division, resulting in a reduction in average cell size. UDP-glc, purple, serves as the intracellular proxy for carbon and is thus at a higher concentration in cells cultured in carbon-rich conditions than in carbon-poor conditions. B. The glucosyltransferase UgtP inhibits division in a carbon-dependent manner. (Top) In a rich carbon source, high intracellular concentrations of UDP-glc stimulate UgtP (green) localization to mid-cell where it interacts directly with FtsZ to inhibit assembly and/or maturation of the FtsZ ring and increase cell size. (Bottom) During growth in a poor carbon source, UgtP expression levels are reduced and the remaining protein is sequestered in randomly positioned foci, permitting division to proceed unimpeded and reducing average cell size.

Figure 4. Developmentally regulated changes in division site selection establish cell type specific gene expression during sporulation in B. subtilis.

FtsZ (red), DNA (blue), origin of replication (green). Activation of the transcription factor Spo0A in response to nutrient limitation and cell crowding induces expression of genes, including spoIIE, required for relocalization of FtsZ from mid-cell to both poles via a spiral intermediate. Through a stochastic process, one FtsZ ring is used for polar septation while the other one is disassembled in response to the onset of mother cell-specific gene expression. (Bottom inset) Activation of forespore-specific gene expression is controlled in part via transient genetic asymmetry. The asymmetrically positioned septum bisects the chromosome, such that only the origin proximal ~30% is in the forespore immediately following septation. Forespore-specific gene expression immediately following septation is thus limited to origin proximal loci until the remainder of the chromosome is pumped into the forespore through the actions of the DNA translocase, SpoIIIE.

Figure 5. The concentration of assembly-competent FtsZ dictates cell size at division

A. (Left) Cytoplasmic FtsZ concentration (dark pink background) is constant throughout the cell cycle, however the total amount of FtsZ increases with cell size. Growth-dependent accumulation of FtsZ to critical levels dictates cell size at division. (Right) Reducing the intracellular concentration of assembly-competent FtsZ (light pink background) increases the size at which cells accumulate sufficient FtsZ to support division. B. Graphic model for the growth rate-dependent control of cell size. Asterisks mark the initiation of constriction in response to accumulation of FtsZ to critical levels. In the slower growing cell, on the left, assembly-competent FtsZ accumulates in direct proportion to cell size, reaching critical levels near the end of the cell cycle and stimulating maturation of the FtsZ ring and division. The faster growing cell, on the right, is born larger than its slow growing counterpart due to the presence of a growth-dependent inhibitor of FtsZ assembly (green). Due to the presence of the inhibitor, the faster growing cell is significantly larger when sufficient assembly-competent FtsZ has accumulated to stimulate maturation of the FtsZ ring and division. For simplicity in both A and B we have depicted FtsZ accumulation dictating maturation of the FtsZ ring and constriction. In the absence of data to suggest otherwise, it is equally likely that the rate-limiting step in cell division is the initiation of FtsZ ring formation. For clarity, we have also drawn the graph such that the newborn cell cultured under conditions supporting rapid growth is larger than the slow growing cell at division. In reality the sizes of these two cells likely overlap to some degree even when the difference in growth rates are at its most extreme.