Cell size varies greatly among different types of cells, but the range in size that a specific cell type can reach is limited. A longstanding question in biology is how cells control their size. Escherichia coli adjusts size and growth rate according to the availability of nutrients so that it grows larger and faster in nutrient-rich media than in nutrient-poor media. Here, we describe how, using classical genetics, we have isolated a remarkably small E. coli mutant that has undergone a 70% reduction in cell volume with respect to wild type. This mutant lacks FabH, an enzyme involved in fatty acid biosynthesis that previously was thought to be essential for the viability of E. coli. We demonstrate that although FabH is not essential in wild-type E. coli, it is essential in cells that are defective in the production of the small-molecule and global regulator ppGpp. Furthermore, we have found that the loss of FabH causes a reduction in the rate of envelope growth and renders cells unable to regulate cell size properly in response to nutrient excess. Therefore we propose a model in which fatty acid biosynthesis plays a central role in regulating the size of E. coli cells in response to nutrient availability.acteria regulate their size and growth rate in response to nutrient availability. For example, Escherichia coli, Salmonella typhimurium, and Bacillus subtilis cells grow larger and faster in nutrient-rich medium than in nutrient-poor medium (1-6). Changes in temperature can alter growth rate but not size (2). Therefore, the size a cell attains depends on the nutritional composition of the growth medium, suggesting that nutrients affect a rate-limiting step(s) that controls size and the rate of growth.Bacteria must coordinate cell size, growth rate, and division in response to nutrient availability. Indeed, when E. coli changes its size, it also changes its generation time inversely; however, it maintains the cell mass-to-DNA ratio constant because it initiates DNA replication whenever it reaches a particular cell mass or a multiple of that mass (6). Interestingly, recent studies have shown that B. subtilis and E. coli use different regulatory mechanisms to couple cell size and DNA replication (4, 7). In E. coli DNA replication is not initiated until the cell reaches an appropriate size, but size does not affect the timing of replication in B. subtilis. Nevertheless, the amount of active DnaA, which unwinds DNA at the origin and thereby triggers replication (8, 9), is relevant in controlling the initiation of replication in both bacteria (7). Furthermore, a metabolic pathway for glucolipid biosynthesis regulates cell size in B. subtilis in response to nutrients under conditions that promote rapid growth (4). In this pathway, the UDP-glucose transferase UgtP inhibits the assembly of the divisome, the division machinery. The levels and localization of UgtP vary with nutrient availability so that assembly of the divisome is delayed under nutrient-rich conditions, resulting in longer cells.We do not understand how nu...

Summary The bacterial cell wall is conserved in prokaryotes, stabilizing cells against osmotic stress. Beta-lactams inhibit cell wall synthesis and induce lysis through a bulge-mediated mechanism; however, little is known about the formation dynamics and stability of these bulges. To capture processes of different timescales, we developed an imaging platform combining automated image analysis with live cell microscopy at high time resolution. Beta-lactam killing of Escherichia coli cells proceeded through four stages: elongation, bulge formation, bulge stagnation and lysis. Both the cell wall and outer membrane (OM) affect the observed dynamics; damaging the cell wall with different beta-lactams and compromising OM integrity cause different modes and rates of lysis. Our results show that the bulge formation dynamics is determined by how the cell wall is perturbed. The OM plays an independent role in stabilizing the bulge once it is formed. The stabilized bulge delays lysis, and allows recovery upon drug removal.

␤-lactam antibiotics inhibit penicillin binding proteins (PBPs) involved in peptidoglycan synthesis. Although inhibition of peptidoglycan biosynthesis is generally thought to induce cell lysis, the pattern and mechanism of cell lysis can vary substantially. ␤-lactams that inhibit FtsI, the only division specific PBP, block cell division and result in growth as filaments. These filaments ultimately lyse through a poorly understood mechanism. Here we find that one such ␤-lactam, cephalexin, can, under certain conditions, lead instead to rapid lysis at nascent division sites through a process that requires the complete and ordered assembly of the divisome, the essential machinery involved in cell division. We propose that this assembly process (in which the localization of cell wall hydrolases depends on properly targeted FtsN, which in turn depends on the presence of FtsI) ensures that the biosynthetic machinery to form new septa is in place before the machinery to degrade septated daughter cells is enabled. ␤-lactams that target FtsI subvert this mechanism by inhibiting FtsI without perturbing the normal assembly of the cell division machinery and the consequent activation of cell wall hydrolases. One seemingly paradoxical implication of our results is that ␤-lactam therapy may be improved by promoting active cell division.amidase ͉ cell wall hydrolase ͉ ftsN ͉ penicillin-binding proteins ͉ peptidoglycan

How cells control their shape and size is a long-standing question in cell biology. Many rod-shaped bacteria elongate their sidewalls by the action of cell wall synthesizing machineries that are associated to actin-like MreB cortical patches. However, little is known about how elongation is regulated to enable varied growth rates and sizes. Here we use total internal reflection fluorescence microscopy and single-particle tracking to visualize MreB isoforms, as a proxy for cell wall synthesis, in Bacillus subtilis and Escherichia coli cells growing in different media and during nutrient upshift. We find that these two model organisms appear to use orthogonal strategies to adapt to growth regime variations: B. subtilis regulates MreB patch speed, while E. coli may mainly regulate the production capacity of MreB-associated cell wall machineries. We present numerical models that link MreB-mediated sidewall synthesis and cell elongation, and argue that the distinct regulatory mechanism employed might reflect the different cell wall integrity constraints in Gram-positive and Gram-negative bacteria.