Biological consequences and advantages of asymmetric bacterial growth

Getting to the point: unipolar growth of Hyphomicrobiales

The governing principles and suites of genes for lateral elongation or incorporation of new cell wall material along the length of a rod-shaped cell are well described. In contrast, relatively little is known about unipolar elongation or incorporation of peptidoglycan at one end of the rod. Recent work in three related model systems of unipolar growth (Agrobacterium tumefaciens, Brucella abortus, and Sinorhizobium meliloti) has clearly established that unipolar growth in the Hyphomicrobiales order relies on a set of genes distinct from the canonical elongasome. Polar incorporation of envelope components relies on homologous proteins shared by the Hyphomicrobiales, reviewed here. Ongoing and future work will reveal how unipolar growth is integrated into the alphaproteobacterial cell cycle and coordinated with other processes such as chromosome segregation and cell division.

Regulation of the cell division hydrolase RipC by the FtsEX system in Mycobacterium tuberculosis

The FtsEX complex regulates, directly or via a protein mediator depending on bacterial genera, peptidoglycan degradation for cell division. In mycobacteria and Gram-positive bacteria, the FtsEX system directly activates peptidoglycan-hydrolases by a mechanism that remains unclear. Here we report our investigation of Mycobacterium tuberculosis FtsEX as a non-canonical regulator with high basal ATPase activity. The cryo-EM structures of the FtsEX system alone and in complex with RipC, as well as the ATP-activated state, unveil detailed information on the signal transduction mechanism, leading to the activation of RipC. Our findings indicate that RipC is recognized through a "Match and Fit" mechanism, resulting in an asymmetric rearrangement of the extracellular domains of FtsX and a unique inclined binding mode of RipC. This study provides insights into the molecular mechanisms of FtsEX and RipC regulation in the context of a critical human pathogen, guiding the design of drugs targeting peptidoglycan remodeling.

Interacting bactofilins impact cell shape of the MreB-less multicellular Rhodomicrobium vannielii

Most non-spherical bacteria rely on the actin-like MreB cytoskeleton to control synthesis of a cell-shaping and primarily rod-like cell wall. Diverging from simple rod shape generally requires accessory cytoskeletal elements, which locally interfere with the MreB-guided cell wall synthesis. Conserved and widespread representatives of this accessory cytoskeleton are bactofilins that polymerize into static, non-polar bundles of filaments. Intriguingly, many species of the Actinobacteria and Rhizobiales manage to grow rod-like without MreB by tip extension, yet some of them still possess bactofilin genes, whose function in cell morphogenesis is unknown. An intricate representative of these tip-growing bacteria is Rhodomicrobium vannielii; a member of the hitherto genetically not tractable and poorly studied Hyphomicrobiaceae within the MreB-less Rhizobiales order. R. vannielii displays complex asymmetric cell shapes and differentiation patterns including filamentous hyphae to produce offspring and to build dendritic multicellular arrays. Here, we introduce techniques to genetically access R. vannielii, and we elucidate the role of bactofilins in its sophisticated morphogenesis. By targeted mutagenesis and fluorescence microscopy, protein interaction studies and peptidoglycan incorporation analysis we show that the R. vannielii bactofilins are associated with the hyphal growth zones and that one of them is essential to form proper hyphae. Another paralog is suggested to represent a novel hybrid and co-polymerizing bactofilin. Notably, we present R. vannielii as a powerful new model to understand prokaryotic cell development and control of multipolar cell growth in the absence of the conserved cytoskeletal element, MreB.

The Effects of Ventilation, Humidity, and Temperature on Bacterial Growth and Bacterial Genera Distribution

BACKGROUND

Bacteria are readily nourished in airtight environments with high humidity, such as storage cabinets, clothing closets, and corners, where ventilation is normally low and humidity is high.

OBJECTIVES

We characterized the role of humidity and ventilation in bacterial growth and genus distribution at different temperatures (26 °C and 34 °C).

METHODS

Fresh pork, which was used as the substrate for bacterial culture, was placed in storage cabinets. Bacterial growth and genera distribution on the surface of pork placed in a storage cabinet under different temperatures (26 °C and 34 °C); relative humidity levels (RH: 50%, 70%, 90%); and ventilation conditions (no ventilation and low, medium, and high levels of ventilation) were assessed by rDNA sequencing.

RESULTS

Increased ventilation and reduced humidity significantly decreased bacterial growth at 26 °C and 34 °C. The contribution of increased ventilation to the reduction in bacterial growth exceeded that of decreased humidity. Ventilation had the greatest effect on reducing bacterial growth compared to the unventilated conditions at 70% RH. At 34 °C, medium and high levels of ventilation were required to reduce bacterial growth. High temperatures greatly increased bacterial growth, but ventilation could reduce the degree of this increase.

Visualization of a Limonene Synthesis Metabolon Inside Living Bacteria by Hyperspectral SRS Microscopy

Monitoring biosynthesis activity at single-cell level is key to metabolic engineering but is still difficult to achieve in a label-free manner. Using hyperspectral stimulated Raman scattering imaging in the 670-900 cm^-1 region, localized limonene synthesis are visualized inside engineered Escherichia coli. The colocalization of limonene and GFP-fused limonene synthase is confirmed by co-registered stimulated Raman scattering and two-photon fluorescence images. The finding suggests a limonene synthesis metabolon with a polar distribution inside the cells. This finding expands the knowledge of de novo limonene biosynthesis in engineered bacteria and highlights the potential of SRS chemical imaging in metabolic engineering research.

Engineering Escherichia coli asymmetry distribution-based synthetic consortium for shikimate production

Microbial consortia constitute a promising tool for achieving high-value chemical bio-production. However, customizing the consortium ratio remains challenging. Herein, an asymmetry distribution-based synthetic consortium (ADSC) was developed to switch cell phenotypes using shikimate synthesis for proof of concept. First, the cell pole-organizing protein PopZ was screened as a mediator of asymmetric protein distribution in Escherichia coli. The ADSC was then constructed to incorporate PopZ-mediated asymmetry distribution and a TetR-based transcription repression switch to achieve the dynamical control of microbial population ratio. Finally, the ADSC was used to decouple cell growth from shikimate synthesis by effectively coordinating the ratio of growing cells and production cells at the consortium level, thereby increasing shikimate titer to 30.1 g/L in the 7.5-L bioreactor with a minimal medium. This titer was further improved to 82.5 g/L when using rich medium fermentation. Our results illustrate a novel approach to control consortium structure through ADSC-mediated regulation, highlighting its potential as an efficient strategy for controlling metabolic state in microbes. This article is protected by copyright. All rights reserved.

Unipolar Peptidoglycan Synthesis in the Rhizobiales Requires an Essential Class A Penicillin-Binding Protein

Members of the Rhizobiales are polarly growing bacteria that lack homologs of the canonical Rod complex. To investigate the mechanisms underlying polar cell wall synthesis, we systematically probed the function of cell wall synthesis enzymes in the plant pathogen Agrobacterium tumefaciens. The development of fluorescent d-amino acid dipeptide (FDAAD) probes, which are incorporated into peptidoglycan by penicillin-binding proteins in A. tumefaciens, enabled us to monitor changes in growth patterns in the mutants. Use of these fluorescent cell wall probes and peptidoglycan compositional analysis demonstrate that a single class A penicillin-binding protein is essential for polar peptidoglycan synthesis. Furthermore, we find evidence of an additional mode of cell wall synthesis that requires ld-transpeptidase activity. Genetic analysis and cell wall targeting antibiotics reveal that the mechanism of unipolar growth is conserved in Sinorhizobium and Brucella. This work provides insights into unipolar peptidoglycan biosynthesis employed by the Rhizobiales during cell elongation.

Label-Free Probing of Electron Transfer Kinetics of Single Microbial Cells on a Single-Layer Graphene via Structural Color Microscopy

Studies of electron transfer at the population level veil the nature of the cell itself; however, in situ probing of the electron transfer dynamics of individual cells is still challenging. Here we propose label-free structural color microscopy for this aim. We demonstrate that Shewanella oneidensis MR-1 cells show unique structural color scattering, changing with the redox state of cytochrome complexes in the outer membrane. It enables quantitatively and noninvasive studies of electron transfer in single microbial cells during bioelectrochemical activities, such as extracellular electron transfer (EET) on a transparent single-layer graphene electrode. Increasing the applied potential leads to the associated EET current, accompanied by more oxidized cytochromes. The high spatiotemporal resolution of the proposed method not only demonstrates the large diversity in EET activity among microbial cells but also reveals the subcellular asymmetric distribution of active cytochromes in a single cell. We anticipate that it provides a potential platform for further exploring the electron transfer mechanism of subcellular structure.

Stable inheritance of Sinorhizobium meliloti cell growth polarity requires an FtsN-like protein and an amidase

In Rhizobiales bacteria, such as Sinorhizobium meliloti, cell elongation takes place only at new cell poles, generated by cell division. Here, we show that the role of the FtsN-like protein RgsS in S. meliloti extends beyond cell division. RgsS contains a conserved SPOR domain known to bind amidase-processed peptidoglycan. This part of RgsS and peptidoglycan amidase AmiC are crucial for reliable selection of the new cell pole as cell elongation zone. Absence of these components increases mobility of RgsS molecules, as well as abnormal RgsS accumulation and positioning of the growth zone at the old cell pole in about one third of the cells. These cells with inverted growth polarity are able to complete the cell cycle but show partially impaired chromosome segregation. We propose that amidase-processed peptidoglycan provides a landmark for RgsS to generate cell polarity in unipolarly growing Rhizobiales.

In Sinorhizobium bacteria, cell elongation takes place only at new cell poles, generated by cell division. Here, Krol et al. show that an FtsN-like protein and a peptidoglycan amidase are crucial for reliable selection of the new cell pole as cell elongation zone.

PLK-1 Regulation of Asymmetric Cell Division in the Early C. elegans Embryo

PLK1 is a conserved mitotic kinase that is essential for the entry into and progression through mitosis. In addition to its canonical mitotic functions, recent studies have characterized a critical role for PLK-1 in regulating the polarization and asymmetric division of the one-cell C. elegans embryo. Prior to cell division, PLK-1 regulates both the polarization of the PAR proteins at the cell cortex and the segregation of cell fate determinants in the cytoplasm. Following cell division, PLK-1 is preferentially inherited to one daughter cell where it acts to regulate the timing of centrosome separation and cell division. PLK1 also regulates cell polarity in asymmetrically dividing Drosophila neuroblasts and during mammalian planar cell polarity, suggesting it may act broadly to connect cell polarity and cell cycle mechanisms.

Generating asymmetry in a changing environment: cell cycle regulation in dimorphic alphaproteobacteria

While many bacteria divide by symmetric binary fission, some alphaproteobacteria have strikingly asymmetric cell cycles, producing offspring that differs significantly in their morphology and reproductive state. To establish this asymmetry, these species employ a complex cell cycle regulatory pathway based on two-component signaling cascades. At the center of this network is the essential DNA-binding response regulator CtrA, which acts as a transcription factor controlling numerous genes with cell cycle-relevant functions as well as a regulator of chromosome replication. The DNA-binding activity of CtrA is controlled at the level of both protein phosphorylation and stability, dependent on an intricate network of regulatory proteins, whose function is tightly coordinated in time and space. CtrA is differentially activated in the two (developing) offspring, thereby establishing distinct transcriptional programs that ultimately determine their distinct cell fates. Phase-separated polar microdomains of changing composition sequester proteins involved in the (in-)activation and degradation of CtrA specifically at each pole. In this review, we summarize the current knowledge of the CtrA pathway and discuss how it has evolved to regulate the cell cycle of morphologically distinct alphaproteobacteria.

Alternative systems for misfolded protein clearance: life beyond the proteasome

Protein misfolding is a major driver of ageing-associated frailty and disease pathology. Although all cells possess multiple, well-characterised protein quality control systems to mitigate the toxicity of misfolded proteins, how they are integrated to maintain protein homeostasis ('proteostasis') in health-and how their disintegration contributes to disease-is still an exciting and fast-paced area of research. Under physiological conditions, the predominant route for misfolded protein clearance involves ubiquitylation and proteasome-mediated degradation. When the capacity of this route is overwhelmed-as happens during conditions of acute environmental stress, or chronic ageing-related decline-alternative routes for protein quality control are activated. In this review, we summarise our current understanding of how proteasome-targeted misfolded proteins are retrafficked to alternative protein quality control routes such as juxta-nuclear sequestration and selective autophagy when the ubiquitin-proteasome system is compromised. We also discuss the molecular determinants of these alternative protein quality control systems, attempt to clarify distinctions between various cytoplasmic spatial quality control inclusion bodies (e.g., Q-bodies, p62 bodies, JUNQ, aggresomes, and aggresome-like induced structures 'ALIS'), and speculate on emerging concepts in the field that we hope will spur future research-with the potential to benefit the rational development of healthy ageing strategies.

Stochastic models coupling gene expression and partitioning in cell division in Escherichia coli

Regulation of future RNA and protein numbers is a key process by which cells continuously best fit the environment. In bacteria, RNA and proteins exist in small numbers and their regulatory processes are stochastic. Consequently, there is cell-to-cell variability in these numbers, even between sister cells. Traditionally, the two most studied sources of this variability are gene expression and RNA and protein degradation, with evidence suggesting that the latter is subject to little regulation, when compared to the former. However, time-lapse microscopy and single molecule fluorescent tagging have produced evidence that cell division can also be a significant source of variability due to asymmetries in the partitioning of RNA and proteins. Relevantly, the impact of this noise differs from noise in production and degradation since, unlike these, it is not continuous. Rather, it occurs at specific time points, at which moment it can introduce major fluctuations. Several models have now been proposed that integrate noise from cell division, in addition to noise in gene expression, to mimic the dynamics of RNA and protein numbers of cell lineages. This is expected to be particularly relevant in genetic circuits, where significant fluctuations in one component protein, at specific time moments, are expected to perturb near-equilibrium states of the circuits, which can have long-lasting consequences. Here we review stochastic models coupling these processes in Escherichia coli, from single genes to small circuits.

A biphasic growth model for cell pole elongation in mycobacteria

Mycobacteria grow by inserting new cell wall material in discrete zones at the cell poles. This pattern implies that polar growth zones must be assembled de novo at each division, but the mechanisms that control the initiation of new pole growth are unknown. Here, we combine time-lapse optical and atomic force microscopy to measure single-cell pole growth in mycobacteria with nanometer-scale precision. We show that single-cell growth is biphasic due to a lag phase of variable duration before the new pole transitions from slow to fast growth. This transition and cell division are independent events. The difference between the lag and interdivision times determines the degree of single-cell growth asymmetry, which is high in fast-growing species and low in slow-growing species. We propose a biphasic growth model that is distinct from previous unipolar and bipolar models and resembles “new end take off” (NETO) dynamics of polar growth in fission yeast.

Mycobacteria grow by inserting new cell wall material at the cell poles. Here, Hannebelle et al. combine time-lapse optical and atomic force microscopy to show that single-cell growth is biphasic due to a lag phase of variable duration before the new pole transitions from slow to fast growth.

Bacterial ageing in the absence of external stressors

Evidence of ageing in the bacterium Escherichia coli was a landmark finding in senescence research, as it suggested that even organisms with morphologically symmetrical fission may have evolved strategies to permit damage accumulation. However, recent work has suggested that ageing is only detectable in this organism in the presence of extrinsic stressors, such as the fluorescent proteins and strong light sources typically used to excite them. Here we combine microfluidics with brightfield microscopy to provide evidence of ageing in E. coli in the absence of these stressors. We report (i) that the doubling time of the lineage of cells that consistently inherits the 'maternal old pole' progressively increases with successive rounds of cell division until it reaches an apparent asymptote, and (ii) that the parental cell divides asymmetrically, with the old pole daughter showing a longer doubling time and slower glucose accumulation than the new pole daughter. Notably, these patterns arise without the progressive accumulation or asymmetric partitioning of observable misfolded-protein aggregates, phenomena previously hypothesized to cause the ageing phenotype. Our findings suggest that ageing is part of the naturally occurring ecologically-relevant phenotype of this bacterium and highlight the importance of alternative mechanisms of damage accumulation in this context. This article is part of a discussion meeting issue 'Single cell ecology'.

¡vIVA la DivIVA!

Reproduction in the bacterial kingdom predominantly occurs through binary fission-a process in which one parental cell is divided into two similarly sized daughter cells. How cell division, in conjunction with cell elongation and chromosome segregation, is orchestrated by a multitude of proteins has been an active area of research spanning the past few decades. Together, the monumental endeavors of multiple laboratories have identified several cell division and cell shape regulators as well as their underlying regulatory mechanisms in rod-shaped Escherichia coli and Bacillus subtilis, which serve as model organisms for Gram-negative and Gram-positive bacteria, respectively. Yet our understanding of bacterial cell division and morphology regulation is far from complete, especially in noncanonical and non-rod-shaped organisms. In this review, we focus on two proteins that are highly conserved in Gram-positive organisms, DivIVA and its homolog GpsB, and attempt to summarize the recent advances in this area of research and discuss their various roles in cell division, cell growth, and chromosome segregation in addition to their interactome and posttranslational regulation.

Microbial ageing and longevity

Longevity reflects the ability to maintain homeostatic conditions necessary for life as an organism ages. A long-lived organism must contend not only with environmental hazards but also with internal entropy and macromolecular damage that result in the loss of fitness during ageing, a phenomenon known as senescence. Although central to many of the core concepts in biology, ageing and longevity have primarily been investigated in sexually reproducing, multicellular organisms. However, growing evidence suggests that microorganisms undergo senescence, and can also exhibit extreme longevity. In this Review, we integrate theoretical and empirical insights to establish a unified perspective on senescence and longevity. We discuss the evolutionary origins, genetic mechanisms and functional consequences of microbial ageing. In addition to having biomedical implications, insights into microbial ageing shed light on the role of ageing in the origin of life and the upper limits to longevity.

Inducible asymmetric cell division and cell differentiation in a bacterium

Multicellular organisms achieve greater complexity through cell divisions that generate different cell types. We engineered a simple genetic circuit that induces asymmetric cell division and subsequent cell differentiation in Escherichia coli. The circuit involves a scaffolding protein, PopZ, that is stably maintained at a single cell pole over multiple asymmetric cell divisions. PopZ was functionalized to degrade the signaling molecule, c-di-GMP. By regulating synthesis of functionalized PopZ via small molecules or light, we can chemically or optogenetically control the relative abundance of two distinct cell types, characterized by either low or high c-di-GMP levels. Differences in c-di-GMP levels can be transformed into genetically programmable differences in protein complex assembly or gene expression, which in turn produce differential behavior or biosynthetic activities. This study shows emergence of complex biological phenomena from a simple genetic circuit and adds programmable bacterial cell differentiation to the genetic toolbox of synthetic biology and biotechnology.

GROWTH POLE RING protein forms a 200-nm-diameter ring structure essential for polar growth and rod shape in Agrobacterium tumefaciens

Polar growth in Agrobacterium pirates and repurposes well-known bacterial cell cycle proteins, such as FtsZ, FtsA, PopZ, and PodJ. Here we identify a heretofore unknown protein that we name GROWTH POLE RING (GPR) due to its striking localization as a hexameric ring at the growth pole during polar growth. GPR also localizes at the midcell late in the cell cycle just before division, where it is then poised to be precisely localized at new growth poles in sibling cells. GPR is 2,115 aa long, with two N-terminal transmembrane domains placing the bulk of the protein in the cytoplasm, N- and C-terminal proline-rich disordered regions, and a large 1,700-aa central region of continuous α-helical domains. This latter region contains 12 predicted adjacent or overlapping apolipoprotein domains that may function to sequester lipids during polar growth. Stable genetic deletion or riboswitch-controlled depletion results in spherical cells that grow poorly; thus, GPR is essential for wild-type growth and morphology. As GPR has no predicted enzymatic domains and it forms a distinct 200-nm-diameter ring, we propose that GPR is a structural component of an organizing center for peptidoglycan and membrane syntheses critical for cell envelope formation during polar growth. GPR homologs are found in numerous Rhizobiales; thus, our results and proposed model are fundamental to understanding polar growth strategy in a variety of bacterial species.

Asymmetric division events promote variability in cell cycle duration in animal cells and Escherichia coli

Asymmetric cell division is a major mechanism generating cell diversity. As cell cycle duration varies among cells in mammalian tissue culture cells, we asked whether their division asymmetry contributes to this variability. We identify among sibling cells an outlier using hierarchical clustering on cell cycle durations of granddaughter cells obtained by lineage tracking of single histone2B-labelled MDCKs. Remarkably, divisions involving outlier cells are not uniformly distributed in lineages, as shown by permutation tests, but appear to emerge from asymmetric divisions taking place at non-stochastic levels: a parent cell influences with 95% confidence and 0.5% error the unequal partitioning of the cell cycle duration in its two progenies. Upon ninein downregulation, this variability propagation is lost, and outlier frequency and variability in cell cycle durations in lineages is reduced. As external influences are not detectable, we propose that a cell-autonomous process, possibly involved in cell specialisation, determines cell cycle duration variability.

We know that variations in cell cycle duration between cells naturally occur but the mechanisms are largely unknown. Here, using lineage tracking, hierarchical clustering and Monte Carlo methods, the authors show that large differences in granddaughter cell cycle duration are driven by asymmetric divisions.

Reciprocal control of motility and biofilm formation by the PdhS2 two-component sensor kinase of Agrobacterium tumefaciens

A core regulatory pathway that directs developmental transitions and cellular asymmetries in Agrobacterium tumefaciens involves two overlapping, integrated phosphorelays. One of these phosphorelays putatively includes four histidine sensor kinase homologues, DivJ, PleC, PdhS1 and PdhS2, and two response regulators, DivK and PleD. In several different alphaproteobacteria, this pathway influences a conserved downstream phosphorelay that ultimately controls the phosphorylation state of the CtrA master response regulator. The PdhS2 sensor kinase reciprocally regulates biofilm formation and swimming motility. In the current study, the mechanisms by which the A. tumefaciens sensor kinase PdhS2 directs this regulation are delineated. PdhS2 lacking a key residue implicated in phosphatase activity is markedly deficient in proper control of attachment and motility phenotypes, whereas a kinase-deficient PdhS2 mutant is only modestly affected. A genetic interaction between DivK and PdhS2 is revealed, unmasking one of several connections between PdhS2-dependent phenotypes and transcriptional control by CtrA. Epistasis experiments suggest that PdhS2 may function independently of the CckA sensor kinase, the cognate sensor kinase for CtrA, which is inhibited by DivK. Global expression analysis of the pdhS2 mutant reveals a restricted regulon, most likely functioning through CtrA to separately control motility and regulate the levels of the intracellular signal cyclic diguanylate monophosphate (cdGMP), thereby affecting the production of adhesive polysaccharides and attachment. We hypothesize that in A. tumefaciens the CtrA regulatory circuit has expanded to include additional inputs through the addition of PdhS-type sensor kinases, likely fine-tuning the response of this organism to the soil microenvironment.

Effect of cardiolipin on the antimicrobial activity of a new amphiphilic aminoglycoside derivative on Pseudomonas aeruginosa

Amphiphilic aminoglycoside derivatives are promising new antibacterials active against Gram-negative bacteria such as Pseudomonas aeruginosa, including colistin resistant strains. In this study, we demonstrated that addition of cardiolipin to the culture medium delayed growth of P. aeruginosa, favored asymmetrical growth and enhanced the efficiency of a new amphiphilic aminoglycoside derivative, the 3’,6-dinonylneamine. By using membrane models mimicking P. aeruginosa plasma membrane composition (POPE:POPG:CL), we demonstrated the ability of 3’6-dinonylneamine to induce changes in the biophysical properties of membrane model lipid systems in a cardiolipin dependent manner. These changes include an increased membrane permeability associated with a reduced hydration and a decreased ability of membrane to mix and fuse as shown by monitoring calcein release, Generalized Polarization of Laurdan and fluorescence dequenching of octadecyl rhodamine B, respectively. Altogether, results shed light on how cardiolipin may be critical for improving antibacterial action of new amphiphilic aminoglycoside derivatives.

Mechanical Division of Cell-Sized Liposomes

Liposomes, self-assembled vesicles with a lipid-bilayer boundary similar to cell membranes, are extensively used in both fundamental and applied sciences. Manipulation of their physical properties, such as growth and division, may significantly expand their use as model systems in cellular and synthetic biology. Several approaches have been explored to controllably divide liposomes, such as shape transformation through temperature cycling, incorporation of additional lipids, and the encapsulation of protein division machinery. However, so far, these methods lacked control, exhibited low efficiency, and yielded asymmetric division in terms of volume or lipid composition. Here, we present a microfluidics-based strategy to realize mechanical division of cell-sized (∼6 μm) liposomes. We use octanol-assisted liposome assembly (OLA) to produce liposomes on chip, which are subsequently flowed against the sharp edge of a wedge-shaped splitter. Upon encountering such a Y-shaped bifurcation, the liposomes are deformed and, remarkably, are able to divide into two stable daughter liposomes in just a few milliseconds. The probability of successful division is found to critically depend on the surface area-to-volume ratio of the mother liposome, which can be tuned through osmotic pressure, and to strongly correlate to the mother liposome size for given microchannel dimensions. The division process is highly symmetric (∼3% size variation between the daughter liposomes) and is accompanied by a low leakage. This mechanical division of liposomes may constitute a valuable step to establish a growth-division cycle of synthetic cells.

Stable Regulation of Cell Cycle Events in Mycobacteria: Insights From Inherently Heterogeneous Bacterial Populations

Model bacteria, such as E. coli and B. subtilis, tightly regulate cell cycle progression to achieve consistent cell size distributions and replication dynamics. Many of the hallmark features of these model bacteria, including lateral cell wall elongation and symmetric growth and division, do not occur in mycobacteria. Instead, mycobacterial growth is characterized by asymmetric polar growth and division. This innate asymmetry creates unequal birth sizes and growth rates for daughter cells with each division, generating a phenotypically heterogeneous population. Although the asymmetric growth patterns of mycobacteria lead to a larger variation in birth size than typically seen in model bacterial populations, the cell size distribution is stable over time. Here, we review the cellular mechanisms of growth, division, and cell cycle progression in mycobacteria in the face of asymmetry and inherent heterogeneity. These processes coalesce to control cell size. Although Mycobacterium smegmatis and Mycobacterium bovis Bacillus Calmette-Guérin (BCG) utilize a novel model of cell size control, they are similar to previously studied bacteria in that initiation of DNA replication is a key checkpoint for cell division. We compare the regulation of DNA replication initiation and strategies used for cell size homeostasis in mycobacteria and model bacteria. Finally, we review the importance of cellular organization and chromosome segregation relating to the physiology of mycobacteria and consider how new frameworks could be applied across the wide spectrum of bacterial diversity.

Recent advances in understanding how rod-like bacteria stably maintain their cell shapes

Cell shape and cell volume are important for many bacterial functions. In recent years, we have seen a range of experimental and theoretical work that led to a better understanding of the determinants of cell shape and size. The roles of different molecular machineries for cell-wall expansion have been detailed and partially redefined, mechanical forces have been shown to influence cell shape, and new connections between metabolism and cell shape have been proposed. Yet the fundamental determinants of the different cellular dimensions remain to be identified. Here, we highlight some of the recent developments and focus on the determinants of rod-like cell shape and size in the well-studied model organisms Escherichia coli and Bacillus subtilis.

Cell Wall Biogenesis During Elongation and Division in the Plant Pathogen Agrobacterium tumefaciens

A great diversity of bacterial cell shapes can be found in nature, suggesting that cell wall biogenesis is regulated both spatially and temporally. Although Agrobacterium tumefaciens has a rod-shaped morphology, the mechanisms underlying cell growth are strikingly different than other well-studied rod-shaped bacteria including Escherichia coli. Technological advances, such as the ability to deplete essential genes and the development of fluorescent D-amino acids, have enabled recent advances in our understanding of cell wall biogenesis during cell elongation and division of A. tumefaciens. In this review, we address how the field has evolved over the years by providing a historical overview of cell elongation and division in rod-shaped bacteria. Next, we summarize the current understanding of cell growth and cell division processes in A. tumefaciens. Finally, we highlight the need for further research to answer key questions related to the regulation of cell wall biogenesis in A. tumefaciens.

The Molecular Basis of Noncanonical Bacterial Morphology

Bacteria come in a wide variety of shapes and sizes. The true picture of bacterial morphological diversity is likely skewed due to an experimental focus on pathogens and industrially relevant organisms. Indeed, most of the work elucidating the genes and molecular processes involved in maintaining bacterial morphology has been limited to rod- or coccal-shaped model systems. The mechanisms of shape evolution, the molecular processes underlying diverse shapes and growth modes, and how individual cells can dynamically modulate their shape are just beginning to be revealed. Here we discuss recent work aimed at advancing our knowledge of shape diversity and uncovering the molecular basis for shape generation in noncanonical and morphologically complex bacteria.

Distinct Spatiotemporal Dynamics of Peptidoglycan Synthesis between Mycobacterium smegmatis and Mycobacterium tuberculosis

Peptidoglycan (PG), a polymer cross-linked by d-amino acid-containing peptides, is an essential component of the bacterial cell wall. We found that a fluorescent d-alanine analog (FDAA) incorporates chiefly at one of the two poles in Mycobacterium smegmatis but that polar dominance varies as a function of the cell cycle in Mycobacterium tuberculosis: immediately after cytokinesis, FDAAs are incorporated chiefly at one of the two poles, but just before cytokinesis, FDAAs are incorporated comparably at both. These observations suggest that mycobacterial PG-synthesizing enzymes are localized in functional compartments at the poles and septum and that the capacity for PG synthesis matures at the new pole in M. tuberculosis Deeper knowledge of the biology of mycobacterial PG synthesis may help in discovering drugs that disable previously unappreciated steps in the process.IMPORTANCE People are dying all over the world because of the rise of antimicrobial resistance to medicines that could previously treat bacterial infections, including tuberculosis. Here, we used fluorescent d-alanine analogs (FDAAs) that incorporate into peptidoglycan (PG)-the synthesis of which is an attractive drug target-combined with high- and super-resolution microscopy to investigate the spatiotemporal dynamics of PG synthesis in M. smegmatis and M. tuberculosis FDAA incorporation predominates at one of the two poles in M. smegmatis In contrast, while FDAA incorporation into M. tuberculosis is also polar, there are striking variations in polar dominance as a function of the cell cycle. This suggests that enzymes involved in PG synthesis are localized in functional compartments in mycobacteria and that M. tuberculosis possesses a mechanism for maturation of the capacity for PG synthesis at the new pole. This may help in discovering drugs that cripple previously unappreciated steps in the process.

Aging and immortality in unicellular species

It has been historically thought that in conditions that permit growth, most unicellular species do not to age. This was particularly thought to be the case for symmetrically dividing species, as such species lack a clear distinction between the soma and the germline. Despite this, studies of the symmetrically dividing species Escherichia coli and Schizosaccharomyces pombe have recently started to challenge this notion. They indicate that E. coli and S. pombe do age, but only when subjected to environmental stress. If true, this suggests that aging may be widespread among microbial species in general, and that studying aging in microbes may inform other long-standing questions in aging. This review examines the recent evidence for and against replicative aging in symmetrically dividing unicellular organisms, the mechanisms that underlie aging, why aging evolved in these species, and how microbial aging fits into the context of other questions in aging.

Epigenetic Phosphorylation Control of Mycobacterium tuberculosis Infection and Persistence

Reversible protein phosphorylation is the most common type of epigenetic posttranslational modification in living cells used as a major regulation mechanism of biological processes. The Mycobacterium tuberculosis genome encodes for 11 serine/threonine protein kinases that are responsible for sensing environmental signals to coordinate a cellular response to ensure the pathogen's infectivity, survival, and growth. To overcome killing mechanisms generated within the host during infection, M. tuberculosis enters a state of nonreplicating persistence that is characterized by arrested growth, limited metabolic activity, and phenotypic resistance to antimycobacterial drugs. In this article we focus our attention on the role of M. tuberculosis serine/threonine protein kinases in sensing the host environment to coordinate the bacilli's physiology, including growth, cell wall components, and central metabolism, to establish a persistent infection.

Probing bacterial cell biology using image cytometry

Advances in automated fluorescence microscopy have made snapshot and time-lapse imaging of bacterial cells commonplace, yet fundamental challenges remain in analysis. The vast quantity of data collected in high-throughput experiments requires a fast and reliable automated method to analyze fluorescence intensity and localization, cell morphology and proliferation as well as other descriptors. Inspired by effective yet tractable methods of population-level analysis using flow cytometry, we have developed a framework and tools for facilitating analogous analyses in image cytometry. These tools can both visualize and gate (generate subpopulations) more than 70 cell descriptors, including cell size, age and fluorescence. The method is well suited to multi-well imaging, analysis of bacterial cultures with high cell density (thousands of cells per frame) and complete cell cycle imaging. We give a brief description of the analysis of four distinct applications to emphasize the broad applicability of the tool.

Building the bacterial cell wall at the pole

Polar growth is the predominant mode of cell wall extension in the Actinobacteria and the alphaproteobacterial clade Rhizobiales. The observation of polar elongation in taxonomically diverse bacteria suggests that polar growth may have evolved independently. Indeed, the regulatory mechanisms governing the assembly of cell wall biosynthesis machinery at the pole are distinct in the Actinobacteria and Rhizobiales. Here we highlight recent advances in our understanding of polar growth mechanisms in bacteria, with an emphasis on Streptomyces and Agrobacterium. This review illustrates that common themes are emerging in the regulation of polar growth in diverse bacteria. Emerging themes include the use of landmark proteins to direct growth to the pole and coordination of polar growth with cell-cycle progression.

Programmable, Pneumatically Actuated Microfluidic Device with an Integrated Nanochannel Array To Track Development of Individual Bacteria

We describe a microfluidic device with an integrated nanochannel array to trap individual bacteria and monitor growth and reproduction of lineages over multiple generations. Our poly(dimethylsiloxane) device comprises a pneumatically actuated nanochannel array that includes 1280 channels with widths from 600 to 1000 nm to actively trap diverse bacteria. Integrated pumps and valves perform on-chip fluid and cell manipulations that provide dynamic control of cell loading and nutrient flow, permitting chemostatic growth for extended periods of time (typically 12 to 20 h). Nanochannels confine bacterial growth to a single dimension, facilitating high-resolution, time-lapse imaging and tracking of individual cells. We use the device to monitor the growth of single bacterial cells that undergo symmetric (Bacillus subtilis) and asymmetric (Caulobacter crescentus) division and reconstruct their lineages to correlate growth measurements through time and among related cells. Furthermore, we monitor the motility state of single B. subtilis cells across multiple generations by the expression of a fluorescent reporter protein and observe that the state of the epigenetic switch is correlated over five generations. Our device allows imaging of cellular lineages with high spatiotemporal resolution to facilitate the analysis of biological processes spanning multiple generations.

Short-Stalked Prosthecomicrobium hirschii Cells Have a Caulobacter-Like Cell Cycle

The dimorphic alphaproteobacterium Prosthecomicrobium hirschii has both short-stalked and long-stalked morphotypes. Notably, these morphologies do not arise from transitions in a cell cycle. Instead, the maternal cell morphology is typically reproduced in daughter cells, which results in microcolonies of a single cell type. In this work, we further characterized the short-stalked cells and found that these cells have a Caulobacter-like life cycle in which cell division leads to the generation of two morphologically distinct daughter cells. Using a microfluidic device and total internal reflection fluorescence (TIRF) microscopy, we observed that motile short-stalked cells attach to a surface by means of a polar adhesin. Cells attached at their poles elongate and ultimately release motile daughter cells. Robust biofilm growth occurs in the microfluidic device, enabling the collection of synchronous motile cells and downstream analysis of cell growth and attachment. Analysis of a draft P. hirschii genome sequence indicates the presence of CtrA-dependent cell cycle regulation. This characterization of P. hirschii will enable future studies on the mechanisms underlying complex morphologies and polymorphic cell cycles.

IMPORTANCE

Bacterial cell shape plays a critical role in regulating important behaviors, such as attachment to surfaces, motility, predation, and cellular differentiation; however, most studies on these behaviors focus on bacteria with relatively simple morphologies, such as rods and spheres. Notably, complex morphologies abound throughout the bacteria, with striking examples, such as P. hirschii, found within the stalked Alphaproteobacteria. P. hirschii is an outstanding candidate for studies of complex morphology generation and polymorphic cell cycles. Here, the cell cycle and genome of P. hirschii are characterized. This work sets the stage for future studies of the impact of complex cell shapes on bacterial behaviors.

Periplasmic Acid Stress Increases Cell Division Asymmetry (Polar Aging) of Escherichia coli

Under certain kinds of cytoplasmic stress, Escherichia coli selectively reproduce by distributing the newer cytoplasmic components to new-pole cells while sequestering older, damaged components in cells inheriting the old pole. This phenomenon is termed polar aging or cell division asymmetry. It is unknown whether cell division asymmetry can arise from a periplasmic stress, such as the stress of extracellular acid, which is mediated by the periplasm. We tested the effect of periplasmic acid stress on growth and division of adherent single cells. We tracked individual cell lineages over five or more generations, using fluorescence microscopy with ratiometric pHluorin to measure cytoplasmic pH. Adherent colonies were perfused continually with LBK medium buffered at pH 6.00 or at pH 7.50; the external pH determines periplasmic pH. In each experiment, cell lineages were mapped to correlate division time, pole age and cell generation number. In colonies perfused at pH 6.0, the cells inheriting the oldest pole divided significantly more slowly than the cells inheriting the newest pole. In colonies perfused at pH 7.50 (near or above cytoplasmic pH), no significant cell division asymmetry was observed. Under both conditions (periplasmic pH 6.0 or pH 7.5) the cells maintained cytoplasmic pH values at 7.2–7.3. No evidence of cytoplasmic protein aggregation was seen. Thus, periplasmic acid stress leads to cell division asymmetry with minimal cytoplasmic stress.

c-di-GMP signalling and the regulation of developmental transitions in streptomycetes

The complex life cycle of streptomycetes involves two distinct filamentous cell forms: the growing (or vegetative) hyphae and the reproductive (or aerial) hyphae, which differentiate into long chains of spores. Until recently, little was known about the signalling pathways that regulate the developmental transitions leading to sporulation. In this Review, we discuss important new insights into these pathways that have led to the emergence of a coherent regulatory network, focusing on the erection of aerial hyphae and the synchronous cell division event that produces dozens of unigenomic spores. In particular, we highlight the role of cyclic di-GMP (c-di-GMP) in controlling the initiation of development, and the role of the master regulator BldD in mediating c-di-GMP signalling.

Mycobacterial Growth

In this work, we review progress made in understanding the molecular underpinnings of growth and division in mycobacteria, concentrating on work published since the last comprehensive review ( Hett and Rubin 2008). We have focused on exciting work making use of new time-lapse imaging technologies coupled with reporter-gene fusions and antimicrobial treatment to generate insights into how mycobacteria grow and divide in a heterogeneous manner. We try to reconcile the different observations reported, providing a model of how they might fit together. We also review the topic of mycobacterial spores, which has generated considerable discussion during the last few years. Resuscitation promoting factors, and regulation of growth and division, have also been actively researched, and we summarize progress in these areas.

Mechanisms of bacterial morphogenesis: evolutionary cell biology approaches provide new insights

How Darwin's "endless forms most beautiful" have evolved remains one of the most exciting questions in biology. The significant variety of bacterial shapes is most likely due to the specific advantages they confer with respect to the diverse environments they occupy. While our understanding of the mechanisms generating relatively simple shapes has improved tremendously in the last few years, the molecular mechanisms underlying the generation of complex shapes and the evolution of shape diversity are largely unknown. The emerging field of bacterial evolutionary cell biology provides a novel strategy to answer this question in a comparative phylogenetic framework. This relatively novel approach provides hypotheses and insights into cell biological mechanisms, such as morphogenesis, and their evolution that would have been difficult to obtain by studying only model organisms. We discuss the necessary steps, challenges, and impact of integrating "evolutionary thinking" into bacterial cell biology in the genomic era.

The essential features and modes of bacterial polar growth

Polar growth represents a surprising departure from the canonical dispersed cell growth model. However, we know relatively little of the underlying mechanisms governing polar growth or the requisite suite of factors that direct polar growth. Underscoring how classic doctrine can be turned on its head, the peptidoglycan layer of polar-growing bacteria features unusual crosslinks and in some species the quintessential cell division proteins FtsA and FtsZ are recruited to the growing poles. Remarkably, numerous medically important pathogens utilize polar growth, accentuating the need for intensive research in this area. Here we review models of polar growth in bacteria based on recent research in the Actinomycetales and Rhizobiales, with emphasis on Mycobacterium and Agrobacterium species.

Illumination of growth, division and secretion by metabolic labeling of the bacterial cell surface

The cell surface is the essential interface between a bacterium and its surroundings. Composed primarily of molecules that are not directly genetically encoded, this highly dynamic structure accommodates the basic cellular processes of growth and division as well as the transport of molecules between the cytoplasm and the extracellular milieu. In this review, we describe aspects of bacterial growth, division and secretion that have recently been uncovered by metabolic labeling of the cell envelope. Metabolite derivatives can be used to label a variety of macromolecules, from proteins to non-genetically-encoded glycans and lipids. The embedded metabolite enables precise tracking in time and space, and the versatility of newer chemoselective detection methods offers the ability to execute multiple experiments concurrently. In addition to reviewing the discoveries enabled by metabolic labeling of the bacterial cell envelope, we also discuss the potential of these techniques for translational applications. Finally, we offer some guidelines for implementing this emerging technology.

Molecular toolbox for genetic manipulation of the stalked budding bacterium Hyphomonas neptunium

The alphaproteobacterium Hyphomonas neptunium proliferates by a unique budding mechanism in which daughter cells emerge from the end of a stalk-like extension emanating from the mother cell body. Studies of this species so far have been hampered by the lack of a genetic system and of molecular tools allowing the regulated expression of target genes. Based on microarray analyses, this work identifies two H. neptunium promoters that are activated specifically by copper and zinc. Functional analyses show that they have low basal activity and a high dynamic range, meeting the requirements for use as a multipurpose expression system. To facilitate their application, the two promoters were incorporated into a set of integrative plasmids, featuring a choice of two different selection markers and various fluorescent protein genes. These constructs enable the straightforward generation and heavy metal-inducible synthesis of fluorescent protein fusions in H. neptunium, thereby opening the door to an in-depth analysis of polar growth and development in this species.

Identification of long-lived proteins retained in cells undergoing repeated asymmetric divisions

Long-lived proteins have been implicated in age-associated decline in metazoa, but they have only been identified in extracellular matrices or postmitotic cells. However, the aging process also occurs in dividing cells undergoing repeated asymmetric divisions. It was not clear whether long-lived proteins exist in asymmetrically dividing cells or whether they are involved in aging. Here we identify long-lived proteins in dividing cells during aging using the budding yeast, Saccharomyces cerevisiae. Yeast mother cells undergo a limited number of asymmetric divisions that define replicative lifespan. We used stable-isotope pulse-chase and total proteome mass-spectrometry to identify proteins that were both long-lived and retained in aging mother cells after ∼ 18 cells divisions. We identified ∼ 135 proteins that we designate as long-lived asymmetrically retained proteins (LARPS). Surprisingly, the majority of LARPs appeared to be stable fragments of their original full-length protein. However, 15% of LARPs were full-length proteins and we confirmed several candidates to be long-lived and retained in mother cells by time-lapse microscopy. Some LARPs localized to the plasma membrane and remained robustly in the mother cell upon cell division. Other full-length LARPs were assembled into large cytoplasmic structures that had a strong bias to remain in mother cells. We identified age-associated changes to LARPs that include an increase in their levels during aging because of their continued synthesis, which is not balanced by turnover. Additionally, several LARPs were posttranslationally modified during aging. We suggest that LARPs contribute to age-associated phenotypes and likely exist in other organisms.

How sisters grow apart: mycobacterial growth and division

Mycobacterium tuberculosis, which is the aetiological agent of tuberculosis, owes much of its success as a pathogen to its unique cell wall and unusual mechanism of growth, which facilitate its adaptation to the human host and could have a role in clinical latency. Asymmetric growth and division increase population heterogeneity, which may promote antibiotic tolerance and the fitness of single cells. In this Review, we describe the unusual mechanisms of mycobacterial growth, cell wall biogenesis and division, and discuss how these processes might affect the survival of M. tuberculosis in vivo and contribute to the persistence of infection.

Peptidoglycan synthesis machinery in Agrobacterium tumefaciens during unipolar growth and cell division

The synthesis of peptidoglycan (PG) in bacteria is a crucial process controlling cell shape and vitality. In contrast to bacteria such as Escherichia coli that grow by dispersed lateral insertion of PG, little is known of the processes that direct polar PG synthesis in other bacteria such as the Rhizobiales. To better understand polar growth in the Rhizobiales Agrobacterium tumefaciens, we first surveyed its genome to identify homologs of (~70) well-known PG synthesis components. Since most of the canonical cell elongation components are absent from A. tumefaciens, we made fluorescent protein fusions to other putative PG synthesis components to assay their subcellular localization patterns. The cell division scaffolds FtsZ and FtsA, PBP1a, and a Rhizobiales- and Rhodobacterales-specific l,d-transpeptidase (LDT) all associate with the elongating cell pole. All four proteins also localize to the septum during cell division. Examination of the dimensions of growing cells revealed that new cell compartments gradually increase in width as they grow in length. This increase in cell width is coincident with an expanded region of LDT-mediated PG synthesis activity, as measured directly through incorporation of exogenous d-amino acids. Thus, unipolar growth in the Rhizobiales is surprisingly dynamic and represents a significant departure from the canonical growth mechanism of E. coli and other well-studied bacilli.

Many rod-shaped bacteria, including pathogens such as Brucella and Mycobacterium, grow by adding new material to their cell poles, and yet the proteins and mechanisms contributing to this process are not yet well defined. The polarly growing plant pathogen Agrobacterium tumefaciens was used as a model bacterium to explore these polar growth mechanisms. The results obtained indicate that polar growth in this organism is facilitated by repurposed cell division components and an otherwise obscure class of alternative peptidoglycan transpeptidases (l,d-transpeptidases). This growth results in dynamically changing cell widths as the poles expand to maturity and contrasts with the tightly regulated cell widths characteristic of canonical rod-shaped growth. Furthermore, the abundance and/or activity of l,d-transpeptidases appears to associate with polar growth strategies, suggesting that these enzymes may serve as attractive targets for specifically inhibiting growth of Rhizobiales, Actinomycetales, and other polarly growing bacterial pathogens.

Modes of cell wall growth differentiation in rod-shaped bacteria

A bacterial cell takes on the challenge to preserve and reproduce its shape at every generation against a substantial internal pressure by surrounding itself with a mechanical support, a peptidoglycan cell wall. The enlargement of the cell wall via net incorporation of precursors into the pre-existing wall conditions bacterial growth and morphology. However, generation, reproduction and/or modification of a specific shape requires that the incorporation takes place at precise locations for a defined time period. Much has been learnt in the past few years about the biochemistry of the peptidoglycan synthesis process, but topological approaches to the understanding of shape generation have been hindered by a lack of appropriate techniques. Recent technological advances are paving the way for substantial progress in understanding the mechanisms of bacterial morphogenesis. Here we review the latest developments, focusing on the impact of new techniques on the precise mapping of cell wall growth sites.