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Bacterial cells in their natural environments encounter rapid and large changes in external osmolality. For instance, enteric bacteria such as Escherichia coli experience a rapid decrease when they exit from host intestines. Changes in osmolality alter the mechanical load on the cell envelope, and previous studies have shown that large osmotic shocks can slow down bacterial growth and impact cytoplasmic diffusion. However, it remains unclear how cells maintain envelope integrity and regulate envelope synthesis in response to osmotic shocks. In this study, we developed an agarose pad-based protocol to assay envelope stiffness by measuring population-averaged cell length before and after a hyperosmotic shock. Pad-based measurements exhibited an apparently larger length change compared with single-cell dynamics in a microfluidic device, which we found was quantitatively explained by a transient increase in division rate after the shock. Inhibiting cell division led to consistent measurements between agarose pad-based and microfluidic measurements. Directly after hyperosmotic shock, FtsZ concentration and Z-ring intensity increased, and the rate of septum constriction increased. These findings establish an agarose pad-based protocol for quantifying cell envelope stiffness, and demonstrate that mechanical perturbations can have profound effects on bacterial physiology. [ABSTRACT FROM AUTHOR]

The actin family of cytoskeletal proteins is essential to the physiology of virtually all archaea, bacteria, and eukaryotes. While X-ray crystallography and electron microscopy have revealed structural homologies among actin-family proteins, these techniques cannot probe molecular-scale conformational dynamics. Here, we use all-atom molecular dynamic simulations to reveal conserved dynamical behaviors in four prokaryotic actin homologs: MreB, FtsA, ParM, and crenactin. We demonstrate that the majority of the conformational dynamics of prokaryotic actins can be explained by treating the four subdomains as rigid bodies. MreB, ParM, and FtsA monomers exhibited nucleotide-dependent dihedral and opening angles, while crenactin monomer dynamics were nucleotide-independent. We further show that the opening angle of ParM is sensitive to a specific interaction between subdomains. Steered molecular dynamics simulations of MreB, FtsA, and crenactin dimers revealed that changes in subunit dihedral angle lead to intersubunit bending or twist, suggesting a conserved mechanism for regulating filament structure. Taken together, our results provide molecular-scale insights into the nucleotide and polymerization dependencies of the structure of prokaryotic actins, suggesting mechanisms for how these structural features are linked to their diverse functions. [ABSTRACT FROM AUTHOR]

Despite their small sizes, bacterial cells within a host-associated microbial community often form highly structured complexes determined by environmental factors and interspecies interactions. Wilbert et al. combined species-specific fluorescent labels and high-resolution microscopy to visualize human tongue dorsum microbiomes and to highlight their structure and dynamics. [ABSTRACT FROM AUTHOR]

Cell shape matters across the kingdoms of life, and cells have the remarkable capacity to define and maintain specific shapes and sizes. But how are the shapes of micron-sized cells determined from the coordinated activities of nanometer-sized proteins? Here, we review general principles that have surfaced through the study of rod-shaped bacterial growth. Imaging approaches have revealed that polymers of the actin homolog MreB play a central role. MreB both senses and changes cell shape, thereby generating a self-organizing feedback system for shape maintenance. At the molecular level, structural and computational studies indicate that MreB filaments exhibit tunable mechanical properties that explain their preference for certain geometries and orientations along the cylindrical cell body. We illustrate the regulatory landscape of rod-shape formation and the connectivity between cell shape, cell growth, and other aspects of cell physiology. These discoveries provide a framework for future investigations into the architecture and construction of microbes. How do microbes maintain their shape? This review takes a closer look at the role played by the actin homolog MreB in controlling rod-shaped bacterial growth. [ABSTRACT FROM AUTHOR]

Background: The determination and regulation of cell morphology are critical components of cell-cycle control, fitness, and development in both single-cell and multicellular organisms. Understanding how environmental factors, chemical perturbations, and genetic differences affect cell morphology requires precise, unbiased, and validated measurements of cell-shape features. Results: Here we introduce two software packages, Morphometrics and BlurLab, that together enable automated, computationally efficient, unbiased identification of cells and morphological features. We applied these tools to bacterial cells because the small size of these cells and the subtlety of certain morphological changes have thus far obscured correlations between bacterial morphology and genotype. We used an online resource of images of the Keio knockout library of nonessential genes in the Gram-negative bacterium Escherichia coli to demonstrate that cell width, width variability, and length significantly correlate with each other and with drug treatments, nutrient changes, and environmental conditions. Further, we combined morphological classification of genetic variants with genetic meta-analysis to reveal novel connections among gene function, fitness, and cell morphology, thus suggesting potential functions for unknown genes and differences in modes of action of antibiotics. Conclusions: Morphometrics and BlurLab set the stage for future quantitative studies of bacterial cell shape and intracellular localization. The previously unappreciated connections between morphological parameters measured with these software packages and the cellular environment point toward novel mechanistic connections among physiological perturbations, cell fitness, and growth. [ABSTRACT FROM AUTHOR]

Bacterial cells are regularly confronted with simultaneous changes in environmental nutrient supply and osmolarity. Despite the importance of osmolarity and osmoregulation in bacterial physiology, the relationship between the cellular response to osmotic perturbations and other stresses has remained largely unexplored. Bacteria cultured in hyperosmotic conditions and bacteria experiencing nutrient stress exhibit similar physiological changes, including metabolic shutdown, increased protein instability, dehydration, and condensation of chromosomal DNA. In this review, we highlight overlapping molecular players between osmotic and nutrient stresses. These connections between two seemingly disparate stress response pathways reinforce the importance of central carbon metabolism as a control point for diverse aspects of homeostatic regulation. We identify important open questions for future research, emphasizing the pressing need to develop and exploit new methods for probing how osmolarity affects phylogenetically diverse species. [ABSTRACT FROM AUTHOR]

The concept of microbial consortia is of great attractiveness in synthetic biology. Despite of all its benefits, however, there are still problems remaining for large-scaled multicellular gene circuits, for example, how to reliably design and distribute the circuits in microbial consortia with limited number of well-behaved genetic modules and wiring quorum-sensing molecules. To manage such problem, here we propose a formalized design process: (i) determine the basic logic units (AND, OR and NOT gates) based on mathematical and biological considerations; (ii) establish rules to search and distribute simplest logic design; (iii) assemble assigned basic logic units in each logic operating cell; and (iv) fine-tune the circuiting interface between logic operators. We in silico analyzed gene circuits with inputs ranging from two to four, comparing our method with the pre-existing ones. Results showed that this formalized design process is more feasible concerning numbers of cells required. Furthermore, as a proof of principle, an Escherichia coli consortium that performs XOR function, a typical complex computing operation, was designed. The construction and characterization of logic operators is independent of “wiring” and provides predictive information for fine-tuning. This formalized design process provides guidance for the design of microbial consortia that perform distributed biological computation. [ABSTRACT FROM AUTHOR]

While cell division is a critical process in cellular proliferation, very few antibiotics have been identified that target the bacterial cell‐division machinery. Recent studies have shown that the small molecule PC190723 inhibits cell division in several Gram‐positive bacteria, with a hypothesized mechanism of action involving direct targeting of the tubulin homolog FtsZ, which is essential for division in virtually all bacterial species. Here, it is shown that PC190723 also inhibits cell division in the Gram‐negative bacterium Escherichia coli if the outer membrane permeability barrier is compromised genetically or chemically. The results show that the equivalent FtsZ mutations conferring PC190723 resistance in Staphylococcus aureus do not protect E. coli against PC190723, and that suppressors of PC190723 sensitivity in E. coli, which do not generically decrease outer membrane permeability, do not map to FtsZ or other division proteins. These suppressors display a wide range of morphological and growth phenotypes, and one exhibits a death phenotype in the stationary phase similar to that of a mutant with disrupted lipid homeostasis. Finally, a complementing FtsZ–msfGFP fusion is used to show that PC190723 does not affect the Z‐ring structure. Taken together, the findings suggest that PC190723 inhibits growth and division in E. coli without targeting FtsZ. This study highlights the importance of utilizing a combination of genetic, chemical, and single‐cell approaches to dissect the mechanisms of action of new antibiotics, which are not necessarily conserved across bacterial species. [ABSTRACT FROM AUTHOR]

Summary Size is a universally defining characteristic of all living cells and tissues and is intrinsically linked with cell genotype, growth, and physiology. Many mutations have been identified to alter cell size, but pleiotropic effects have largely hampered our ability to probe how cell size specifically affects fundamental cellular properties, such as DNA content and intracellular localization. To systematically interrogate the impact of cell morphology on bacterial physiology, we used fluorescence-activated cell sorting to enrich a library of hundreds of Escherichia coli mutants in the essential cytoskeletal protein MreB for subtle changes in cell shape, cumulatively spanning ∼5-fold variation in average cell volume. Critically, pleiotropic effects in the mutated library are most likely minimized because only one gene was mutated and because growth rate was unaffected, thereby allowing us to query the general effects of morphology on cellular physiology over a large range of cell sizes with high resolution. We discovered linear scaling of the abundance of DNA and the key division protein FtsZ with cell volume, a strong dependency of sensitivity to specific antibiotics on cell width, and a simple correlation between MreB localization pattern and cell width. Our systematic, quantitative approach reveals complex and dynamic links between bacterial morphology and physiology and should be generally applicable for probing size-related genotype-phenotype relationships. [ABSTRACT FROM AUTHOR]

Summary Essential gene functions underpin the core reactions required for cell viability, but their contributions and relationships are poorly studied in vivo. Using CRISPR interference, we created knockdowns of every essential gene in Bacillus subtilis and probed their phenotypes. Our high-confidence essential gene network, established using chemical genomics, showed extensive interconnections among distantly related processes and identified modes of action for uncharacterized antibiotics. Importantly, mild knockdown of essential gene functions significantly reduced stationary-phase survival without affecting maximal growth rate, suggesting that essential protein levels are set to maximize outgrowth from stationary phase. Finally, high-throughput microscopy indicated that cell morphology is relatively insensitive to mild knockdown but profoundly affected by depletion of gene function, revealing intimate connections between cell growth and shape. Our results provide a framework for systematic investigation of essential gene functions in vivo broadly applicable to diverse microorganisms and amenable to comparative analysis. [ABSTRACT FROM AUTHOR]

The key bacterial cell division protein FtsZ can adopt multiple conformations, and prevailing models suggest that transitions of FtsZ subunits from the closed to open state are necessary for filament formation and stability. Using all-atom molecular dynamics simulations, we analyzed state transitions of Staphylococcus aureus FtsZ as a monomer, dimer, and hexamer. We found that monomers can adopt intermediate states but preferentially adopt a closed state that is robust to forced reopening. Dimer subunits transitioned between open and closed states, and dimers with both subunits in the closed state remained highly stable, suggesting that open-state conformations are not necessary for filament formation. Mg 2+ strongly stabilized the conformation of GTP-bound subunits and the dimer filament interface. Our hexamer simulations indicate that the plus end subunit preferentially closes and that other subunits can transition between states without affecting inter-subunit stability. We found that rather than being correlated with subunit opening, inter-subunit stability was strongly correlated with catalytic site interactions. By leveraging deep-learning models, we identified key intrasubunit interactions governing state transitions. Our findings suggest a greater range of possible monomer and filament states than previously considered and offer new insights into the nuanced interplay between subunit states and the critical role of nucleotide hydrolysis and Mg 2+ in FtsZ filament dynamics.

The human gut contains diverse communities of bacteriophage, whose interactions with the broader microbiome and potential roles in human health are only beginning to be uncovered. Here, we combine multiple types of data to quantitatively estimate gut phage population dynamics and lifestyle characteristics in human subjects. Unifying results from previous studies, we show that an average human gut contains a low ratio of phage particles to bacterial cells (~1:100), but a much larger ratio of phage genomes to bacterial genomes (~4:1), implying that most gut phage are effectively temperate (e.g., integrated prophage, phage-plasmids, etc.). By integrating imaging and sequencing data with a generalized model of temperate phage dynamics, we estimate that phage induction and lysis occurs at a low average rate (~0.001-0.01 per bacterium per day), imposing only a modest fitness burden on their bacterial hosts. Consistent with these estimates, we find that the phage composition of a diverse synthetic community in gnotobiotic mice can be quantitatively predicted from bacterial abundances alone, while still exhibiting phage diversity comparable to native human microbiomes. These results provide a foundation for interpreting existing and future studies on links between the gut virome and human health.

Human gut commensal bacteria are routinely exposed to various stresses, including therapeutic drugs, and collateral effects are difficult to predict. To systematically interrogate community-level effects of drug perturbations, we screened stool-derived in vitro communities with 707 clinically relevant small molecules. Across ∼5,000 community-drug interaction conditions, compositional and metabolomic responses were predictably impacted by nutrient competition, with certain species exhibiting improved growth due to adverse impacts on competitors. Changes to community composition were generally reversed by reseeding with the original community, although occasionally species promotion was long-lasting, due to higher-order interactions, even when the competitor was reseeded. Despite strong selection pressures, emergence of resistance within communities was infrequent. Finally, while qualitative species responses to drug perturbations were conserved across community contexts, nutrient competition quantitatively affected their abundances, consistent with predictions of consumer-resource models. Our study reveals that quantitative understanding of the interaction landscape, particularly nutrient competition, can be used to anticipate and potentially mitigate side effects of drug treatment on the gut microbiota.

While microbial communities inhabit a wide variety of complex natural environments, in vitro culturing enables highly controlled conditions and high-throughput interrogation for generating mechanistic insights. In vitro assemblies of gut commensals have recently been introduced as models for the intestinal microbiota, which plays fundamental roles in host health. However, a protocol for 16S rRNA sequencing and analysis of in vitro samples that optimizes financial cost, time/effort, and accuracy/reproducibility has yet to be established. Here, we systematically identify protocol elements that have significant impact, introduce bias, and/or can be simplified. Our results indicate that community diversity and composition are generally unaffected by substantial protocol streamlining. Additionally, we demonstrate that a strictly aerobic halophile is an effective spike-in for estimating absolute abundances in communities of anaerobic gut commensals. This time- and money-saving protocol should accelerate discovery by increasing 16S rRNA data reliability and comparability and through the incorporation of absolute abundance estimates., Competing Interests: The authors declare no competing interests., (© 2022.)

CRISPR interference (CRISPRi) has facilitated the study of essential genes in diverse organisms using both high-throughput and targeted approaches. Despite the promise of this technique, no comprehensive arrayed CRISPRi library targeting essential genes exists for the model bacterium Escherichia coli, or for any Gram-negative species. Here, we built and characterized such a library. Each of the ∼500 strains in our E. coli library contains an inducible, chromosomally integrated single guide RNA (sgRNA) targeting an essential (or selected nonessential) gene and can be mated with a pseudo-Hfr donor strain carrying a dcas9 cassette to create a CRISPRi knockdown strain. Using this system, we built an arrayed library of CRISPRi strains and performed population and single-cell growth and morphology measurements as well as targeted follow-up experiments. These studies found that inhibiting translation causes an extended lag phase, identified new modulators of cell morphology, and revealed that the morphogene mreB is subject to transcriptional feedback regulation, which is critical for the maintenance of morphology. Our findings highlight canonical and noncanonical roles for essential genes in numerous aspects of cellular homeostasis. IMPORTANCE Essential genes make up only ∼5 to 10% of the genetic complement in most organisms but occupy much of their protein synthesis and account for almost all antibiotic targets. Despite the importance of essential genes, their intractability has, until recently, hampered efforts to study them. CRISPRi has facilitated the study of essential genes by allowing inducible and titratable depletion. However, all large-scale CRISPRi studies in Gram-negative bacteria thus far have used plasmids to express CRISPRi components and have been constructed in pools, limiting their utility for targeted assays and complicating the determination of antibiotic effects. Here, we use a modular method to construct an arrayed library of chromosomally integrated CRISPRi strains targeting the essential genes of the model bacterium Escherichia coli. This library enables targeted studies of essential gene depletions and high-throughput determination of antibiotic targets and facilitates studies targeting the outer membrane, an essential component that serves as the major barrier to antibiotics.

Bacterial growth under nutrient-rich and starvation conditions is intrinsically tied to the environmental history and physiological state of the population. While high-throughput technologies have enabled rapid analyses of mutant libraries, technical and biological challenges complicate data collection and interpretation. Here, we present a framework for the execution and analysis of growth measurements with improved accuracy over that of standard approaches. Using this framework, we demonstrate key biological insights that emerge from consideration of culturing conditions and history. We determined that quantification of the background absorbance in each well of a multiwell plate is critical for accurate measurements of maximal growth rate. Using mathematical modeling, we demonstrated that maximal growth rate is dependent on initial cell density, which distorts comparisons across strains with variable lag properties. We established a multiple-passage protocol that alleviates the substantial effects of glycerol on growth in carbon-poor media, and we tracked growth rate-mediated fitness increases observed during a long-term evolution of Escherichia coli in low glucose concentrations. Finally, we showed that growth of Bacillus subtilis in the presence of glycerol induces a long lag in the next passage due to inhibition of a large fraction of the population. Transposon mutagenesis linked this phenotype to the incorporation of glycerol into lipoteichoic acids, revealing a new role for these envelope components in resuming growth after starvation. Together, our investigations underscore the complex physiology of bacteria during bulk passaging and the importance of robust strategies to understand and quantify growth. IMPORTANCE How starved bacteria adapt and multiply under replete nutrient conditions is intimately linked to their history of previous growth, their physiological state, and the surrounding environment. While automated equipment has enabled high-throughput growth measurements, data interpretation and knowledge gaps regarding the determinants of growth kinetics complicate comparisons between strains. Here, we present a framework for growth measurements that improves accuracy and attenuates the effects of growth history. We determined that background absorbance quantification and multiple passaging cycles allow for accurate growth rate measurements even in carbon-poor media, which we used to reveal growth-rate increases during long-term laboratory evolution of Escherichia coli Using mathematical modeling, we showed that maximum growth rate depends on initial cell density. Finally, we demonstrated that growth of Bacillus subtilis with glycerol inhibits the future growth of most of the population, due to lipoteichoic acid synthesis. These studies highlight the challenges of accurate quantification of bacterial growth behaviors., (Copyright © 2020 Atolia et al.)

The MreB actin-like cytoskeleton assembles into dynamic polymers that coordinate cell shape in many bacteria. In contrast to most other cytoskeleton systems, few MreB-interacting proteins have been well characterized. Here, we identify a small protein from Caulobacter crescentus, an assembly inhibitor of MreB (AimB). AimB overexpression mimics inhibition of MreB polymerization, leading to increased cell width and MreB delocalization. Furthermore, aimB appears to be essential, and its depletion results in decreased cell width and increased resistance to A22, a small-molecule inhibitor of MreB assembly. Molecular dynamics simulations suggest that AimB binds MreB at its monomer-monomer protofilament interaction cleft and that this interaction is favored for C. crescentus MreB over Escherichia coli MreB because of a closer match in the degree of opening with AimB size, suggesting coevolution of AimB with MreB conformational dynamics in C. crescentus. We support this model through functional analysis of point mutants in both AimB and MreB, photo-cross-linking studies with site-specific unnatural amino acids, and species-specific activity of AimB. Together, our findings are consistent with AimB promoting MreB dynamics by inhibiting monomer-monomer assembly interactions, representing a new mechanism for regulating actin-like polymers and the first identification of a non-toxin MreB assembly inhibitor. Because AimB has only 104 amino acids and small proteins are often poorly characterized, our work suggests the possibility of more bacterial cytoskeletal regulators to be found in this class. Thus, like FtsZ and eukaryotic actin, MreB may have a rich repertoire of regulators to tune its precise assembly and dynamics., (Copyright © 2020 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

Despite their small size and lack of membrane-based DNA encapsulation, prokaryotic cells still organize and scale their nucleoid in specific subcellular regions. Two studies show that the DNA-free regions in prokaryotes are full of large biomolecules, which exclude DNA via entropic forces., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

In the rod-shaped bacterium Escherichia coli, the actin-like protein MreB localizes in a curvature-dependent manner and spatially coordinates cell-wall insertion to maintain cell shape, although the molecular mechanism by which cell width is regulated remains unknown. Here we demonstrate that the membrane protein RodZ regulates the biophysical properties of MreB and alters the spatial organization of E. coli cell-wall growth. The relative expression levels of MreB and RodZ change in a manner commensurate with variations in growth rate and cell width, and RodZ systematically alters the curvature-based localization of MreB and cell width in a concentration-dependent manner. We identify MreB mutants that alter the bending properties of MreB filaments in molecular dynamics simulations similar to RodZ binding, and show that these mutants rescue rod-like shape in the absence of RodZ alone or in combination with wild-type MreB. Thus, E. coli can control its shape and dimensions by differentially regulating RodZ and MreB to alter the patterning of cell-wall insertion, highlighting the rich regulatory landscape of cytoskeletal molecular biophysics.

Background: The determination and regulation of cell morphology are critical components of cell-cycle control, fitness, and development in both single-cell and multicellular organisms. Understanding how environmental factors, chemical perturbations, and genetic differences affect cell morphology requires precise, unbiased, and validated measurements of cell-shape features., Results: Here we introduce two software packages, Morphometrics and BlurLab, that together enable automated, computationally efficient, unbiased identification of cells and morphological features. We applied these tools to bacterial cells because the small size of these cells and the subtlety of certain morphological changes have thus far obscured correlations between bacterial morphology and genotype. We used an online resource of images of the Keio knockout library of nonessential genes in the Gram-negative bacterium Escherichia coli to demonstrate that cell width, width variability, and length significantly correlate with each other and with drug treatments, nutrient changes, and environmental conditions. Further, we combined morphological classification of genetic variants with genetic meta-analysis to reveal novel connections among gene function, fitness, and cell morphology, thus suggesting potential functions for unknown genes and differences in modes of action of antibiotics., Conclusions: Morphometrics and BlurLab set the stage for future quantitative studies of bacterial cell shape and intracellular localization. The previously unappreciated connections between morphological parameters measured with these software packages and the cellular environment point toward novel mechanistic connections among physiological perturbations, cell fitness, and growth.

Mathematical modeling of genetic circuits generally assumes that gene expression is at steady state when measurements are performed. However, conventional methods of measurement do not necessarily guarantee that this assumption is satisfied. In this study, we reveal a bi-plateau mode of gene expression at the single-cell level in bacterial batch cultures. The first plateau is dynamically active, where gene expression is at steady state; the second plateau, however, is dynamically inactive. We further demonstrate that the predictability of assembled genetic circuits in the first plateau (steady state) is much higher than that in the second plateau where conventional measurements are often performed. By taking the nature of steady state into consideration, our method of measurement promises to directly capture the intrinsic property of biological parts/circuits regardless of circuit-host or circuit-environment interactions.

Synthetic biology aims to rationally construct biological devices with required functionalities. Methods that automate the design of genetic devices without post-hoc adjustment are therefore highly desired. Here we provide a method to predictably design genetic toggle switches with predetermined bistability. To accomplish this task, a biophysical model that links ribosome binding site (RBS) DNA sequence to toggle switch bistability was first developed by integrating a stochastic model with RBS design method. Then, to parametrize the model, a library of genetic toggle switch mutants was experimentally built, followed by establishing the equivalence between RBS DNA sequences and switch bistability. To test this equivalence, RBS nucleotide sequences for different specified bistabilities were in silico designed and experimentally verified. Results show that the deciphered equivalence is highly predictive for the toggle switch design with predetermined bistability. This method can be generalized to quantitative design of other probabilistic genetic devices in synthetic biology.