Mechanomicrobiology: how bacteria sense and respond to forces

Effect of preferential growth of Shewanella oneidensis MR-1 on microbial corrosion of constituent phases of 2205 duplex stainless steel

Understanding selective growth of electroactive bacteria on surface of constituent phases (ferrite/ austenite) of 2205 DSS is required for mitigating microbiologically influenced corrosion (MIC). In this study, the preferential attachment of bacteria and its impact on corrosion of single phase were investigated under anaerobic condition using Shewanella oneidensis MR-1. Single-ferrite phase was more susceptible to biofilm formation compared with single-austenite phase. Atomic force microscope (AFM) revealed that the surface of ferrite phase coupon was fully covered with S. oneidensis MR-1 biofilm whereas few S. oneidensis MR-1 cells were observed on the surface of austenite phase. After 14 d of incubation, the maximum biofilm thicknesses on 2205 DSS, ferrite and austenite phase were 15.5 ± 1.0 µm, 13.8 ± 3.2 µm, and 10.2 ± 0.8 µm, respectively. S. oneidensis MR-1 accelerated the pitting corrosion of materials. The maximum pits depth on single ferrite and austenite phase in sterile medium (3.2 µm vs 2.2 µm with mean values 2.5 µm vs 1.7 µm) were relatively small than those in biotic medium (6.0 µm vs 4.5 µm with mean values 4.5 µm vs 3.8 µm). Synergistic effects of Cr and Ni enhanced the stability of passive film on austenite phase.

Bacterial clustering biomaterials as anti-infective therapies

In Nature, bacterial clustering by host-released peptides or nucleic acids is an evolutionarily conserved immune defense strategy employed to prevent adhesion of pathogenic microbes, which is prerequisite for most infections. Synthetic anti-adhesion strategies present as non-lethal means of targeting bacteria and may potentially be used to avoid resistance against antimicrobial therapies. From bacteria-agglutinating biomolecules discovered in nature to synthetic designs involving peptides, cationic polymers and nanoparticles, the modes of actions appear broad and unconsolidated. Herein, we present a critical review and update of the state-of-the-art in synthetic bacteria-clustering designs with proposition of a more streamlined nomenclature and classification. Overall, this review aims to consolidate the conceptual framework in the field of bacterial clustering and highlight its potentials as an avenue for discovering novel antibacterial biomaterials.

Evaluation of protective immune response of live-attenuated candidate vaccines ΔcpxA and ΔcpxR against Vibrio alginolyticus in pearl gentian grouper

The grouper farming industry was severely influenced by vibriosis. In this study, we developed two live-attenuated vaccine (LAV) candidates against Vibrio alginolyticus infection in pearl gentian groupers using cpxA or cpxR mutant strains of V. alginolyticus (ΔcpxA and ΔcpxR). Groupers were administrated with ΔcpxA and ΔcpxR at the dose of 1.0 × 104 CFU/fish (safety dose) to evaluate the immune protect effect of LAV. The increasing median lethal dose (LD50) of ΔcpxA and ΔcpxR indicated the decreased virulence of bacteria to groupers. Our results suggested that two LAVs achieved over 70 % relative percent survival (RPS) after groupers were challenged by V. alginolyticus on 14 days post-immunization. The immune protection was mainly attributed to the up-regulation of immune-related gene expression (IL-6, IL-12, TNF-α, TLR2, TLR5S, CD4, MHC-Iα, IFN-γ2 and NF-κB), the higher activities of catalase (CAT), lysozyme (LZM), superoxide dismutase (SOD), and the increasing production of total protein (TP) in serum. The research indicated that the vaccination of fish with ΔcpxA and ΔcpxR can induce the innate and acquired immunity and survival rate of groupers after bacterial infection, so they can be considered as the promising candidates of vaccine for grouper industry.

Effect of oat β-glucan on the freezing resistance of yeast and the underlying mechanism

The objective of this study was to investigate the protective effects of oat β-glucan (OβG) on yeast subjected to freeze-thaw cycle-induced stress. A range of analytical techniques were employed to identify the underlying molecular mechanisms, including flow cytometry, gas chromatography-mass spectrometry, and quantitative real-time PCR. Following three freeze-thaw cycles, the survival rate of yeast that had been supplemented with 0.5 % OβG was found to be significantly higher than that of the control sample, increasing from 36.21 % to 56.81 %. The addition of 0.5 % OβG resulted in a remarkable reduction in apoptosis, an improvement in cell membrane integrity, and an increase in superoxide dismutase, catalase activity and glutathione content compared to the control group. Furthermore, a noticeable increment in the intracellular trehalose content was observed, from 4.10 mg/g to 7.48 mg/g. OβG modulated the expression of trehalose metabolism-related genes (ATH1, NTH1, NTH2) throughout the freeze-thaw cycle. Therefore, it could be concluded that OβG protected yeast cells against excessive reactive oxygen species and minimised oxidative damage to cellular membranes by upregulating antioxidant enzyme activity and total antioxidant capacity. Moreover, the supplementation of OβG was also found to be effective in increasing intracellular trehalose levels, thereby enhancing the freezing resistance of yeast cells.

Modifying the antibacterial performance of Cu surfaces by topographic patterning in the micro- and nanometer scale

Antimicrobial surfaces are a promising approach to reduce the spread of pathogenic microorganisms in various critical environments. To achieve high antimicrobial functionality, it is essential to consider the material-specific bactericidal mode of action in conjunction with bacterial surface interactions. This study investigates the effect of altered contact conditions on the antimicrobial efficiency of Cu surfaces against Escherichia coli and Staphylococcus aureus. The fabrication of line-like periodic surface patterns in the scale range of single bacterial cells was achieved utilizing ultrashort pulsed direct laser interference patterning. These patterns create both favorable and unfavorable topographies for bacterial adhesion. The variation in bacteria/surface interaction is monitored in terms of strain-specific bactericidal efficiency and the role of corrosive forces driving quantitative Cu ion release. The investigation revealed that bacterial deactivation on Cu surfaces can be either enhanced or decreased by intentional topography modifications, independent of Cu ion emission, with strain-specific deviations in effective pattern scales observed. The results of this study indicate the potential of targeted topographic surface functionalization to optimize antimicrobial surface designs, enabling strain-specific decontamination strategies.

Metareview: a survey of active matter reviews

In the past years, the amount of research on active matter has grown extremely rapidly, a fact that is reflected in particular by the existence of more than 1000 reviews on this topic. Moreover, the field has become very diverse, ranging from theoretical studies of the statistical mechanics of active particles to applied work on medical applications of microrobots and from biological systems to artificial swimmers. This makes it very difficult to get an overview over the field as a whole. Here, we provide such an overview in the form of a metareview article that surveys the existing review articles and books on active matter. Thereby, this article provides a useful starting point for finding literature about a specific topic.

Decoding bacterial communication: Intracellular signal transduction, quorum sensing, and cross-kingdom interactions

This review provides a comprehensive analysis of the intricate architecture of bacterial sensing systems, with a focus on signal transduction mechanisms and their critical roles in microbial physiology. It highlights quorum sensing (QS), quorum quenching (QQ), and quorum sensing interference (QSI) as fundamental processes driving bacterial communication, influencing gene expression, biofilm formation, and interspecies interactions. The analysis explores the importance of diffusible signal factors (DSFs) and secondary messengers such as cAMP and c-di-GMP in modulating microbial behaviors. Additionally, cross-kingdom signaling, where bacterial signals impact host-pathogen dynamics and ecological balance, is systematically reviewed. This review introduces "signalomics", an novel interdisciplinary framework integrating genomics, proteomics, and metabolomics to offer a holistic framework for understanding microbial communication and evolution. These findings hold significant implications for various domains, including food preservation, agriculture, and human health.

Bacterial Swimming and Accumulation on Endothelial Cell Surfaces

Flagellar-driven locomotion plays a critical role in the bacterial attachment and colonization of surfaces, contributing to the risks of contamination and infection. Extensive efforts to uncover the underlying principles governing bacterial motility near surfaces have relied on idealized assumptions about surrounding artificial surfaces. However, in the context of living systems, the role of cells from tissues and organs becomes increasingly critical, particularly in bacterial swimming and adhesion, yet it remains poorly understood. Here, we propose using biological surfaces composed of vascular endothelial cells to experimentally investigate bacterial motion and interaction behaviors. Our results reveal that bacterial trapping observed on inorganic surfaces is counteractively manifested with reduced radii of circular motion on cellular surfaces. Additionally, two distinct modes of bacterial adhesion were identified: tight and loose adhesion. Interestingly, the presence of living cells enhances bacterial surface enrichment, and imposed flow intensifies this accumulation via a bias-swimming effect. These results surprisingly indicate that physical effects remain the dominant factor regulating bacterial motility and accumulation at the single-cell-layer level in vitro, bridging the gap between simplified hydrodynamic mechanisms and complex biological surfaces with relevance to biofilm formation and bacterial contamination.

Disentangling the feedback loops driving spatial patterning in microbial communities

The properties of multispecies biofilms are determined by how species are arranged in space. How these patterns emerge is a complex and largely unsolved problem. Here, we synthesize the known factors affecting pattern formation, identify the interdependencies and feedback loops coupling them, and discuss approaches to disentangle their effects. Finally, we propose an interdisciplinary research program that could create a predictive understanding of pattern formation in microbial communities.

Atomic force microscopy combined with microfluidics for label-free sorting and automated nanomechanics of circulating tumor cells in liquid biopsy

Liquid biopsies are expected to advance cancer management, and particularly physical cues are gaining attention for indicating tumorigenesis and metastasis. Atomic force microscopy (AFM) has become a standard and important tool for detecting the mechanical properties of single living cells, but studies of developing AFM-based methods to efficiently measure the mechanical properties of circulating tumor cells (CTCs) in liquid biopsy for clinical utility are still scarce. Herein, we present a proof-of-concept study based on the complementary combination of AFM and microfluidics, which allows label-free sorting of individual CTCs and subsequent automated AFM measurements of the mechanical properties of CTCs. With the use of a microfluidic system containing contraction-expansion microchannels, specific cancer cell types were separated and harvested in a marker-independent manner. Subsequently, automated AFM indentation and force spectroscopy experiments were performed on the enriched cells under the precise guidance of the label-free identification of cells using a deep learning optical image recognition model. The effectiveness of the presented method was verified on three experimental sample systems, including mixed microspheres with different sizes, a mixture of different types of cancer cells, and a mixture of cancer cells and blood cells. The study illustrates a feasible framework based on the integration of AFM and microfluidics for non-destructive and efficient nanomechanical phenotyping of CTCs in bodily fluids, which offers additional possibilities for the clinical applications of AFM-based nanomechanical analysis and will also benefit the field of mechanobiology as well as cancer liquid biopsy.

Biofilm dynamics in space and their potential for sustainable space exploration - A comprehensive review

Microbial biofilms are universal. The intricate tapestry of biofilms has remarkable implications for the environment, health, and industrial processes. The field of space microbiology is actively investigating the effects of microgravity on microbes, and discoveries are constantly being made. Recent evidence suggests that extraterrestrial environments also fuel the biofilm formation. Understanding the biofilm mechanics under microgravitational conditions is crucial at this stage and could have an astounding impact on inter-planetary missions. This review systematically examines the existing understanding of biofilm development in space and provides insight into how molecules, physiology, or environmental factors influence biofilm formation during microgravitational conditions. In addition, biocontrol strategies targeting the formation and dispersal of biofilms in space environments are explored. In particular, the article highlights the potential benefits of using microbial biofilms in space for bioremediation, life support systems, and biomass production applications.

Force Nanoscopy Demonstrates Stress-Activated Adhesion between Staphylococcus aureus Iron-Regulated Surface Determinant Protein B and Host Toll-like Receptor 4

The Staphylococcus aureus iron-regulated surface determinant protein B (IsdB) has recently been shown to bind to toll-like receptor 4 (TLR4), thereby inducing a strong inflammatory response in innate immune cells. Currently, two unsolved questions are (i) What is the molecular mechanism of the IsdB-TLR4 interaction? and (ii) Does it also play a role in nonimmune systems? Here, we use single-molecule experiments to demonstrate that IsdB binds TLR4 with both weak and extremely strong forces and that the mechanostability of the molecular complex is dramatically increased by physical stress, sustaining forces up to 2000 pN, at a loading rate of 105 pN/s. We also show that TLR4 binding by IsdB mediates time-dependent bacterial adhesion to endothelial cells, pointing to the role of this bond in cell invasion. Our findings point to a function for IsdB in pathogen-host interactions, that is, mediating strong bacterial adhesion to host endothelial cells under fluid shear stress, unknown until now. In nanomedicine, this stress-dependent adhesion represents a potential target for innovative therapeutics against S. aureus-resistant strains.

Flow-induced bending of flagella controls bacterial surface behavior

Bacteria engage in surface-specific behaviors that are assumed to be driven by biological signaling. However, surface behaviors could be controlled by mechanical reorientation of bacterial appendages. Here, we use microfluidics and flagellar labeling to discover how shear force bends flagella to control surface behavior of the human pathogen Pseudomonas aeruginosa. By imaging flagellar rotation and using mutants with paralyzed flagella, we establish that flagellar rotation promotes surface departure in host-relevant shear regimes. Our single-cell experiments reveal two distinct subpopulations in flow: cells with their flagellum positioned upstream and cells with their flagellum positioned downstream. Shear force bends upstream flagella around the cell and blocks rotation. In contrast, downstream flagella can continue to rotate after surface arrival. Cells with downstream flagella depart the surface more frequently than cells with upstream flagella, indicating how flow direction can determine bacterial cell fate on surfaces. Together, our results demonstrate how the geometric relationship between flow and cell appendages can generate subpopulations and control surface behaviors.

Towards improved biofilm models

Biofilms are complex microbial communities that have a critical function in many natural ecosystems, industrial settings as well as in recurrent and chronic infections. Biofilms are highly heterogeneous and dynamic assemblages that display complex responses to varying environmental factors, and those properties present substantial challenges for their study and control. In recent years, there has been a growing interest in developing improved biofilm models to offer more precise and comprehensive representations of these intricate systems. However, an objective assessment for ascertaining the ability of biofilms in model systems to recapitulate those in natural environments has been lacking. In this Perspective, we focus on medical biofilms to delve into the current state-of-the-art in biofilm modelling, emphasizing the advantages and limitations of different approaches and addressing the key challenges and opportunities for future research. We outline a framework for quantitatively assessing model accuracy. Ultimately, this Perspective aims to provide a comprehensive and critical overview of medically focused biofilm models, with the intent of inspiring future research aimed at enhancing the biological relevance of biofilm models.

Programming Surface Motility and Modulating Physiological Behaviors of Bacteria via Biosurfactant-Mimetic Polyurethanes

Modulating microbial motility and physiology can enhance the production of bacterial macromolecules and small molecules. Herein, a platform of water-soluble and amphiphilic peptidomimetic polyurethanes is reported as a means of regulating bacterial surface behavior and the concomitant production of extracellular polymeric substances (EPS). It is demonstrated that carboxyl (-COOH)-containing polyurethanes exhibited 17-fold and 80-fold enhancements in Pseudomonas aeruginosa (P. aeruginosa) swarming and twitching areas, respectively. Conversely, an amine (-NH2)-functionalized polyurethane reduces the P. aeruginosa swarming area by 58%. Similar influences on the surface motility of Escherichia coli (E. coli) and a nonswarming P. aeruginosa mutant strain are also observed. Notably, -COOH polyurethanes completely wet the agar hydrogel surface and promote bacterial surface proliferation, resulting in enhanced EPS and rhamnolipid production. The programming of bacterial spatial migration into designed patterns is achieved by leveraging the opposing influences of -NH2 and -COOH polyurethanes. The results highlight the potential of this synthetic polyurethane platform and potentially other polymer systems as an exciting approach to control bacterial surface behaviors and influence the production of engineered living materials.

Biofilms as more than the sum of their parts: lessons from developmental biology

Although our understanding of both bacterial cell physiology and the complex behaviors exhibited by bacterial biofilms is expanding rapidly, we cannot yet sum the behaviors of individual cells to understand or predict biofilm behavior. This is both because cell physiology in biofilms is different from planktonic growth and because cell behavior in biofilms is spatiotemporally patterned. We use developmental biology as a guide to examine this phenotypic patterning, discussing candidate cues that may encode spatiotemporal information and possible roles for phenotypic patterning in biofilms. We consider other questions that arise from the comparison between biofilm and eukaryotic development, including what defines normal biofilm development and the nature of biofilm cell types and fates. We conclude by discussing what biofilm development can tell us about developmental processes, emphasizing the additional challenges faced by bacteria in biofilm development compared with their eukaryotic counterparts.

Novel ellipsoid-like granules exhibit enhanced anammox performance compared to sphere-like granules

Anammox granular sludge (AnGS) serves as an important platform for cost-effective nitrogen removal from wastewater. Different to the traditionally sphere-like granules, a novel type of AnGS in a unique ellipsoid-like shape was obtained through enhancing shear force. The ellipsoid-like AnGS significantly exhibited a smaller aspect ratio (-25.1 %) and granular size (-11.8 %), compared to traditional sphere-like AnGS (p < 0.01). Comprehensive comparisons showed that ellipsoid-like AnGS possessed a significantly higher extracellular polymeric substances (EPS) content and strength, as well as an enhanced mass transfer and a higher viable bacteria proportion due to the larger substrate permeable zone (p < 0.01). Additionally, the anammox bacterial abundance (Candidatus Kuenenia) was 12.2 % higher in ellipsoid-like AnGS than in sphere-like AnGS. All these characteristics of ellipsoid-like AnGS jointly increased the specific anammox activity by 29.0 % and nitrogen removal capacity by 22.6 %, compared to sphere-like AnGS. Further fluid field simulation suggested the enhanced flow shear on the side surface of AnGS likely drove the formation of ellipsoid-like AnGS. The higher shear force on the side surface led to an increase of EPS content (especially hydrophobic protein) and elastic modulus, thus constraining lateral expansion. This study sheds light on impacts of granular shape, an overlooked morphological factor, on anammox performance. The ellipsoid-like AnGS presented herein also offers a unique and promising aggregate to enhance anammox performance.

Effect of shear rate on early Shewanella oneidensis adhesion dynamics monitored by deep learning

Understanding pioneer bacterial adhesion is essential to appreciate bacterial colonization and consider appropriate control strategies. This bacterial entrapment at the wall is known to be controlled by many physical, chemical or biological factors, including hydrodynamic conditions. However, due to the nature of early bacterial adhesion, i.e. a short and dynamic process with low biomass involved, such investigations are challenging. In this context, our study aimed to evaluate the effect of wall shear rate on the early bacterial adhesion dynamics. Firstly, at the population scale by assessing bacterial colonization kinetics and the mechanisms responsible for wall transfer under shear rates using a time-lapse approach. Secondly, at the individual scale, by implementing an automated image processing method based on deep learning to track each individual pioneer bacterium on the wall. Bacterial adhesion experiments are performed on a model bacterium (Shewanella oneidensis MR-1) at different shear rates (0 to1250 s-1) in a microfluidic system mounted under a microscope equipped with a CCD camera. Image processing was performed using a trained neural network (YOLOv8), which allowed information extraction, i.e. bacterial wall residence time and orientation for each adhered bacterium during pioneer colonization (14 min). Collected from over 20,000 bacteria, our results showed that adhered bacteria had a very short residence time at the wall, with over 70 % remaining less than 1 min. Shear rates had a non-proportional effect on pioneer colonization with a bell-shape profile suggesting that intermediate shear rates improved both bacterial wall residence time as well as colonization rate and level. This lack of proportionality highlights the dual effect of wall shear rate on early bacterial colonization; initially increasing it improves bacterial colonization up to a threshold, beyond which it leads to higher bacterial wall detachment. The present study provides quantitative data on the individual dynamics of just adhered bacteria within a population when exposed to different rates of wall shear.

Multi-omics insights implicate the remodeling of the intestinal structure and microbiome in aging

Background

Aging can impair the ability of elderly individuals to fight infections and trigger persistent systemic inflammation, a condition known as inflammaging. However, the mechanisms underlying the development of inflammaging remain unknown.

Methods

We conducted 16S rRNA sequencing of intestinal contents from young and old C57BL/6J mice to elucidate changes in gut microbiota diversity and microbial community composition after aging. Aging-related differential bacterial taxa were then identified, and their abundance trends were validated in human samples. The variances in intestinal barrier function and circulating endotoxin between groups were also assessed. Furthermore, widely targeted metabolomics was conducted to characterize metabolic profiles after aging and to investigate the key metabolic pathways enriched by the differential metabolites.

Results

Our findings demonstrated an increase in relative proportion of pathogenic bacteria with age, a trend also revealed in healthy populations of different age groups. Additionally, aging individuals exhibited reduced intestinal barrier function and increased circulating endotoxin levels. Widely targeted metabolomics revealed a significant increase in various secondary bile acid metabolites after aging, positively correlated with the relative abundance of several aging-related bacterial taxa. Furthermore, old group had lower levels of various anti-inflammatory or beneficial metabolites. Enrichment analysis identified the starch and sucrose metabolism pathway as potentially the most significantly impacted signaling pathway during aging.

Conclusion

This study aimed to provide insights into the complex interactions involved in organismal inflammaging through microbial multi-omics. These findings lay a solid foundation for future research aimed at identifying novel biomarkers for the clinical diagnosis of aging-related diseases or potential therapeutic targets.

Nanovibrational Stimulation of Escherichia coli Mitigates Surface Adhesion by Altering Cell Membrane Potential

Mechanical forces shape living matter from the macro- to the microscale as both eukaryotic and prokaryotic cells are force wielders and sensors. However, whereas such forces have been used to control mechanically dependent behaviors in mammalian cells, we lack the same level of understanding in bacteria. Surface adhesion, the initial stages of biofilm formation and surface biofouling, is a mechanically dependent process, which makes it an ideal target for mechano-control. In this study, we employed nanometer surface vibrations to mechanically stimulate bacteria and investigate their effect on adhesion. We discovered that vibrational stimulation at the nanoscale consistently reduces surface adhesion by altering cell membrane potential. Our findings identify a link between bacteria electrophysiology and surface adhesion and provide evidence that the nanometric mechanical "tickling" of bacteria can inhibit surface adhesion.

Free-swimming bacteria transcriptionally respond to shear flow

Surface-attached cells can sense and respond to shear flow, but planktonic (free-swimming) cells are typically assumed to be oblivious to any flow that carries them. Here, we find that planktonic bacteria can transcriptionally respond to flow, inducing expression changes that are beneficial in flow. Specifically, we use microfluidic experiments and quantitative modeling to show that in the presence of flow, planktonic Pseudomonas aeruginosa induce shear rate-dependent genes that promote growth in low-oxygen environments. Untangling this mechanism revealed that in flow, motile P. aeruginosa spatially redistribute, leading to cell density changes that activate quorum sensing, which in turn enhances the oxygen uptake rate. In diffusion-limited environments, including those commonly encountered by bacteria, flow-induced cell density gradients also independently generate oxygen gradients that alter gene expression. Mutants deficient in this flow-responsive mechanism exhibit decreased fitness in flow, suggesting that this dynamic coupling of biological and mechanical processes can be physiologically significant.

Preventing bacterial adhesion to skin by altering their physicochemical cell surface properties specifically

The adhesion of bacteria to surfaces is associated with physicochemical and biological interactions. The present investigations provide new results about the differential adhesion levels of skin bacteria using a representative 3D skin model which mainly relies on the different physicochemical properties of the respective surfaces. Modulation of the adhesion of bacteria and thus their colonization, may occur by adjusting the physicochemical properties of the epidermal and bacterial surfaces. Lewis acid and hydrophobicity were the most strongly correlated parameters with the antiadhesion properties of the tested compounds. Modulation of physicochemical properties appears to be the primary driver of reduced Staphylococcus aureus adhesion in this study, with no significant changes observed in the expression of genes associated with classical adhesion pathways.

Microbiology of human spaceflight: microbial responses to mechanical forces that impact health and habitat sustainability

SUMMARYUnderstanding the dynamic adaptive plasticity of microorganisms has been advanced by studying their responses to extreme environments. Spaceflight research platforms provide a unique opportunity to study microbial characteristics in new extreme adaptational modes, including sustained exposure to reduced forces of gravity and associated low fluid shear force conditions. Under these conditions, unexpected microbial responses occur, including alterations in virulence, antibiotic and stress resistance, biofilm formation, metabolism, motility, and gene expression, which are not observed using conventional experimental approaches. Here, we review biological and physical mechanisms that regulate microbial responses to spaceflight and spaceflight analog environments from both the microbe and host-microbe perspective that are relevant to human health and habitat sustainability. We highlight instrumentation and technology used in spaceflight microbiology experiments, their limitations, and advances necessary to enable next-generation research. As spaceflight experiments are relatively rare, we discuss ground-based analogs that mimic aspects of microbial responses to reduced gravity in spaceflight, including those that reduce mechanical forces of fluid flow over cell surfaces which also simulate conditions encountered by microorganisms during their terrestrial lifecycles. As spaceflight mission durations increase with traditional astronauts and commercial space programs send civilian crews with underlying health conditions, microorganisms will continue to play increasingly critical roles in health and habitat sustainability, thus defining a new dimension of occupational health. The ability of microorganisms to adapt, survive, and evolve in the spaceflight environment is important for future human space endeavors and provides opportunities for innovative biological and technological advances to benefit life on Earth.

Electrical Signaling Beyond Neurons

Neural action potentials (APs) are difficult to interpret as signal encoders and/or computational primitives. Their relationships with stimuli and behaviors are obscured by the staggering complexity of nervous systems themselves. We can reduce this complexity by observing that "simpler" neuron-less organisms also transduce stimuli into transient electrical pulses that affect their behaviors. Without a complicated nervous system, APs are often easier to understand as signal/response mechanisms. We review examples of nonneural stimulus transductions in domains of life largely neglected by theoretical neuroscience: bacteria, protozoans, plants, fungi, and neuron-less animals. We report properties of those electrical signals-for example, amplitudes, durations, ionic bases, refractory periods, and particularly their ecological purposes. We compare those properties with those of neurons to infer the tasks and selection pressures that neurons satisfy. Throughout the tree of life, nonneural stimulus transductions time behavioral responses to environmental changes. Nonneural organisms represent the presence or absence of a stimulus with the presence or absence of an electrical signal. Their transductions usually exhibit high sensitivity and specificity to a stimulus, but are often slow compared to neurons. Neurons appear to be sacrificing the specificity of their stimulus transductions for sensitivity and speed. We interpret cellular stimulus transductions as a cell's assertion that it detected something important at that moment in time. In particular, we consider neural APs as fast but noisy detection assertions. We infer that a principal goal of nervous systems is to detect extremely weak signals from noisy sensory spikes under enormous time pressure. We discuss neural computation proposals that address this goal by casting neurons as devices that implement online, analog, probabilistic computations with their membrane potentials. Those proposals imply a measurable relationship between afferent neural spiking statistics and efferent neural membrane electrophysiology.

Gut mechanoimmunology: Shaping immune response through physical cues

The gut immune system embodies a complex interplay between the gut mucosal barrier, the host's immune cells, and gut microbiota. These components exist within a dynamic environment characterized by a variety of physical cues, e.g., compression, tension, shear stress, stiffness, and viscoelasticity. The physical cues can be modified under specific pathological conditions. Given their dynamic nature, comprehending the specific effects of these physical cues on the gut immune system is critical for pathological and therapeutic studies of intestinal immune-related diseases. This review aims to discuss how physical cues influence gut immunology by affecting the gut mucosal barrier, host immune cells, and gut microbiota, defining this concept as gut mechanoimmunology. This review seeks to highlight that an enhanced understanding of gut mechanoimmunology carries therapeutic implications, not only for intestinal diseases but also for extraintestinal diseases.

VirB11, a traffic ATPase, mediated flagella assembly and type IV pilus morphogenesis to control the motility and virulence of Xanthomonas albilineans

Xanthomonas albilineans (Xal) is a gram-negative bacterial pathogen responsible for developing sugarcane leaf scald disease, which engenders significant economic losses within the sugarcane industry. In the current study, homologous recombination exchange was carried out to induce mutations within the virB/D4-like type IV secretion system (T4SS) genes of Xal. The results revealed that the virB11-deletion mutant (ΔvirB11) exhibited a loss in swimming and twitching motility. Application of transmission electron microscopy analysis further demonstrated that the ΔvirB11 failed to develop flagella formation and type IV pilus morphology and exhibited reduced swarming behaviour and virulence. However, these alterations had no discernible impact on bacterial growth. Comparative transcriptome analysis between the wild-type Xal JG43 and the deletion-mutant ΔvirB11 revealed 123 differentially expressed genes (DEGs), of which 28 and 10 DEGs were notably associated with flagellar assembly and chemotaxis, respectively. In light of these findings, we postulate that virB11 plays an indispensable role in regulating the processes related to motility and chemotaxis in Xal.

Hosts manipulate lifestyle switch and pathogenicity heterogeneity of opportunistic pathogens in the single-cell resolution

Host-microbe interactions are virtually bidirectional, but how the host affects their microbiome is poorly understood. Here, we report that the host is a critical modulator to regulate the lifestyle switch and pathogenicity heterogeneity of the opportunistic pathogens Serratia marcescens utilizing the Drosophila and bacterium model system. First, we find that Drosophila larvae efficiently outcompete S. marcescens and typically drive a bacterial switch from pathogenicity to commensalism toward the fly. Furthermore, Drosophila larvae reshape the transcriptomic and metabolic profiles of S. marcescens characterized by a lifestyle switch. More importantly, the host alters pathogenicity and heterogeneity of S. marcescens in the single-cell resolution. Finally, we find that larvae-derived AMPs are required to recapitulate the response of S. marcescens to larvae. Altogether, our findings provide an insight into the pivotal roles of the host in harnessing the life history and heterogeneity of symbiotic bacterial cells, advancing knowledge of the reciprocal relationships between the host and pathogen.

Interfacial Stresses within Droplets and Channels Influence Bacterial Physiology: A Perspective

Bacterial cells frequently experience fluid motion in their natural environments, like water bodies, aerosols, fomites, human capillaries, etc., a phenomenon that researchers have largely overlooked. Nevertheless, some reports have suggested that the interfacial stresses caused by fluid motion inside evaporating droplets or shear flows within capillaries may trigger physiological and morphological changes in the bacterial cells. Remarkably, the virulence of bacterial cells exhibits significant alterations in response to fluctuations in stress levels and external environmental factors. The dynamics of bacterial systems are analogous to colloidal systems but with the distinction that bacterial systems exhibit responsiveness, necessitating thorough exploration in dynamic environments. In this perspective, we discuss the important issue pertaining to bacterial survival, virulence, and disease transmission. Furthermore, we delineate a pathway and underscore emerging opportunities that demand exploration to unveil new avenues in the domains of bacterial pathogenicity, drug development, and strategies for disease mitigation.

Flagellar stator genes control a trophic shift from obligate to facultative predation and biofilm formation in a bacterial predator

The bacterial predator Bdellovibrio bacteriovorus is considered to be obligatorily prey (host)-dependent (H-D), and thus unable to form biofilms. However, spontaneous host-independent (H-I) variants grow axenically and can form robust biofilms. A screen of 350 H-I mutants revealed that single mutations in stator genes fliL or motA were sufficient to generate flagellar motility-defective H-I strains able to adhere to surfaces but unable to develop biofilms. The variants showed large transcriptional shifts in genes related to flagella, prey-invasion, and cyclic-di-GMP (CdG), as well as large changes in CdG cellular concentration relative to the H-D parent. The introduction of the parental fliL allele resulted in a full reversion to the H-D phenotype, but we propose that specific interactions between stator proteins prevented functional complementation by fliL paralogs. In contrast, specific mutations in a pilus-associated protein (Bd0108) mutant background were necessary for biofilm formation, including secretion of extracellular DNA (eDNA), proteins, and polysaccharides matrix components. Remarkably, fliL disruption strongly reduced biofilm development. All H-I variants grew similarly without prey, showed a strain-specific reduction in predatory ability in prey suspensions, but maintained similar high efficiency in prey biofilms. Population-wide allele sequencing suggested additional routes to host independence. Thus, stator and invasion pole-dependent signaling control the H-D and the H-I biofilm-forming phenotypes, with single mutations overriding prey requirements, and enabling shifts from obligate to facultative predation, with potential consequences on community dynamics. Our findings on the facility and variety of changes leading to facultative predation also challenge the concept of Bdellovibrio and like organisms being obligate predators.

IMPORTANCE

The ability of bacteria to form biofilms is a central research theme in biology, medicine, and the environment. We show that cultures of the obligate (host-dependent) "solitary" predatory bacterium Bdellovibrio bacteriovorus, which cannot replicate without prey, can use various genetic routes to spontaneously yield host-independent (H-I) variants that grow axenically (as a single species, in the absence of prey) and exhibit various surface attachment phenotypes, including biofilm formation. These routes include single mutations in flagellar stator genes that affect biofilm formation, provoke motor instability and large motility defects, and disrupt cyclic-di-GMP intracellular signaling. H-I strains also exhibit reduced predatory efficiency in suspension but high efficiency in prey biofilms. These changes override the requirements for prey, enabling a shift from obligate to facultative predation, with potential consequences on community dynamics.

A Model for Mechanical Stress Limited Bacterial Growth and Resporulation in Confinement

In this study, we propose a self-limiting growth model forBacillus subtilisspores confined within porous polyacrylamide (PA) hydrogels. We observed thatB. subtilisspores germinate into vegetative cells within the hydrogel matrix, forming spherical colonies. These colonies expand until the mechanical stress they exert on their environment surpasses the yield stress of the hydrogel, leading to formation of a nonpermeable layer that halts nutrient diffusion and forces the bacteria to resporulate. These novel observations suggest a model to explain why bacterial growth in confined environments and material interfaces may be limited, providing insight for natural phenomena and biotechnological applications involving bacterial encapsulation.

Mechanobiology of bacterial biofilms: Implications for orthopedic infection

Postoperative bacterial infections are prevalent complications in both human and veterinary orthopedic surgery, particularly when a biofilm develops. These infections often result in delayed healing, early revision, permanent functional loss, and, in severe cases, amputation. The diagnosis and treatment pose significant challenges, and bacterial biofilm further amplifies the therapeutic difficulty as it confers protection against the host immune system and against antibiotics which are usually administered as a first-line therapeutic option. However, the inappropriate use of antibiotics has led to the emergence of numerous multidrug-resistant organisms, which largely compromise the already imperfect treatment efficiency. In this context, the study of bacterial biofilm formation allows to better target antibiotic use and to evaluate alternative therapeutic strategies. Exploration of the roles played by mechanical factors on biofilm development is of particular interest, especially because cartilage and bone tissues are reactive environments that are subjected to mechanical load. This review delves into the current landscape of biofilm mechanobiology, exploring the role of mechanical factors on biofilm development through a multiscale prism starting from bacterial microscopic scale to reach biofilm mesoscopic size and finally the macroscopic scale of the fracture site or bone-implant interface.

A bacterial sense of touch: T4P retraction motor as a means of surface sensing by Pseudomonas aeruginosa PA14

Most microbial cells found in nature exist in matrix-covered, surface-attached communities known as biofilms. This mode of growth is initiated by the ability of the microbe to sense a surface on which to grow. The opportunistic pathogen Pseudomonas aeruginosa (Pa) PA14 utilizes a single polar flagellum and type 4 pili (T4P) to sense surfaces. For Pa, T4P-dependent "twitching" motility is characterized by effectively pulling the cell across a surface through a complex process of cooperative binding, pulling, and unbinding. T4P retraction is powered by hexameric ATPases. Pa cells that have engaged a surface increase production of the second messenger cyclic AMP (cAMP) over multiple generations via the Pil-Chp system. This rise in cAMP allows cells and their progeny to become better adapted for surface attachment and activates virulence pathways through the cAMP-binding transcription factor Vfr. While many studies have focused on mechanisms of T4P twitching and regulation of T4P production and function by the Pil-Chp system, the mechanism by which Pa senses and relays a surface-engagement signal to the cell is still an open question. Here we review the current state of the surface sensing literature for Pa, with a focus on T4P, and propose an integrated model of surface sensing whereby the retraction motor PilT senses and relays the signal to the Pil-Chp system via PilJ to drive cAMP production and adaptation to a surface lifestyle.

Fluid flow drives phenotypic heterogeneity in bacterial growth and adhesion on surfaces

Bacteria often thrive in surface-attached communities, where they can form biofilms affording them multiple advantages. In this sessile form, fluid flow is a key component of their environments, renewing nutrients and transporting metabolic products and signaling molecules. It also controls colonization patterns and growth rates on surfaces, through bacteria transport, attachment and detachment. However, the current understanding of bacterial growth on surfaces neglects the possibility that bacteria may modulate their division behavior as a response to flow. Here, we employed single-cell imaging in microfluidic experiments to demonstrate that attached Escherichia coli cells can enter a growth arrest state while simultaneously enhancing their adhesion underflow. Despite utilizing clonal populations, we observed a non-uniform response characterized by bistable dynamics, with co-existing subpopulations of non-dividing and actively dividing bacteria. As the proportion of non-dividing bacteria increased with the applied flow rate, it resulted in a reduction in the average growth rate of bacterial populations on flow-exposed surfaces. Dividing bacteria exhibited asymmetric attachment, whereas non-dividing counterparts adhered to the surface via both cell poles. Hence, this phenotypic diversity allows bacterial colonies to combine enhanced attachment with sustained growth, although at a reduced rate, which may be a significant advantage in fluctuating flow conditions.

Bacterial envelope polysaccharide cues settlement and metamorphosis in the biofouling tubeworm Hydroides elegans

Metamorphosis for many marine invertebrates is triggered by external cues, commonly produced by bacteria. For larvae of Hydroides elegans, lipopolysaccharide (LPS) from the biofilm-dwelling bacterium Cellulophaga lytica induces metamorphosis. To determine whether bacterial LPS is a common metamorphosis-inducing factor for this species, we compare larval responses to LPS from 3 additional inductive Gram-negative marine biofilm bacteria with commercially available LPS from 3 bacteria not known to induce metamorphosis. LPS from all the inductive bacteria trigger metamorphosis, while LPS from non-inductive isolated marine bacteria do not. We then ask, which part of the LPS is the inductive element, the lipid (Lipid-A) or the polysaccharide (O-antigen), and find it is the latter for all four inductive bacteria. Finally, we examine the LPS subunits from two strains of the same bacterial species, one inductive and the other not, and find the LPS and O-antigen to be inductive from only the inductive bacterial strain.

Side-view optical microscopy-assisted atomic force microscopy for thickness-dependent nanobiomechanics

The mechanical properties of biomaterials play an important role in regulating life processes, and thus accurately delineating the mechanical properties of biomaterials is critical to understand their functionality. Particularly, atomic force microscopy (AFM) has become a powerful and standard tool for characterizing and analyzing the nanomechanical properties of biomaterials, and providing a capability to visualize the thickness of the specimen during AFM-based force spectroscopy experiments benefits the biomedical applications of AFM. Here, we present a study of side-view optical microscopy-assisted AFM based on the integration of AFM and a detachable side-view optical microscopy module, which is able to image in real time the AFM indentation process from the side-view perspective and consequently facilitates the utilization of AFM-based indentation assay to precisely detect the mechanical properties of a specimen by taking its thickness into account. The effectiveness of side-view optical microscopy-assisted AFM was confirmed on four different types of biomaterial systems, including microfabricated structures, hydrogels, living cells, and cell spheroids, and the experimental results significantly show that the mechanical properties of samples at the micro/nanoscale are closely related to their thickness, vividly illustrating side-view optical microscopy-assisted AFM as a promising approach for accurate nanomechanics of biomaterial systems. The study provides additional possibilities for measuring the thickness-dependent nanomechanical properties of biomaterials by AFM, which will enable AFM-based force spectroscopy technology to address more biological issues with enhanced precision and will benefit the field of mechanobiology.

Nanoscale Spatiotemporal Dynamics of Microbial Adhesion: Unveiling Stepwise Transitions with Plasmonic Imaging

Understanding bacterial adhesion at the nanoscale is crucial for elucidating biofilm formation, enhancing biosensor performance, and designing advanced biomaterials. However, the dynamics of the critical transition from reversible to irreversible adhesion has remained elusive due to analytical constraints. Here, we probed this adhesion transition, unveiling nanoscale, step-like bacterial approaches to substrates using a plasmonic imaging technique. This method reveals the discontinuous nature of adhesion, emphasizing the complex interplay between bacterial extracellular polymeric substances (EPS) and substrates. Our findings not only deepen our understanding of bacterial adhesion but also have significant implications for the development of theoretical models for biofilm management. By elucidating these nanoscale step-like adhesion processes, our work provides avenues for the application of nanotechnology in biosensing, biofilm control, and the creation of biomimetic materials.

Analysis of the urine flow characteristics inside catheters for intermittent catheter selection

In this study, we conducted a numerical analysis on catheter sizes using computational fluid dynamics to assess urinary flow rates during intermittent catheterization (IC). The results revealed that the fluid (urine) movement within a catheter is driven by intravesical pressure, with friction against the catheter walls being the main hindrance to fluid movement. Higher-viscosity fluids experienced increased friction with increasing intravesical pressure, resulting in reduced fluid velocity, whereas lower-viscosity fluids experienced reduced friction under similar pressure, leading to increased fluid velocity. Regarding urine characteristics, the results indicated that bacteriuria, with lower viscosity, exhibited higher flow rates, whereas glucosuria exhibited the lowest flow rates. Additionally, velocity gradients decreased with increasing catheter diameters, reducing friction and enhancing fluid speed, while the friction increased with decreasing diameters, reducing fluid velocity. These findings confirm that flow rates increased with larger catheter sizes. Furthermore, in terms of specific gravity, the results showed that a 12Fr catheter did not meet the ISO-suggested average flow rate (50 cc/min). The significance of this study lies in its application of fluid dynamics to nursing, examining urinary flow characteristics in catheterization. It is expected to aid nurses in selecting appropriate catheters for intermittent catheterization based on urinary test results.

Fluid flow overcomes antimicrobial resistance by boosting delivery

Antimicrobial resistance is an emerging global threat to humanity. As resistance outpaces development, new perspectives are required. For decades, scientists have prioritized chemical optimization, while largely ignoring the physical process of delivery. Here, we used biophysical simulations and microfluidic experiments to explore how fluid flow delivers antimicrobials into communities of the highly resistant pathogen Pseudomonas aeruginosa . We discover that increasing flow overcomes bacterial resistance towards three chemically distinct antimicrobials: hydrogen peroxide, gentamicin, and carbenicillin. Without flow, resistant P. aeruginosa cells generate local zones of depletion by neutralizing all three antimicrobials through degradation or chemical modification. As flow increases, delivery overwhelms neutralization, allowing antimicrobials to regain effectiveness against resistant bacteria. Additionally, we discover that cells on the edge of a community shield internal cells, and cell-cell shielding is abolished in higher flow regimes. Collectively, our quantitative experiments reveal the unexpected result that physical flow and chemical dosage are equally important to antimicrobial effectiveness. Thus, our results should inspire the incorporation of flow into the discovery, development, and implementation of antimicrobials, and could represent a new strategy to combat antimicrobial resistance.

Agent-based modeling of stress anisotropy driven nematic ordering in growing biofilms

Living active collectives have evolved with remarkable self-patterning capabilities to adapt to the physical and biological constraints crucial for their growth and survival. However, the intricate process by which complex multicellular patterns emerge from a single founder cell remains elusive. In this study, we utilize an agent-based model, validated through single-cell microscopy imaging, to track the three-dimensional (3D) morphodynamics of cells within growing bacterial biofilms encased by agarose gels. The confined growth conditions give rise to a spatiotemporally heterogeneous stress landscape within the biofilm. In the core of the biofilm, where high hydrostatic and low shear stresses prevail, cell packing appears disordered. In contrast, near the gel-cell interface, a state of high shear stress and low hydrostatic stress emerges, driving nematic ordering, albeit with a time delay inherent to shear stress relaxation. Strikingly, we observe a robust spatiotemporal correlation between stress anisotropy and nematic ordering within these confined biofilms. This correlation suggests a mechanism whereby stress anisotropy plays a pivotal role in governing the spatial organization of cells. The reciprocity between stress anisotropy and cell ordering in confined biofilms opens new avenues for innovative 3D mechanically guided patterning techniques for living active collectives, which hold significant promise for a wide array of environmental and biomedical applications.

Interplay between environmental yielding and dynamic forcing modulates bacterial growth

Many bacterial habitats-ranging from gels and tissues in the body to cell-secreted exopolysaccharides in biofilms-are rheologically complex, undergo dynamic external forcing, and have unevenly distributed nutrients. How do these features jointly influence how the resident cells grow and proliferate? Here, we address this question by studying the growth of Escherichia coli dispersed in granular hydrogel matrices with defined and highly tunable structural and rheological properties, under different amounts of external forcing imposed by mechanical shaking, and in both aerobic and anaerobic conditions. Our experiments establish a general principle: that the balance between the yield stress of the environment that the cells inhabit, σy, and the external stress imposed on the environment, σ, modulates bacterial growth by altering transport of essential nutrients to the cells. In particular, when σy<σ, the environment is easily fluidized and mixed over large scales, providing nutrients to the cells and sustaining complete cellular growth. By contrast, when σy>σ, the elasticity of the environment suppresses large-scale fluid mixing, limiting nutrient availability and arresting cellular growth. Our work thus reveals a new mechanism, beyond effects that change cellular behavior via local forcing, by which the rheology of the environment may modulate microbial physiology in diverse natural and industrial settings.

High-throughput measurement of elastic moduli of microfibers by rope coiling

There are many fields where it is of interest to measure the elastic moduli of tiny fragile fibers, such as filamentous bacteria, actin filaments, DNA, carbon nanotubes, and functional microfibers. The elastic modulus is typically deduced from a sophisticated tensile test under a microscope, but the throughput is low and limited by the time-consuming and skill-intensive sample loading/unloading. Here, we demonstrate a simple microfluidic method enabling the high-throughput measurement of the elastic moduli of microfibers by rope coiling using a localized compression, where sample loading/unloading are not needed between consecutive measurements. The rope coiling phenomenon occurs spontaneously when a microfiber flows from a small channel into a wide channel. The elastic modulus is determined by measuring either the buckling length or the coiling radius. The throughput of this method, currently 3,300 fibers per hour, is a thousand times higher than that of a tensile tester. We demonstrate the feasibility of the method by testing a nonuniform fiber with axially varying elastic modulus. We also demonstrate its capability for in situ inline measurement in a microfluidic production line. We envisage that high-throughput measurements may facilitate potential applications such as screening or sorting by mechanical properties and real-time control during production of microfibers.

Surface (bio)-functionalization of metallic materials: How to cope with real interfaces?

Metallic materials are an important class of biomaterials used in various medical devices, owing to a suitable combination of their mechanical properties. The (bio)-functionalization of their surfaces is frequently performed for biocompatibility requirements, as it offers a powerful way to control their interaction with biological systems. This is particularly important when physicochemical processes and biological events, mainly involving proteins and cells, are initiated at the host-material interface. This review addresses the state of "real interfaces" in the context of (bio)-functionalization of metallic materials, and the necessity to cope with it to avoid frequent improper evaluation of the procedure used. This issue is, indeed, well-recognized but often neglected and emerges from three main issues: (i) ubiquity of surface contamination with organic compounds, (ii) reactivity of metallic surfaces in biological medium, and (iii) discrepancy in (bio)-functionalization procedures between expectations and reality. These disturb the assessment of the strategies adopted for surface modifications and limit the possibilities to provide guidelines for their improvements. For this purpose, X-ray photoelectrons spectroscopy (XPS) comes to the rescue. Based on significant progresses made in methodological developments, and through a large amount of data compiled to generate statistically meaningful information, and to insure selectivity, precision and accuracy, the state of "real interfaces" is explored in depth, while looking after the two main constituents: (i) the bio-organic adlayer, in which the discrimination between the compounds of interest (anchoring molecules, coupling agents, proteins, etc) and organic contaminants can be made, and (ii) the metallic surface, which undergoes dynamic processes due to their reactivity. Moreover, through one of the widespread (bio)-functionalization strategy, given as a case study, a particular attention is devoted to describe the state of the interface at different stages (composition, depth distribution of contaminants and (bio)compounds of interest) and the mode of protein retention. It is highlighted, in particular, that the occurrence or improvement of bioactivity does not demonstrate that the chemical schemes worked in reality. These aspects are particularly essential to make progress on the way to choose the suitable (bio)-functionalization strategy and to provide guidelines to improve its efficiency.

Mathematical modeling of mechanosensitive reversal control in Myxococcus xanthus

Adjusting motility patterns according to environmental cues is important for bacterial survival. Myxococcus xanthus, a bacterium moving on surfaces by gliding and twitching mechanisms, modulates the reversal frequency of its front-back polarity in response to mechanical cues like substrate stiffness and cell-cell contact. In this study, we propose that M. xanthus's gliding machinery senses environmental mechanical cues during force generation and modulates cell reversal accordingly. To examine our hypothesis, we expand an existing mathematical model for periodic polarity reversal in M. xanthus, incorporating the experimental data on the intracellular dynamics of the gliding machinery and the interaction between the gliding machinery and a key polarity regulator. The model successfully reproduces the dependence of cell reversal frequency on substrate stiffness observed in M. xanthus gliding. We further propose reversal control networks between the gliding and twitching motility machineries to explain the opposite reversal responses observed in wild type M. xanthus cells that possess both motility mechanisms. These results provide testable predictions for future experimental investigations. In conclusion, our model suggests that the gliding machinery in M. xanthus can function as a mechanosensor, which transduces mechanical cues into a cell reversal signal.

Flagella are required to coordinately activate competition and host colonization factors in response to a mechanical signal

Bacteria employ antagonistic strategies to eliminate competitors of an ecological niche. Contact-dependent mechanisms, such as the type VI secretion system (T6SS), are prevalent in host-associated bacteria, yet we know relatively little about how T6SS+ strains make contact with competitors in highly viscous environments, such as host mucus. To better understand how cells respond to and contact one another in such environments, we performed a genome-wide transposon mutant screen of the T6SS-wielding beneficial bacterial symbiont, Vibrio fischeri, and identified two sets of genes that are conditionally required for killing. LPS/capsule and flagellar-associated genes do not affect T6SS directly and are therefore not required for interbacterial killing when cell contact is forced yet are necessary for killing in high-viscosity liquid (hydrogel) where cell-cell contact must be biologically mediated. Quantitative transcriptomics revealed that V. fischeri significantly increases expression of both T6SS genes and cell surface modification factors upon transition from low- to high-viscosity media. Consistent with coincubation and fluorescence microscopy data, flagella are not required for T6SS expression in hydrogel. However, flagella play a key role in responding to the physical environment by promoting expression of the surface modification genes identified in our screen, as well as additional functional pathways important for host colonization including uptake of host-relevant iron and carbon sources, and nitric oxide detoxification enzymes. Our findings suggest that flagella may act as a mechanosensor for V. fischeri to coordinately activate competitive strategies and host colonization factors, underscoring the significance of the physical environment in directing complex bacterial behaviors.

Single-cell force spectroscopy of fluid flow-tuned cell adhesion for dissecting hemodynamics in tumor metastasis

Cell adhesion plays an important role in regulating the metastasis of cancer cells, and atomic force microscopy (AFM)-based single-cell force spectroscopy (SCFS) has become an important method to directly measure the adhesion forces of individual cells. Particularly, bodily fluid flow environments strongly affect the functions and behaviors of metastatic cells for successful dissemination. Nevertheless, the interactions between fluidic flow medium environment and cell adhesion remain poorly understood. In this work, AFM-based SCFS was exploited to examine the effects of fluidic flow environment on cellular adhesion. A fluidic cell culture medium device was used to simulate the fluidic flow environment experienced by cancer cells during metastasis, which was combined with AFM-based SCFS assay. A single living cancer cell was attached to the AFM tipless cantilever to prepare the single-cell probe for performing SCFS experiments on the mesothelial cells grown under the fluidic flow medium conditions, and the effects of experimental parameters (retraction speed, contact time, loading force) on the measured cellular adhesion forces were analyzed. Experimental results of SCFS assay show that cellular adhesion forces significantly decrease after growth in fluidic flow medium, whereas cellular adhesion forces increase after growth in static culture medium. Experiments performed with the use of spherical probes coated with cell adhesion-associated biomolecules also show the weakening of cell adhesion after growth in fluidic flow cell culture medium, which was subsequently confirmed by the confocal fluorescence microscopy experiments of cell adhesion molecules, vividly illustrating the remarkable effects of fluidic flow environment on cellular adhesion. The study provides a new approach to detect adhesion force dynamics involved in the interactions between cells and the fluidic flow environment at the single-cell level, which will facilitate dissecting the role of hemodynamics in tumor metastasis.

Elasticity tunes mechanical stress localization around active topological defects

Mechanical stresses are increasingly found to be associated with various biological functionalities. At the same time, topological defects are being identified across a diverse range of biological systems and are points of localized mechanical stress. It is therefore important to ask how mechanical stress localization around topological defects is controlled. Here, we use continuum simulations of nonequilibrium, fluctuating and active nematics to explore the patterns of stress localization, as well as their extent and intensity around topological defects. We find that by increasing the orientational elasticity of the material, the isotropic stress pattern around topological defects is changed substantially, from a stress dipole characterized by symmetric compression-tension regions around the core of the defect, to a localized stress monopole at the defect position. Moreover, we show that elastic anisotropy alters the extent and intensity of the stresses, and can result in the dominance of tension or compression around defects. Finally, including both nonequilibrium fluctuations and active stress generation, we find that the elastic constant tunes the relative effect of each, leading to the flipping of tension and compression regions around topological defects. This flipping of the tension-compression regions only by changing the elastic constant presents an interesting, simple, way of switching the dynamic behavior in active matter by changing a passive material property. We expect these findings to motivate further exploration tuning stresses in active biological materials by varying material properties of the constituent units.

Atomic force microscopy-mediated mechanobiological profiling of complex human tissues

Tissue mechanobiology is an emerging field with the overarching goal of understanding the interplay between biophysical and biochemical responses affecting development, physiology, and disease. Changes in mechanical properties including stiffness and viscosity have been shown to describe how cells and tissues respond to mechanical cues and modify critical biological functions. To quantitatively characterize the mechanical properties of tissues at physiologically relevant conditions, atomic force microscopy (AFM) has emerged as a highly versatile biomechanical technology. In this review, we describe the fundamental principles of AFM, typical AFM modalities used for tissue mechanics, and commonly used elastic and viscoelastic contact mechanics models to characterize complex human tissues. Furthermore, we discuss the application of AFM-based mechanobiology to characterize the mechanical responses within complex human tissues to track their developmental, physiological/functional, and diseased states, including oral, hearing, and cancer-related tissues. Finally, we discuss the current outlook and challenges to further advance the field of tissue mechanobiology. Altogether, AFM-based tissue mechanobiology provides a mechanistic understanding of biological processes governing the unique functions of tissues.

Biofilms as self-shaping growing nematics

Active nematics are the nonequilibrium analogue of passive liquid crystals. They consist of anisotropic units that consume free energy to drive emergent behaviour. Like liquid crystal molecules in displays, ordering and dynamics in active nematics are sensitive to boundary conditions. However, unlike passive liquid crystals, active nematics have the potential to regulate their boundaries through self-generated stresses. Here, we show how a three-dimensional, living nematic can actively shape itself and its boundary to regulate its internal architecture through growth-induced stresses, using bacterial biofilms confined by a hydrogel as a model system. We show that biofilms exhibit a sharp transition in shape from domes to lenses upon changing environmental stiffness or cell-substrate friction, which is explained by a theoretical model that considers the competition between confinement and interfacial forces. The growth mode defines the progression of the boundary, which in turn determines the trajectories and spatial distribution of cell lineages. We further demonstrate that the evolving boundary and corresponding stress anisotropy define the orientational ordering of cells and the emergence of topological defects in the biofilm interior. Our findings may provide strategies for the development of programmed microbial consortia with emergent material properties.

Mechanobiology: How pathogens use mechanics to modulate host interactions

Mechanobiology explores how cells sense and respond to mechanical cues and how mechanics guide cell function, physiology, and disease. In this issue of Cell, Thacker and colleagues reveal how the tuberculosis-causing pathogen exploits the mechanical behavior of cord-like structures to promote infection, impacting immune response, antibiotic susceptibility, and treatment strategies.