A modified motor-clutch model reveals that neuronal growth cones respond faster to soft substrates.

Neuronal growth cones sense a variety of cues including chemical and mechanical ones to establish functional connections during nervous system development. Substrate-cytoskeletal coupling is an established model for adhesion-mediated growth cone advance; however, the detailed molecular and biophysical mechanisms underlying the mechanosensing and mechanotransduction process remain unclear. Here, we adapted a motor-clutch model to better understand the changes in clutch and cytoskeletal dynamics, traction forces, and substrate deformation when a growth cone interacts with adhesive substrates of different stiffnesses. Model parameters were optimized using experimental data from Aplysia growth cones probed with force-calibrated glass microneedles. We included a reinforcement mechanism at both motor and clutch level. Furthermore, we added a threshold for retrograde F-actin flow that indicates when the growth cone is strongly coupled to the substrate. Our modeling results are in strong agreement with experimental data with respect to the substrate deformation and the latency time after which substrate-cytoskeletal coupling is strong enough for the growth cone to advance. Our simulations show that it takes the shortest time to achieve strong coupling when substrate stiffness was low at 4 pN/nm. Taken together, these results suggest that Aplysia growth cones respond faster and more efficiently to soft than stiff substrates.

Probiotics: insights and new opportunities for Clostridioides difficile intervention.

Clostridioides difficile infection (CDI) is a life-threatening disease caused by the Gram-positive, opportunistic intestinal pathogen C. difficile. Despite the availability of antimicrobial drugs to treat CDI, such as vancomycin, metronidazole, and fidaxomicin, recurrence of infection remains a significant clinical challenge. The use of live commensal microorganisms, or probiotics, is one of the most investigated non-antibiotic therapeutic options to balance gastrointestinal (GI) microbiota and subsequently tackle dysbiosis. In this review, we will discuss major commensal probiotic strains that have the potential to prevent and/or treat CDI and its recurrence, reassess the efficacy of probiotics supplementation as a CDI intervention, delve into lessons learned from probiotic modulation of the immune system, explore avenues like genome-scale metabolic network reconstructions, genome sequencing, and multi-omics to identify novel strains and understand their functionality, and discuss the current regulatory framework, challenges, and future directions.

Curcumin: A natural derivative with antibacterial activity against Clostridium difficile.

OBJECTIVES: The rapid emergence of hypervirulent Clostridium difficile (C. difficile) isolates and the paucity of effective anti-clostridial antibiotics call for extensive research to identify new treatment options. This study aimed to test the anti-clostridial activity of bioactive extracts of turmeric, which is a natural herb widely known for its profound medicinal properties. METHODS: The MICs of turmeric derivatives were determined against 27 C. difficile strains, including hypervirulent (BI/NAP1/027) and clinical toxigenic isolates. Additionally, their ability to inhibit C. difficile toxin production and spore formation was investigated. Furthermore, the safety profiles of turmeric derivatives regarding their effects on human gut microflora - such as Bacteroides, Lactobacillus and Bifidobacterium - were evaluated. RESULTS: Curcuminoids, the major phytoconstituents of turmeric - including curcumin, demethoxycurcumin and bisdemethoxycurcumin - inhibited growth of C. difficile at concentrations ranging from 4 to 32µg/mL. Additionally, curcuminoids showed no negative effect on major populating species of the human gut. Curcumin was more effective than fidaxomicin in inhibiting C. difficile toxin production, but less so in inhibiting spore formation. CONCLUSION: The findings suggest that curcumin has potential as an anti-clostridial agent. More work is needed to further investigate the efficacy of curcumin as a stand-alone drug or as a supplement of current drugs of choice, as it has no antagonistic activities but might overcome their drawbacks.

Mapping heterogeneity of cellular mechanics by multi-harmonic atomic force microscopy.

The goal of mechanobiology is to understand the links between changes in the physical properties of living cells and normal physiology and disease. This requires mechanical measurements that have appropriate spatial and temporal resolution within a single cell. Conventional atomic force microscopy (AFM) methods that acquire force curves pointwise are used to map the heterogeneous mechanical properties of cells. However, the resulting map acquisition time is much longer than that required to study many dynamic cellular processes. Dynamic AFM (dAFM) methods using resonant microcantilevers are compatible with higher-speed, high-resolution scanning; however, they do not directly acquire force curves and they require the conversion of a limited number of instrument observables to local mechanical property maps. We have recently developed a technique that allows commercial AFM systems equipped with direct cantilever excitation to quantitatively map the viscoelastic properties of live cells. The properties can be obtained at several widely spaced frequencies with nanometer-range spatial resolution and with fast image acquisition times (tens of seconds). Here, we describe detailed procedures for quantitative mapping, including sample preparation, AFM calibration, and data analysis. The protocol can be applied to different biological samples, including cells and viruses. The transition from dAFM imaging to quantitative mapping should be easily achievable for experienced AFM users, who will be able to set up the protocol in <30 min.

Neurite elongation is highly correlated with bulk forward translocation of microtubules.

During the development of the nervous system and regeneration following injury, microtubules (MTs) are required for neurite elongation. Whether this elongation occurs primarily through tubulin assembly at the tip of the axon, the transport of individual MTs, or because MTs translocate forward in bulk is unclear. Using fluorescent speckle microscopy (FSM), differential interference contrast (DIC), and phase contrast microscopy, we tracked the movement of MTs, phase dense material, and docked mitochondria in chick sensory and Aplysia bag cell neurons growing rapidly on physiological substrates. In all cases, we find that MTs and other neuritic components move forward in bulk at a rate that on average matches the velocity of neurite elongation. To better understand whether and why MT assembly is required for bulk translocation, we disrupted it with nocodazole. We found this blocked the forward bulk advance of material along the neurite and was paired with a transient increase in axonal tension. This indicates that disruption of MT dynamics interferes with neurite outgrowth, not by disrupting the net assembly of MTs at the growth cone, but rather because it alters the balance of forces that power the bulk forward translocation of MTs.

Substrate Deformation Predicts Neuronal Growth Cone Advance.

Although pulling forces have been observed in axonal growth for several decades, their underlying mechanisms, absolute magnitudes, and exact roles are not well understood. In this study, using two different experimental approaches, we quantified retrograde traction force in Aplysia californica neuronal growth cones as they develop over time in response to a new adhesion substrate. In the first approach, we developed a novel method, to our knowledge, for measuring traction forces using an atomic force microscope (AFM) with a cantilever that was modified with an Aplysia cell adhesion molecule (apCAM)-coated microbead. In the second approach, we used force-calibrated glass microneedles coated with apCAM ligands to guide growth cone advance. The traction force exerted by the growth cone was measured by monitoring the microneedle deflection using an optical microscope. Both approaches showed that Aplysia growth cones can develop traction forces in the 10(0)-10(2) nN range during adhesion-mediated advance. Moreover, our results suggest that the level of traction force is directly correlated to the stiffness of the microneedle, which is consistent with a reinforcement mechanism previously observed in other cell types. Interestingly, the absolute level of traction force did not correlate with growth cone advance toward the adhesion site, but the amount of microneedle deflection did. In cases of adhesion-mediated growth cone advance, the mean needle deflection was 1.05 ± 0.07 µm. By contrast, the mean deflection was significantly lower (0.48 ± 0.06 µm) when the growth cones did not advance. Our data support a hypothesis that adhesion complexes, which can undergo micron-scale elastic deformation, regulate the coupling between the retrogradely flowing actin cytoskeleton and apCAM substrates, stimulating growth cone advance if sufficiently abundant.

Quantifying mechanical force in axonal growth and guidance.

Mechanical force plays a fundamental role in neuronal development, physiology, and regeneration. In particular, research has shown that force is involved in growth cone-mediated axonal growth and guidance as well as stretch-induced elongation when an organism increases in size after forming initial synaptic connections. However, much of the details about the exact role of force in these fundamental processes remain unknown. In this review, we highlight: (1) standing questions concerning the role of mechanical force in axonal growth and guidance; and (2) different experimental techniques used to quantify forces in axons and growth cones. We believe that satisfying answers to these questions will require quantitative information about the relationship between elongation, forces, cytoskeletal dynamics, axonal transport, signaling, substrate adhesion, and stiffness contributing to directional growth advance. Furthermore, we address why a wide range of force values have been reported in the literature, and what these values mean in the context of neuronal mechanics. We hope that this review will provide a guide for those interested in studying the role of force in development and regeneration of neuronal networks.

Near-real-time analysis of the phenotypic responses of Escherichia coli to 1-butanol exposure using Raman Spectroscopy.

Raman spectroscopy was used to study the time course of phenotypic responses of Escherichia coli (DH5α) to 1-butanol exposure (1.2% [vol/vol]). Raman spectroscopy is of interest for bacterial phenotyping because it can be performed (i) in near real time, (ii) with minimal sample preparation (label-free), and (iii) with minimal spectral interference from water. Traditional off-line analytical methodologies were applied to both 1-butanol-treated and control cells to draw correlations with Raman data. Here, distinct sets of Raman bands are presented that characterize phenotypic traits of E. coli with maximized correlation to off-line measurements. In addition, the observed time course phenotypic responses of E. coli to 1.2% (vol/vol) 1-butanol exposure included the following: (i) decreased saturated fatty acids levels, (ii) retention of unsaturated fatty acids and low levels of cyclopropane fatty acids, (iii) increased membrane fluidity following the initial response of increased rigidity, and (iv) no changes in total protein content or protein-derived amino acid composition. For most phenotypic traits, correlation coefficients between Raman spectroscopy and traditional off-line analytical approaches exceeded 0.75, and major trends were captured. The results suggest that near-real-time Raman spectroscopy is suitable for approximating metabolic and physiological phenotyping of bacterial cells subjected to toxic environmental conditions.

Deriving metabolic engineering strategies from genome-scale modeling with flux ratio constraints.

Optimized production of bio-based fuels and chemicals from microbial cell factories is a central goal of systems metabolic engineering. To achieve this goal, a new computational method of using flux balance analysis with flux ratios (FBrAtio) was further developed in this research and applied to five case studies to evaluate and design metabolic engineering strategies. The approach was implemented using publicly available genome-scale metabolic flux models. Synthetic pathways were added to these models along with flux ratio constraints by FBrAtio to achieve increased (i) cellulose production from Arabidopsis thaliana; (ii) isobutanol production from Saccharomyces cerevisiae; (iii) acetone production from Synechocystis sp. PCC6803; (iv) H2 production from Escherichia coli MG1655; and (v) isopropanol, butanol, and ethanol (IBE) production from engineered Clostridium acetobutylicum. The FBrAtio approach was applied to each case to simulate a metabolic engineering strategy already implemented experimentally, and flux ratios were continually adjusted to find (i) the end-limit of increased production using the existing strategy, (ii) new potential strategies to increase production, and (iii) the impact of these metabolic engineering strategies on product yield and culture growth. The FBrAtio approach has the potential to design "fine-tuned" metabolic engineering strategies in silico that can be implemented directly with available genomic tools.

Peptide-guided surface-enhanced Raman scattering probes for localized cell composition analysis.

The ability to control the localization of surface-enhanced Raman scattering (SERS) nanoparticle probes in bacterial cells is critical to the development of analytical techniques that can nondestructively determine cell composition and phenotype. Here, selective localization of SERS probes was achieved at the outer bacterial membrane by using silver nanoparticles functionalized with synthetic hydrophobic peptides.