Surface ultrastructure and elasticity in growing tips and mature regions of Aspergillus hyphae describe wall maturation

Analysis of Cell Wall Mechanics in Fission Yeast

The growth and shape of fungal cells, such as fission yeast, are strongly constrained by the mechanics of their cell wall (CW). The cell wall encases the plasma membrane and defines instantaneous cell shapes by opposing turgor pressure-derived stress on the cell surface. Measuring cell wall mechanical properties may thus bring key insights into the regulation of cell morphogenesis, cell growth, but also cell surface integrity and survival. The fission yeast cell wall has a thickness of a few tens to hundreds of nanometers, and bulk elasticity similar to that of rubber (tens of MPa). These mechanical properties vary locally around single cells, for instance, at the new vs. old growing ends, or birth scars, and may also largely depend on growth conditions and life cycle phases. While cell wall thickness and mechanics have been traditionally measured by complex methodologies including electron microscopy and atomic force microscopy, we here propose a method based on light microscopy to infer with medium-throughput cell wall mechanical properties, as well as turgor pressure in time and space in living cells. This analysis will enhance our appreciation of the mechanical regulation of fission yeast cell morphogenesis and may be directly transferable to the study of other fungal cells.

Highly parallel bending tests for fungal hyphae enabled by two-photon polymerization of microfluidic mold

Filamentous microorganisms exhibit a complex macro-morphology constituted of branched and cross-linked hyphae. Fully resolved mechanical models of such mycelial compounds rely heavily on accurate input data for mechanical properties of individual hyphae. Due to their irregular shape and high adaptability to environmental factors, the measurement of these intrinsic properties remains challenging. To overcome previous shortcomings of microfluidic bending tests, a novel system for the precise measurement of the individual bending stiffness of fungal hyphae is presented in this study. Utilizing two-photon polymerization, microfluidic molds were fabricated with a multi-material approach, enabling the creation of 3D cell traps for spore immobilization. Unlike previous works applying the methodology of microfluidic bending tests, the hyphae were deflected in the vertical center of the microfluidic channel, eliminating the adverse influence of nearby walls on measurements. This lead to a significant increase in measurement yield compared to the conventional design. The accuracy and reproducibility of bending tests was ensured through validation of the measurement flow using micro-particle image velocimetry. Our results revealed that the bending stiffness of hyphae of Aspergillus niger is approximately three to four times higher than that reported for Candida albicans hyphae. At the same time, the derived longitudinal Young's Modulus of the hyphal cell wall yields a comparable value for both organisms. The methodology established in this study provides a powerful tool for studying the effects of cultivation conditions on the intrinsic mechanical properties of single hyphae. Applying the results to resolved numerical models of mycelial compounds promises to shed light on their response to hydrodynamic stresses in biotechnological cultivation, which influences their expressed macro-morphology and in turn, product yields.

Dynamics of interaction and internalisation of the antifungal protein PeAfpA into Penicillium digitatum morphotypes

Antifungal proteins (AFPs) as the highly active PeAfpA from Penicillium expansum or PdAfpB from Penicillium digitatum exert promising antifungal activity, but their mode of action is not fully understood. We characterised the interaction of PeAfpA against P. digitatum, comparing it to the less active PdAfpB. Despite similar effect on conidia germination, PeAfpA did not induce a burst of reactive oxygen species as PdAfpB. Live-cell fluorescence microscopy revealed complex dynamics of interaction and internalisation of both proteins with distinct P. digitatum morphotypes (quiescent conidia, swollen conidia, germlings and hyphae). Labelled PeAfpA co-localised at the cell wall of quiescent conidia, where its localisation was punctate and not uniformly distributed. This pattern changed during germination to a uniform distribution with increased intensity. Conidia from mutants of genes involved in melanin biosynthesis (pksP/alb1 or arp2) showed an altered distribution of PeAfpA but later mimicked the wild type trend of changes during germination. In swollen conidia and germlings, PeAfpA remained attached to the cell wall. In hyphae, PeAfpA was internalised through the growing hyphal tip after binding to the cell wall, in a non-endocytic but energy-dependent process that caused vacuolisation, which preceded cell death. These results may help the development of biofungicides based on AFPs.

Activation of CWI pathway through high hydrostatic pressure, enhancing glycerol efflux via the aquaglyceroporin Fps1 in Saccharomyces cerevisiae

The fungal cell wall is the initial barrier for the fungi against diverse external stresses, such as osmolarity changes, harmful drugs, and mechanical injuries. This study explores the roles of osmoregulation and the cell-wall integrity (CWI) pathway in response to high hydrostatic pressure in the yeast Saccharomyces cerevisiae. We demonstrate the roles of the transmembrane mechanosensor Wsc1 and aquaglyceroporin Fps1 in a general mechanism to maintain cell growth under high-pressure regimes. The promotion of water influx into cells at 25 MPa, as evident by an increase in cell volume and a loss of the plasma membrane eisosome structure, activates the CWI pathway through the function of Wsc1. Phosphorylation of Slt2, the downstream mitogen-activated protein kinase, was increased at 25 MPa. Glycerol efflux increases via Fps1 phosphorylation, which is initiated by downstream components of the CWI pathway, and contributes to the reduction in intracellular osmolarity under high pressure. The elucidation of the mechanisms underlying adaptation to high pressure through the well-established CWI pathway could potentially translate to mammalian cells and provide novel insights into cellular mechanosensation.

A Novel Monoclonal Antibody 1D2 That Broadly Inhibits Clinically Important Aspergillus Species

Aspergillus fumigatus is a ubiquitous airborne fungus, is the predominant cause (>90%) of invasive aspergillosis (IA) in immunosuppressed patients and has a high mortality. New approaches to prevention and treatment are needed because of the poor efficacy, toxicity and side effects of the current anti-Aspergillus drugs on patients. Thus, we aim to explore a new avenue to combat Aspergillus infection by using a novel monoclonal antibody (mAb) 1D2 against a glycoprotein on the cell wall of Aspergillus. The ability of this mAb to inhibit attachment, germination, and growth of Aspergillus conidia and hyphae in vitro were examined. A dose-dependent growth inhibition of Aspergillus conidia in the presence of mAb 1D2 was found. The mAb 1D2 inhibited attachment of Aspergillus conidia to an untreated slide surface and fibronectin-treated surface compared to an unrelated mAb 6B10. When conidia were exposed to 1D2 concomitantly with inoculation into culture media, the mAb prevented the swelling and germination of conidia. This inhibitory ability of 1D2 was less apparent if it was added two hours after inoculation. Damage to hyphae was also observed when 1D2 was added to Aspergillus hyphae that had been incubated in media overnight. These in vitro results indicate that mAb 1D2 broadly inhibits clinically important Aspergillus species and has a promising therapeutic effect both as prophylaxis to inhibit an Aspergillus infection as well as a treatment.

Cells under pressure: how yeast cells respond to mechanical forces

In their natural habitats, unicellular fungal microbes are exposed to a myriad of mechanical cues such as shear forces from fluid flow, osmotic changes, and contact forces arising from microbial expansion in confined niches. While the rigidity of the cell wall is critical to withstand such external forces and balance high internal turgor pressure, it poses mechanical challenges during physiological processes such as cell growth, division, and mating that require cell wall remodeling. Thus, even organisms as simple as yeast have evolved complex signaling networks to sense and respond to intrinsic and extrinsic mechanical forces. In this review, we summarize the type and origin of mechanical forces experienced by unicellular yeast and discuss how these forces reorganize cell polarity and how pathogenic fungi exploit polarized assemblies to track weak spots in host tissues for successful penetration. We then describe mechanisms of force-sensing by conserved sets of mechanosensors. Finally, we elaborate downstream mechanotransduction mechanisms that orchestrate appropriate cellular responses, leading to improved mechanical fitness.

Mechanical interaction between hyphae during three-dimensional growth

The microscopic development of a mycelium is of importance in all aspects of fungal biology and biotechnology. However, the mechanics of three-dimensional (3D) hyphal growth has been not explored. Using light-sheet fluorescence microscopy, we follow the 3D growth of Trichoderma atroviride in liquid medium and observe two direct collision events among hyphae. In both cases, a hypha undergoing tip extension collides with the side of another hypha, causing mechanical deformation that remains after the collision. From these data we estimate that the force developed by hyphae during tip elongation is at least 260 pN.

Engineering Nanogels for Drug Delivery to Pathogenic Fungi Aspergillus fumigatus by Tuning Polymer Amphiphilicity

Invasive aspergillosis is a serious threat to immunodeficient and critically ill patients caused mainly by the fungus Aspergillus fumigatus. Here, poly(glycidol)-based nanogels (NGs) are proposed as delivery vehicles for antifungal agents for sustained drug release. NGs are formed by simple self-assembly of random copolymers, followed by oxidative cross-linking of thiol functionalities. We investigate the impact of copolymer amphiphilicity on NG interaction with mature fungal hyphae in order to select the optimal drug delivery system for model antifungal drug amphotericin B. The results show that drug-loaded NGs decrease minimal inhibitory concentration (MIC) for around four times and slow down the fungal biofilm synthesis at concentrations lower than MIC. Our results suggest that amphiphilicity of nanoparticle's polymer matrix is an important factor in understanding the action of nanocarriers toward fungal cells and should be considered in the development of nanoparticle-based antifungal therapy.

How Microbes Use Force To Control Adhesion

Microbial adhesion and biofilm formation are usually studied using molecular and cellular biology assays, optical and electron microscopy, or laminar flow chamber experiments. Today, atomic force microscopy (AFM) represents a valuable addition to these approaches, enabling the measurement of forces involved in microbial adhesion at the single-molecule level. In this minireview, we discuss recent discoveries made applying state-of-the-art AFM techniques to microbial specimens in order to understand the strength and dynamics of adhesive interactions. These studies shed new light on the molecular mechanisms of adhesion and demonstrate an intimate relationship between force and function in microbial adhesins.

Alginates along the filament of the brown alga Ectocarpus help cells cope with stress

Ectocarpus is a filamentous brown alga, which cell wall is composed mainly of alginates and fucans (80%), two non-crystalline polysaccharide classes. Alginates are linear chains of epimers of 1,4-linked uronic acids, β-D-mannuronic acid (M) and α-L-guluronic acid (G). Previous physico-chemical studies showed that G-rich alginate gels are stiffer than M-rich alginate gels when prepared in vitro with calcium. In order to assess the possible role of alginates in Ectocarpus, we first immunolocalised M-rich or G-rich alginates using specific monoclonal antibodies along the filament. As a second step, we calculated the tensile stress experienced by the cell wall along the filament, and varied it with hypertonic or hypotonic solutions. As a third step, we measured the stiffness of the cell along the filament, using cell deformation measurements and atomic force microscopy. Overlapping of the three sets of data allowed to show that alginates co-localise with the stiffest and most stressed areas of the filament, namely the dome of the apical cell and the shanks of the central round cells. In addition, no major distinction between M-rich and G-rich alginate spatial patterns could be observed. Altogether, these results support that both M-rich and G-rich alginates play similar roles in stiffening the cell wall where the tensile stress is high and exposes cells to bursting, and that these roles are independent from cell growth and differentiation.

The mechanical properties of microbial surfaces and biofilms

Microbes can modify their surface structure as an adaptive mechanism for survival and dissemination in the environment or inside the host. Altering their ability to respond to mechanical stimuli is part of this adaptive process. Since the 1990s, powerful micromanipulation tools have been developed that allow mechanical studies of microbial cell surfaces, exploring little known aspects of their dynamic behavior. This review concentrates on the study of mechanical and rheological properties of bacteria and fungi, focusing on their cell surface dynamics and biofilm formation.

Gazing at Cell Wall Expansion under a Golden Light

In plants, cell growth is constrained by a stiff cell wall, at least this is the way textbooks usually present it. Accordingly, many studies have focused on the elasticity and plasticity of the cell wall as prerequisites for expansion during growth. With their specific evolutionary history, cell wall composition, and environment, brown algae present a unique configuration offering a new perspective on the involvement of the cell wall, viewed as an inert material yet with intrinsic mechanical properties, in growth. In light of recent findings, we explore here how much of the functional relationship between cell wall chemistry and intrinsic mechanics on the one hand, and growth on the other hand, has been uncovered in brown algae.

Review of Progress in Atomic Force Microscopy

The study of biological samples is one of the most attractive and innovative fields of application of atomic force microscopy AFM. Recent breakthroughs in software and hardware have revolutionized this field and this paper reports on recent trends and describes examples of applications on biological samples. Originally developed for high-resolution imaging purposes, the AFM also has unique capabilities as a nano-indentor to probe the dynamic visco-elastic material properties of living cells in culture. In particular, AFM elastography combines imaging and indentation modalities to map the spatial distribution of cell mechanical properties, which in turn reflect the structure and function of the underlying structure. This paper describes the progress and development of atomic force microscopy as applied to animal and plant cell structures.

Enhanced method for High Spatial Resolution surface imaging and analysis of fungal spores using Scanning Electron Microscopy

Efficient, fast and new micro-analytical methods for characterization of ultrastructures of fungal spores with electron microscopy are very much required and essential. SEM analysis of biological materials, especially fungi, requires optimal preparation of the specimen and often requires the usage of dried samples which demands a challenging sample preparation. In the present investigation, we described a fast and improved method for the preparation of fungal specimen for scanning electron microscopy (SEM). The fungus, Curvularia lunata was grown on the surface of sterile Whatman No.1 filter paper which was overlaid on Potato Dextrose Agar (PDA) medium, gold coated immediately after removal from the growth medium and subjected to imaging. Generally, SEM imaging is done with samples that were fixed with chemical fixatives, dehydrated and gold coated specimens, but here we describe an easy and more efficient sample preparation for SEM which enabled enhanced image quality and precision visualization of fungal cells, especially the spores. The developed method has enabled the analysis of even the robust samples like fungal spores that to eliminating special temperature requirement. The ultimate goal was to develop an improved protocol/method applied to analysis of fungal spores with greater coverage about fungal specimen preparation. This method permits the use of rapid sample preparation and will allow us to imaging of individual spore or conidia structures in the context of fungal cell architecture which clarifies our understanding in fungal taxonomy/biology.

Atomic Force Microscopy: A Promising Tool for Deciphering the Pathogenic Mechanisms of Fungi in Cystic Fibrosis

During the past decades, atomic force microscopy (AFM) has emerged as a powerful tool in microbiology. Although most of the works concerned bacteria, AFM also permitted major breakthroughs in the understanding of physiology and pathogenic mechanisms of some fungal species associated with cystic fibrosis. Complementary to electron microscopies, AFM offers unprecedented insights to visualize the cell wall architecture and components through three-dimensional imaging with nanometer resolution and to follow their dynamic changes during cell growth and division or following the exposure to drugs and chemicals. Besides imaging, force spectroscopy with piconewton sensitivity provides a direct means to decipher the forces governing cell-cell and cell-substrate interactions, but also to quantify specific and non-specific interactions between cell surface components at the single-molecule level. This nanotool explores new ways for a better understanding of the structures and functions of the cell surface components and therefore may be useful to elucidate the role of these components in the host-pathogen interactions as well as in the complex interplay between bacteria and fungi in the lung microbiome.

High-resolution imaging of the microbial cell surface

Microorganisms, or microbes, can function as threatening pathogens that cause disease in humans, animals, and plants; however, they also act as litter decomposers in natural ecosystems. As the outermost barrier and interface with the environment, the microbial cell surface is crucial for cell-to-cell communication and is a potential target of chemotherapeutic agents. Surface ultrastructures of microbial cells have typically been observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Owing to its characteristics of low-temperature specimen preparation and superb resolution (down to 1 nm), cryo-field emission SEM has revealed paired rodlets, referred to as hydrophobins, on the cell walls of bacteria and fungi. Recent technological advances in AFM have enabled high-speed live cell imaging in liquid at the nanoscale level, leading to clear visualization of cell-drug interactions. Platinum-carbon replicas from freeze-fractured fungal spores have been observed using transmission electron microscopy, revealing hydrophobins with varying dimensions. In addition, AFM has been used to resolve bacteriophages in their free state and during infection of bacterial cells. Various microscopy techniques with enhanced spatial resolution, imaging speed, and versatile specimen preparation are being used to document cellular structures and events, thus addressing unanswered biological questions.

Antibacterial Effects of Biosynthesized Silver Nanoparticles on Surface Ultrastructure and Nanomechanical Properties of Gram-Negative Bacteria viz. Escherichia coli and Pseudomonas aeruginosa

Understanding the interactions of silver nanoparticles (AgNPs) with the cell surface is crucial for the evaluation of bactericidal activity and for advanced biomedical and environmental applications. Biosynthesis of AgNPs was carried out through in situ reduction of silver nitrate (AgNO3) by cell free protein of Rhizopus oryzae and the synthesized AgNPs was characterized by UV-vis spectroscopy, high resolution transmission electron microscopy (HRTEM), dynamic light scattering (DLS), ζ-potential analysis, and FTIR spectroscopy. The HRTEM measurement confirmed the formation of 7.1 ± 1.2 nm AgNPs, whereas DLS study demonstrated average hydrodynamic size of AgNPs as 9.1 ± 1.6 nm. The antibacterial activity of the biosynthesized AgNPs (ζ = -17.1 ± 1.2 mV) was evaluated against Gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa. The results showed that AgNPs exhibited concentration dependent antibacterial activity and 100% killing of E. coli and P. aeruginosa achieved when the cells were treated with 4.5 and 2.7 μg/mL AgNPs, respectively for 4 h. Furthermore, the intracellular reactive oxygen species (ROS) production suppressed the antioxidant defense and exerted mechanical damage to the membrane. AgNPs also induced surface charge neutralization and altered of the cell membrane permeability causing nonviability of the cells. Atomic force microscopy (AFM) studies depicted alteration of ultrastructural and nanomechanical properties of the cell surface following interaction with AgNPs, whereas FTIR spectroscopic analysis demonstrated that cell membrane of the treated cells underwent an order-to-disorder transition during the killing process and chemical composition of the cell membrane including fatty acids, proteins, and carbohydrates was decomposed following interaction with AgNPs.

Oxidative stress and metabolic perturbations in Escherichia coli exposed to sublethal levels of 2,4-dichlorophenoxyacetic acid

The chlorophenoxy herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is used extensively worldwide despite its known toxicity and our limited understanding of how it affects non-target organisms. Escherichia coli is a suitable model organism to investigate toxicity and adaptation mechanisms in bacteria exposed to xenobiotic chemicals. We developed a methodical platform that uses atomic force microscopy, metabolomics and biochemical assays to quantify the response of E. coli exposed to sublethal levels of 2,4-D. This herbicide induced a filamentous phenotype in E. coli BL21 and a similar phenotype was observed in a selection of genotypically diverse E. coli strains (A0, A1, B1, and D) isolated from the environment. The filamentous phenotype was observed at concentrations 1000 times below field levels and was reversible upon supplementation with polyamines. Cells treated with 2,4-D had more compliant envelopes, significantly remodeled surfaces that were rougher and altered vital metabolic pathways including oxidative phosphorylation, the ABC transport system, peptidoglycan biosynthesis, amino acid, nucleotide and sugar metabolism. Most of the observed effects could be attributed to oxidative stress, consistent with increases in reactive oxygen species as a function of 2,4-D exposure. This study provides direct evidence that 2,4-D at sublethal levels induces oxidative stress and identifies the associated metabolic changes in E. coli.

Stipe cell wall architecture varies with the stipe elongation of the mushroom Coprinopsis cinerea

A large amount of granular protrusions overlie the outer cell wall surfaces in both elongating and non-elongating stipe regions but overlie the inner cell wall surfaces only in non-elongating stipe regions. Removal of granular protrusions using alkali, amorphous materials overlying on both the inner and outer cell wall surfaces were explored in the non-elongating stipe regions. β-1,3-Glucanase treatment not only removed above those granular protrusions and underlying amorphous materials on the wall surfaces but also removed wall matrices embedding chitin microfibrils on the cell walls of most stipe regions, except for the outer cell wall surfaces of the non-elongating stipe regions where most of the wall matrices remained. The chitin microfibrils were closely and transversely arranged on both the inner and outer cell wall surfaces in the elongating apical stipe region, whereas they were loosely and transversely arranged on the inner cell wall surfaces and further became sparser and even randomly arranged on the outer cell wall surface in the non-elongating stipe regions. We propose that the surface deposition of granular protrusions and amorphous materials and the change of microfibril architecture and wall matrices may cause loss of wall plasticity and cessation of stipe elongation.

Atomic force microscopy of a ctpA mutant in Rhizobium leguminosarum reveals surface defects linking CtpA function to biofilm formation

Atomic force microscopy was used to investigate the surface ultrastructure, adhesive properties and biofilm formation of Rhizobium leguminosarum and a ctpA mutant strain. The surface ultrastructure of wild-type R. leguminosarum consists of tightly packed surface subunits, whereas the ctpA mutant has much larger subunits with loose lateral packing. The ctpA mutant strain is not capable of developing fully mature biofilms, consistent with its altered surface ultrastructure, greater roughness and stronger adhesion to hydrophilic surfaces. For both strains, surface roughness and adhesive forces increased as a function of calcium ion concentration, and for each, biofilms were thicker at higher calcium concentrations.

Quantifying the importance of galactofuranose in Aspergillus nidulans hyphal wall surface organization by atomic force microscopy

The fungal wall mediates cell-environment interactions. Galactofuranose (Galf), the five-member ring form of galactose, has a relatively low abundance in Aspergillus walls yet is important for fungal growth and fitness. Aspergillus nidulans strains deleted for Galf biosynthesis enzymes UgeA (UDP-glucose-4-epimerase) and UgmA (UDP-galactopyranose mutase) lacked immunolocalizable Galf, had growth and sporulation defects, and had abnormal wall architecture. We used atomic force microscopy and force spectroscopy to image and quantify cell wall viscoelasticity and surface adhesion of ugeAΔ and ugmAΔ strains. We compared the results for ugeAΔ and ugmAΔ strains with the results for a wild-type strain (AAE1) and the ugeB deletion strain, which has wild-type growth and sporulation. Our results suggest that UgeA and UgmA are important for cell wall surface subunit organization and wall viscoelasticity. The ugeAΔ and ugmAΔ strains had significantly larger surface subunits and lower cell wall viscoelastic moduli than those of AAE1 or ugeBΔ hyphae. Double deletion strains (ugeAΔ ugeBΔ and ugeAΔ ugmAΔ) had more-disorganized surface subunits than single deletion strains. Changes in wall surface structure correlated with changes in its viscoelastic modulus for both fixed and living hyphae. Wild-type walls had the largest viscoelastic modulus, while the walls of the double deletion strains had the smallest. The ugmAΔ strain and particularly the ugeAΔ ugmAΔ double deletion strain were more adhesive to hydrophilic surfaces than the wild type, consistent with changes in wall viscoelasticity and surface organization. We propose that Galf is necessary for full maturation of A. nidulans walls during hyphal extension.

Atomic force microscopy: a tool to analyze the structural organization of pathogenic protozoa

The fine structure of parasitic protozoa has been the subject of intense investigation with the use of electron microscopy. The recent development of atomic force microscopy (AFM) and all of the techniques associated with AFM has created new ways to further analyze the structure of cells. In this review, the various, presently-available modalities of AFM are discussed, as well as the results obtained in analysis of: (i) the structure of intact and detergent-extracted protozoa; (ii) the surface of infected cells; (iii) the structure of parasite macromolecules; (iv) the measurement of surface potential; and (v) force spectroscopy, the measurement of elasticity and ligand-receptor interactions.

Nanoscale imaging of microbial pathogens using atomic force microscopy

The nanoscale exploration of microbes using atomic force microscopy (AFM) is an exciting research field that has expanded rapidly in the past years. Using AFM topographic imaging, investigators can visualize the surface structure of live cells under physiological conditions and with unprecedented resolution. In doing so, the effect of drugs and chemicals on the fine cell surface architecture can be monitored. Real-time imaging offers a means to follow dynamic events such as cell growth and division. In parallel, chemical force microscopy (CFM), in which AFM tips are modified with specific functional groups, allows researchers to measure interaction forces, such as hydrophobic forces, and to resolve nanoscale chemical heterogeneities on cells, on a scale of only approximately 25 functional groups. Lastly, molecular recognition imaging using spatially resolved force spectroscopy, dynamic recognition imaging or immunogold detection, enables microscopists to localize specific receptors, such as cell adhesion proteins or antibiotic binding sites. These noninvasive nanoscale analyses provide new avenues in pathogenesis research, particularly for investigating the action mode of antimicrobial drugs, and for elucidating the molecular basis of pathogen-host interactions.

Structural analysis of biofilms and pellets of Aspergillus niger by confocal laser scanning microscopy and cryo scanning electron microscopy

Biomass organization of Aspergillus niger biofilms and pellets stained with fluorescein isothiocyanate were analyzed by means of confocal laser scanning microscopy and detectable differences between both types of growth were found. Three-dimensional surface plot analysis of biofilm structure revealed interstitial voids and vertical growth compared with pellets. Growth was lower in biofilm and according to fluorescence profile obtained, biomass density increased at the surface (0-20 microm). However, a decrease in fluorescence intensity was observed through optical sections of pellets even though growth was significantly higher than biofilms. Cryo scanning electron microscopy also showed structural differences. While biofilms showed a spatially ordered mycelium and well structured hyphal channels, pellets were characterized by an entangled and notoriously compacted mycelium. These findings revealed common structural characteristics between A. niger biofilms and those found in other microbial biofilms. Thus, biofilm microstructure may represent a key determinant of biofilm growth and physiology of filamentous fungi.

Structural and nanomechanical properties of Termitomyces clypeatus cell wall and its interaction with chromium(VI)

Alterations of cell surface properties accompanying the complex life cycle of Termitomyces clypeatus have been monitored using atomic force microscopy (AFM). A new hyphae/mycelium is developed on cell division, and the cell wall of the mycelium undergoes a process of internal reorganization (or maturation) followed by morphological and chemical alterations. The changes of the surface ultrastructures during the growth process are correlated to the corresponding changes in relative viscoelasticity and rigidity of the cell wall by employing force spectroscopy. The cell wall rigidity and elasticity are found to be 0.34+/-0.02 N/m and 27.5+/-2.1 MPa, respectively, at the early logarithmic phase, on maturation increase to reach 0.81+/-0.08 N/m and 92.5+/-12 MPa, respectively, at the stationary phase, and thereafter decrease to 0.62+/-0.06 N/m and 61.6+/-6.6 MPa at the death phase. The alterations of the ultrastructural and nanomechanical properties of the cell surface as functions of growth phases affect the interaction involving chromium and T. clypeatus.

Adsorption behavior of mercury on functionalized aspergillus versicolor mycelia: atomic force microscopic study

The adsorption characteristics of mercury on Aspergillus versicolor mycelia have been studied under varied environments. The mycelia are functionalized by carbon disulfide (CS(2)) treatment under alkaline conditions to examine the enhance uptake capacity and explore its potentiality in pollution control management. The functionalized A. versicolor mycelia have been characterized by scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDXA), attenuated total reflection infrared (ATR-IR), and atomic force microscopy (AFM) probing. SEM and AFM images exhibit the formation of nanoparticles on the mycelial surface. ATR-IR profile confirms the functionalization of the mycelia following chemical treatment. ATR-IR and EDXA results demonstrate the binding of the sulfur groups of the functionalized mycelia to the mercury and consequent formation metal sulfide. AFM study reveals that the mycelial surface is covered by a layer of densely packed domain like structures. Sectional analysis yields significant increase in average roughness (R(rms)) value (20.5 +/- 1.82 nm) compared to that of the pristine mycelia (4.56 +/- 0.82 nm). Surface rigidity (0.88 +/- 0.06 N/m) and elasticity (92.6 +/- 10.2 MPa) obtained from a force distance curve using finite element modeling are found to increase significantly with respect to the corresponding values of (0.65 +/- 0.05 N/m and 32.8 +/- 4.5 MPa) of the nonfunctionalized mycelia. The maximum mercury adsorption capacity of the functionalized mycelia is observed to be 256.5 mg/g in comparison to 80.71 mg/g for the pristine mycelia.

Interaction of chromium with resistant strain Aspergillus versicolor: investigation with atomic force microscopy and other physical studies

The interaction of chromium and a chromate resistant Aspergillus versicolor strain has been studied by atomic force (AFM) and transmission electron (TEM) microscopies. The nanomechanical properties such as cell wall rigidity and elasticity were measured by force spectroscopy and found to be 0.61 +/- 0.08 N/m, and 20.5 +/- 2.1 MPa, respectively. On chromium binding, ultrastuctural changes of the cell wall along with the formation of layered structures on the cell wall were observed. TEM and AFM micrographs demonstrate the accumulation of chromium on the cell wall, which were rough and irregular compared with the smooth pristine mycelia. The surface roughness, cell wall rigidity and elasticity increased to 35.5 +/- 3.5 nm, 0.88 +/- 0.05 N/m, and 62.5 +/- 3.5 MPa, respectively, from the corresponding values of 5.2 +/- 0.68 nm, 0.61 +/- 0.02 N/m, and 20.5 +/- 2.1 MPa for the pristine mycelia. X-ray photoelectron spectroscopy and Fourier transform infrared studies suggest that bound chromium was reduced to its trivalent state by the cell wall components. The reduced chromium species on the cell surface further electrostatically bind chromate ions forming layered structure on the cell wall.

Surface characteristics of the entomopathogenic fungus Beauveria (Cordyceps) bassiana

Marked differences in surface characteristics were observed among three types of single-cell propagules produced by the entomopathogenic fungus Beauveria bassiana. Atomic force microscopy (AFM) revealed the presence of bundles or fascicles in aerial conidia absent from in vitro blastospores and submerged conidia. Contact angle measurements using polar and apolar test liquids placed on cell layers were used to calculate surface tension values and the free energies of interaction of the cell types with surfaces. These analyses indicated that the cell surfaces of aerial conidia were hydrophobic, whereas those of blastospores and submerged conidia were hydrophilic. Zeta potential determinations of the electrostatic charge distribution across the surface of the cells varied from +22 to -30 mV for 16-day aerial conidia at pH values ranging from 3 to 9, while the net surface charge ranged from +10 to -13 mV for submerged conidia, with much less variation observed for blastospores, +4 to -4 mV, over the same pH range. Measurements of hydrophobicity using microbial adhesion to hydrocarbons (MATH) indicated that the surfaces of aerial conidia were hydrophobic, and those of blastospores hydrophilic, whereas submerged conidia displayed cell surface characteristics on the borderline between hydrophobic and hydrophilic. Insect pathology assays using tobacco budworm (Heliothis virescens) larvae revealed some variation in virulence among aerial conidia, in vitro blastospores and submerged conidia, using both topical application and haemocoel injection of the fungal cells.

Morphological patterns of Aspergillus niger biofilms and pellets related to lignocellulolytic enzyme productivities

AIMS

To study the morphological patterns of Aspergillus niger during biofilm formation on polyester cloth by using cryo-scanning electron microscopy related to lignocellulolytic enzyme productivity.

METHODS AND RESULTS

Biofilm and pellet samples obtained from flask cultures were examined at -80 degrees C in a LEO PV scanning electron microscope. Spore adhesion depends on both its rough surface and adhesive substances that form a pad between spore and support. An extracellular matrix surrounding germ tubes and hyphae was also seen. Biofilm mycelia showed an orderly distribution forming surface and inner channels, while pellets showed highly intertwined superficial hyphae and a densely packed deep mycelium. Morphological differences between both types of culture correlated with differences in enzyme volumetric and specific productivities. Biofilm cultures produced higher filter paper cellulase, endoglucanase, beta-glucosidase and xylanase volumetric and specific productivities than submerged cultures.

CONCLUSIONS

Fungal biofilms are morphologically efficient systems for enzyme production. Favourable physiological aspects are shared with solid state fermentation, but fungal biofilms present better possibilities for process control and scale-up.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of this study support the importance of morphology in the productivity of fungal submerged processes, placing biofilms in a preferential category.

Nanomicrobiology

Recent advances in atomic force microscopy (AFM) are revolutionizing our views of microbial surfaces. While AFM imaging is very useful for visualizing the surface of hydrated cells and membranes on the nanoscale, force spectroscopy enables researchers to locally probe biomolecular forces and physical properties. These unique capabilities allow us to address a number of questions that were inaccessible before, such as how does the surface architecture of microbes change as they grow or interact with drugs, and what are the molecular forces driving their interaction with antibiotics and host cells? Here, we provide a flavor of recent achievements brought by AFM imaging and single molecule force spectroscopy in microbiology.

Life under pressure: hydrostatic pressure in cell growth and function

H(2)O is one of the most essential molecules for cellular life. Cell volume, osmolality and hydrostatic pressure are tightly controlled by multiple signaling cascades and they drive crucial cellular functions ranging from exocytosis and growth to apoptosis. Ion fluxes and cell shape restructuring induce asymmetries in osmotic potential across the plasma membrane and lead to localized hydrodynamic flow. Cells have evolved fascinating strategies to harness the potential of hydrodynamic flow to perform crucial functions. Plants exploit hydrodynamics to drive processes including gas exchange, leaf positioning, nutrient acquisition and growth. This paradigm is extended by recent work that reveals an important role for hydrodynamics in pollen tube growth.

Nanoscale exploration of microbial surfaces using the atomic force microscope

Atomic force microscopy (AFM) has recently opened a variety of novel possibilities for imaging and manipulating microbial surfaces in their native environment. While AFM imaging offers a means to visualize surface structures at high resolution and in physiological conditions, AFM force spectroscopy enables researchers to probe a variety of properties, including the unfolding pathways of single-membrane proteins, the elasticity of cell walls and surface macromolecules, and the molecular forces responsible for cell–cell and cell–solid interactions. These nanoscale analyses enable us to answer a number of questions that were difficult to address previously, such as: how does the surface architecture of microbes change as they grow or interact with antibiotics; what is the force required to unfold and extract a single membrane protein; and what are the molecular forces driving the interaction between a pathogen and a host or biomaterial surface? This review will expand on these issues.

Fungal surface remodelling visualized by atomic force microscopy

Most fungal growth is localized to the tips of hyphae, however, early stages of spore germination and the growth of certain morphological mutant strains exhibit non-polarized expansion. We used atomic force microscopy (AFM) to document changes in Aspergillus nidulans wall surfaces during non-polarized growth: spore germination, and growth in a strain containing the hypA1 temperature sensitive morphogenesis defect. We compared wall surface structures of both wild-type and mutant A. nidulans following growth at 28 degrees and 42 degrees C, the latter being the restrictive temperature for hypA1. There was no appreciable difference in surface ultrastructure between wild-type and hypA1 spores, or hyphal walls grown at 28 degrees C. When dry mature A. nidulans conidia were wetted they lost their hydrophobin coat, indicating an intermediate stage between dormancy and swelling. The surface structure of hypA1 germlings grown at 42 degrees C was less organized than wild-type hyphae grown under the same conditions, and had a larger range of subunit sizes. AFM images of hyphal wall surface changes following a shift in growth temperature from restrictive (42 degrees C) to permissive (28 degrees C), showed a gradient of sizes for wall surface features similar to the trend observed for wild-type cells at branch points. Changes associated with the hyphal wall structure for A. nidulans hypA1 offer insight into the events associated with fungal germination, and wall remodelling.