Surface rigidity change of Escherichia coli after filamentous bacteriophage infection

Determining the Young's Modulus of the Bacterial Cell Envelope

Bacteria experience substantial physical forces in their natural environment, including forces caused by osmotic pressure, growth in constrained spaces, and fluid shear. The cell envelope is the primary load-carrying structure of bacteria, but the mechanical properties of the cell envelope are poorly understood; reports of Young's modulus of the cell envelope of Escherichia coli range from 2 to 18 MPa. We developed a microfluidic system to apply mechanical loads to hundreds of bacteria at once and demonstrated the utility of the approach for evaluating whole-cell stiffness. Here, we extend this technique to determine Young's modulus of the cell envelope of E. coli and of the pathogens Vibrio cholerae and Staphylococcus aureus. An optimization-based inverse finite element analysis was used to determine the cell envelope Young's modulus from observed deformations. The Young's modulus values of the cell envelope were 2.06 ± 0.04 MPa for E. coli, 0.84 ± 0.02 MPa for E. coli treated with a chemical (A22) known to reduce cell stiffness, 0.12 ± 0.03 MPa for V. cholerae, and 1.52 ± 0.06 MPa for S. aureus (mean ± SD). The microfluidic approach allows examination of hundreds of cells at once and is readily applied to Gram-negative and Gram-positive organisms as well as rod-shaped and cocci cells, allowing further examination of the structural causes behind differences in cell envelope Young's modulus among bacterial species and strains.

Surveying membrane landscapes: a new look at the bacterial cell surface

Recent studies applying advanced imaging techniques are changing the way we understand bacterial cell surfaces, bringing new knowledge on everything from single-cell heterogeneity in bacterial populations to their drug sensitivity and mechanisms of antimicrobial resistance. In both Gram-positive and Gram-negative bacteria, the outermost surface of the bacterial cell is being imaged at nanoscale; as a result, topographical maps of bacterial cell surfaces can be constructed, revealing distinct zones and specific features that might uniquely identify each cell in a population. Functionally defined assembly precincts for protein insertion into the membrane have been mapped at nanoscale, and equivalent lipid-assembly precincts are suggested from discrete lipopolysaccharide patches. As we review here, particularly for Gram-negative bacteria, the applications of various modalities of nanoscale imaging are reawakening our curiosity about what is conceptually a 3D cell surface landscape: what it looks like, how it is made and how it provides resilience to respond to environmental impacts.

Effect of Flexibility and Size of Nanofabricated Topographies on the Mechanobactericidal Efficacy of Polymeric Surfaces

Driven by the growing threat of antimicrobial resistance, the design of intrinsically bactericidal surfaces has been gaining significant attention. Proposed surface topography designs are often inspired by naturally occurring nanopatterns on insect wings that mechanically damage bacteria via membrane deformation. The stability of and the absence of chemicals in such surfaces support their facile and sustainable employment in avoiding surface-born pathogen transmission. Recently, the deflection of controllably nanofabricated pillar arrays has been shown to strongly affect bactericidal activity, with the limits of mechanical effectiveness of such structures remaining largely unexplored. Here, we examine the limits of softer, commonly used polymeric materials and investigate the interplay between pillar nanostructure sizing and flexibility for effective antibacterial functionality. A facile, scalable, UV nanoimprint lithography method was used to fabricate nanopillar array topographies of variable sizes and flexibilities. It was found that bacterial death on nanopillars in the range of diameters ≤100 nm and Young's moduli ≥1.3 GPa is increased by 3.5- to 5.6-fold, while thicker or softer pillars did not reduce bacterial viability. To further support our findings, we performed a finite element analysis of pillar deformation. It revealed that differences in the amount of stress exerted on bacterial membranes, generated from the stored elastic energy in flexible pillars, contribute to the observed bactericidal performance.

Bursting out: linking changes in nanotopography and biomechanical properties of biofilm-forming Escherichia coli to the T4 lytic cycle

The bacteriophage infection cycle has been extensively studied, yet little is known about the nanostructure and mechanical changes that lead to bacterial lysis. Here, atomic force microscopy was used to study in real time and in situ the impact of the canonical phage T4 on the nanotopography and biomechanics of irreversibly attached, biofilm-forming E. coli cells. The results show that in contrast to the lytic cycle in planktonic cells, which ends explosively, anchored cells that are in the process of forming a biofilm undergo a more gradual lysis, developing distinct nanoscale lesions (~300 nm in diameter) within the cell envelope. Furthermore, it is shown that the envelope rigidity and cell elasticity decrease (>50% and >40%, respectively) following T4 infection, a process likely linked to changes in the nanostructure of infected cells. These insights show that the well-established lytic pathway of planktonic cells may be significantly different from that of biofilm-forming cells. Elucidating the lysis paradigm of these cells may advance biofilm removal and phage therapeutics.

Microscopic Investigation of the Combined Use of Antibiotics and Biosurfactants on Methicillin Resistant Staphylococcus aureus

One current strategy to deal with the serious issue of antibiotic resistance is to use biosurfactants, weak antimicrobials in their own right, with antibiotics in order to extend the efficacy of antibiotics. Although an adjuvant effect has been observed, the underlying mechanisms are poorly understood. To investigate the nature of the antibiotic and biosurfactant interaction, we undertook a scanning electron microscopy (SEM) and atomic force microscopy (AFM) microscopic study of the effects of the tetracycline antibiotic, combined with sophorolipid and rhamnolipid biosurfactants, on Methicillin-resistant Staphylococcus aureus using tetracycline concentrations below and above the minimum inhibitory concentration (MIC). Control and treated bacterial samples were prepared with an immersion technique by adsorbing the bacteria onto glass substrates grafted with the poly-cationic polymer polyethyleneimine. Bacterial surface morphology, hydrophobic and hydrophilic surface characters as well as the local bacterial cell stiffness were measured following combined antibiotic and biosurfactant treatment. The sophorolipid biosurfactant stands alone insofar as, when used with the antibiotic at sub-MIC concentration, it resulted in bacterial morphological changes, larger diameters (from 758 ± 75 to 1276 ± 220 nm, p-value = 10^-4) as well as increased bacterial core stiffness (from 205 ± 46 to 396 ± 66 mN/m, p-value = 5 × 10^-5). This investigation demonstrates that such combination of microscopic analysis can give useful information which could complement biological assays to understand the mechanisms of synergy between antibiotics and bioactive molecules such as biosurfactants.

The Mechanical Properties of Bacteria and Why they Matter

I review recent techniques to measure the mechanical properties of bacterial cells and their subcellular components, and then discuss what these techniques have revealed about the constitutive mechanical properties of whole bacterial cells and subcellular material, as well as the molecular basis for these properties.

Investigation of Formation of Bacterial Biofilm upon Dead Siblings

Biocides can effectively kill bacteria; however, whether the dead bacterial cells left on the surface influence the later growth of biofilm is unknown. In this study, we have cultured Pseudomonas aeruginosa (PAO1) biofilm on their dead siblings and have investigated their evolution by using magnetic force modulation atomic force microscopy (MF-AFM). The time dependence of the biofilm thickness indicates that the deposited dead siblings can slow down the growth of PAO1 biofilm. The biofilm growing on dead bacteria layers is softer in comparison with those upon alive siblings, as reflected by the static elastic modulus ( E) and dynamic stiffness ( k_d) scaled to the disturbing frequency ( f) as k_d = k_d,0 f^γ, where k_d,0 is the scaling factor and γ is the power-law exponent. We reveal that the smaller population instead of the variation of extracellular polymeric substances (EPS) within the biofilm upon the dead siblings is responsible for the softer biofilm. The present study provides a better understanding of the biofilm formation, thus, making it significant for designing antimicrobial medical materials and antifouling coatings.

The Effects of β-Lactam Antibiotics on Surface Modifications of Multidrug-Resistant Escherichia coli: A Multiscale Approach

Possible multidrug-resistant (MDR) mechanisms of four resistant strains of Escherichia coli to a model β-lactam, ampicillin, were investigated using contact angle measurements of wettability, crystal violet assays of permeability, biofilm formation, fluorescence imaging, and nanoscale analyses of dimensions, adherence, and roughness. Upon exposure to ampicillin, one of the resistant strains, E. coli A5, changed its phenotype from elliptical to spherical, maintained its roughness and biofilm formation abilities, decreased its length and surface area, maintained its cell wall integrity, increased its hydrophobicity, and decreased its nanoscale adhesion to a model surface of silicon nitride. Such modifications are suggested to allow these cells to conserve energy during metabolic dormancy. In comparison, resistant strains E. coli D4, A9, and H5 elongated their cells, increased their roughness, increased their nanoscale adhesion forces, became more hydrophilic, and increased their biofilm formation upon exposure to ampicillin. These results suggest that these strains resisted ampicillin through biofilm formation that possibly introduces diffusion limitations to antibiotics. Investigations of how MDR bacterial cells modify their surfaces in response to antibiotics can guide research efforts aimed at designing more effective antibiotics and new treatment strategies for MDR bacterial infections.

Longitudinal Study of the Effects of Environmental pH on the Mechanical Properties of Aspergillus niger

The regulation of environmental pH is key to the health of an ecosystem, influencing the metabolic activity, growth, and development of organisms within it. Although pH values can be measured by a wide range of readily available technologies ranging from fluorescent dyes and nanosensors, these cannot reveal the history of environmental pH from before monitoring begins. This information is sometimes crucial for piecing together what has happened to an ecosystem, and our long-term goal is therefore to develop technologies capable of obtaining it. Here, we propose monitoring environmental pH over time by tracking mechanical properties of a common fungus. As a first step toward obtaining a time history of pH, we evaluate the effect of pH upon the effective indentation modulus of spores and hyphae of Aspergillus niger. We report that the indentation modulus of this phosphorus-solubilizing fungus, obtained through atomic force microscopy and nanoindentation, correlated with environmental acidity. We observed a significant, monotonic increase in moduli over the course of incubation in an acidic environment, but no change in moduli over time for incubation in a neutral environment. Results show promise for using our scheme to detect and track environmental pH over time, and more broadly for using a microorganism's mechanical properties as a biomarker for environmental detection.

Bacterial Cell Mechanics

Cellular mechanical properties play an integral role in bacterial survival and adaptation. Historically, the bacterial cell wall and, in particular, the layer of polymeric material called the peptidoglycan were the elements to which cell mechanics could be primarily attributed. Disrupting the biochemical machinery that assembles the peptidoglycan (e.g., using the β-lactam family of antibiotics) alters the structure of this material, leads to mechanical defects, and results in cell lysis. Decades after the discovery of peptidoglycan-synthesizing enzymes, the mechanisms that underlie their positioning and regulation are still not entirely understood. In addition, recent evidence suggests a diverse group of other biochemical elements influence bacterial cell mechanics, may be regulated by new cellular mechanisms, and may be triggered in different environmental contexts to enable cell adaptation and survival. This review summarizes the contributions that different biomolecular components of the cell wall (e.g., lipopolysaccharides, wall and lipoteichoic acids, lipid bilayers, peptidoglycan, and proteins) make to Gram-negative and Gram-positive bacterial cell mechanics. We discuss the contribution of individual proteins and macromolecular complexes in cell mechanics and the tools that make it possible to quantitatively decipher the biochemical machinery that contributes to bacterial cell mechanics. Advances in this area may provide insight into new biology and influence the development of antibacterial chemotherapies.

Microrheology of growing Escherichia coli biofilms investigated by using magnetic force modulation atomic force microscopy

Microrheology of growing biofilms provides insightful information about its structural evolution and properties. In this study, the authors have investigated the microrheology of Escherichia coli (strain HCB1) biofilms at different indentation depth (δ) by using magnetic force modulation atomic force microscopy as a function of disturbing frequency (f). As δ increases, the dynamic stiffness (k_s) for the biofilms in the early stage significantly increases. However, it levels off when the biofilms are matured. The facts indicate that the biofilms change from inhomogeneous to homogeneous in structure. Moreover, k_s is scaled to f, which coincides with the rheology of soft glasses. The exponent increases with the incubation time, indicating the fluidization of biofilms. In contrast, the upper layer of the matured biofilms is solidlike in that the storage modulus is always larger than the loss modulus, and its viscoelasticity is slightly influenced by the shear stress.

Influence of csgD and ompR on Nanomechanics, Adhesion Forces, and Curli Properties of E. coli

Curli are bacterial appendages involved in the adhesion of cells to surfaces; their synthesis is regulated by many genes such as csgD and ompR. The expression of the two curli subunits (CsgA and CsgB) in Escherichia coli (E. coli) is regulated by CsgD; at the same time, csgD transcription is under the control of OmpR. Therefore, both genes are involved in the control of curli production. In this work, we elucidated the role of these genes in the nanomechanical and adhesive properties of E. coli MG1655 (a laboratory strain not expressing significant amount of curli) and its curli-producing mutants overexpressing OmpR and CsgD, employing atomic force microscopy (AFM). Nanomechanical analysis revealed that the expression of these genes gave origin to cells with a lower Young's modulus (E) and turgidity (P0), whereas the adhesion forces were unaffected when genes involved in curli formation were expressed. AFM was also employed to study the primary structure of the curli expressed through the freely jointed chain (FJC) model for polymers. CsgD increased the number of curli on the surface more than OmpR did, and the overexpression of both genes did not result in a greater number of curli. Neither of the genes had an impact on the structure (total length of the polymer and number and length of Kuhn segments) of the curli. Our results further suggest that, despite the widely assumed role of curli in cell adhesion, cell adhesion force is also dictated by surface properties because no relation between the number of curli expressed on the surface and cell adhesion was found.

Nanomechanical properties of the sea-water bacterium Paracoccus seriniphilus--a scanning force microscopy approach

The measurement of force-distance curves on a single bacterium provides a unique opportunity to detect properties such as the turgor pressure under various environmental conditions. Marine bacteria are very interesting candidates for the production of pharmaceuticals, but are only little studied so far. Therefore, the elastic behavior of Paracoccus seriniphilus, an enzyme producing marine organism, is presented in this study. After a careful evaluation of the optimal measurement conditions, the spring constant and the turgor pressure are determined as a function of ionic strength and pH. Whereas the ionic strength changes the turgor pressure passively, the results give a hint that the change to acidic pH increases the turgor pressure by an active mechanism. Furthermore, it could be shown, that P. seriniphilus has adhesive protrusions outside its cell wall.

'Big things in small packages: the genetics of filamentous phage and effects on fitness of their host'

This review synthesizes recent and past observations on filamentous phages and describes how these phages contribute to host phentoypes. For example, the CTXφ phage of Vibrio cholerae encodes the cholera toxin genes, responsible for causing the epidemic disease, cholera. The CTXφ phage can transduce non-toxigenic strains, converting them into toxigenic strains, contributing to the emergence of new pathogenic strains. Other effects of filamentous phage include horizontal gene transfer, biofilm development, motility, metal resistance and the formation of host morphotypic variants, important for the biofilm stress resistance. These phages infect a wide range of Gram-negative bacteria, including deep-sea, pressure-adapted bacteria. Many filamentous phages integrate into the host genome as prophage. In some cases, filamentous phages encode their own integrase genes to facilitate this process, while others rely on host-encoded genes. These differences are mediated by different sets of 'core' and 'accessory' genes, with the latter group accounting for some of the mechanisms that alter the host behaviours in unique ways. It is increasingly clear that despite their relatively small genomes, these phages exert signficant influence on their hosts and ultimately alter the fitness and other behaviours of their hosts.

Single virus detection by means of atomic force microscopy in combination with advanced image analysis

In the present contribution virions of five different virus species, namely Varicella-zoster virus, Porcine teschovirus, Tobacco mosaic virus, Coliphage M13 and Enterobacteria phage PsP3, are investigated using atomic force microscopy (AFM). From the resulting height images quantitative features like maximal height, area and volume of the viruses could be extracted and compared to reference values. Subsequently, these features were accompanied by image moments, which quantify the morphology of the virions. Both types of features could be utilized for an automatic discrimination of the five virus species. The accuracy of this classification model was 96.8%. Thus, a virus detection on a single-particle level using AFM images is possible. Due to the application of advanced image analysis the morphology could be quantified and used for further analysis. Here, an automatic recognition by means of a classification model could be achieved in a reliable and objective manner.

Nanomechanical response of bacterial cells to cationic antimicrobial peptides

The effectiveness of antimicrobial compounds can be easily screened, however their mechanism of action is much more difficult to determine. Many compounds act by compromising the mechanical integrity of the bacterial cell envelope, and our study introduces an AFM-based creep deformation technique to evaluate changes in the time-dependent mechanical properties of Pseudomonas aeruginosa PAO1 bacterial cells upon exposure to two different but structurally related antimicrobial peptides. We observed a distinctive signature for the loss of integrity of the bacterial cell envelope following exposure to the peptides. Measurements performed before and after exposure, as well as time-resolved measurements and those performed at different concentrations, revealed large changes to the viscoelastic parameters that are consistent with differences in the membrane permeabilizing effects of the peptides. The AFM creep deformation measurement provides new, unique insight into the kinetics and mechanism of action of antimicrobial peptides on bacteria.

Nanoscale imaging and mechanical analysis of Fc receptor-mediated macrophage phagocytosis against cancer cells

Fc receptor-mediated macrophage phagocytosis against cancer cells is an important mechanism in the immune therapy of cancers. Traditional research about macrophage phagocytosis was based on optical microscopy, which cannot reveal detailed information because of the 200-nm-resolution limit. Quantitatively investigating the macrophage phagocytosis at micro- and nanoscale levels is still scarce. The advent of atomic force microscopy (AFM) offers an excellent analytical instrument for quantitatively investigating the biological processes at single-cell and single-molecule levels under native conditions. In this work, we combined AFM and fluorescence microscopy to visualize and quantify the detailed changes in cell morphology and mechanical properties during the process of Fc receptor-mediated macrophage phagocytosis against cancer cells. Lymphoma cells were discernible by fluorescence staining. Then, the dynamic process of phagocytosis was observed by time-lapse optical microscopy. Next, AFM was applied to investigate the detailed cellular behaviors during macrophage phagocytosis under the guidance of fluorescence recognition. AFM imaging revealed the distinct features in cellular ultramicrostructures for the different steps of macrophage phagocytosis. AFM cell mechanical property measurements indicated that the binding of cancer cells to macrophages could make macrophages become stiffer. The experimental results provide novel insights in understanding the Fc-receptor-mediated macrophage phagocytosis.

Deformation of filamentous Escherichia coli cells in a microfluidic device: a new technique to study cell mechanics

The mechanical properties of bacterial cells are determined by their stress-bearing elements. The size of typical bacterial cells, and the fact that different time and length scales govern their behavior, necessitate special experimental techniques in order to probe their mechanical properties under various spatiotemporal conditions. Here, we present such an experimental technique to study cell mechanics using hydrodynamic forces in a microfluidic device. We demonstrate the application of this technique by calculating the flexural rigidity of non-growing Escherichia coli cells. In addition, we compare the deformation of filamentous cells under growing and non-growing conditions during the deformation process. We show that, at low forces, the force needed to deform growing cells to the same extent as non-growing cells is approximately two times smaller. Following previous works, we interpret these results as the outcome of the difference between the elastic response of non-growing cells and the plastic-elastic response of growing cells. Finally, we observe some heterogeneity in the response of individual cells to the applied force. We suggest that this results from the individuality of different bacterial cells.

Impedimetric microbial sensor for real-time monitoring of phage infection of Escherichia coli

We describe an impedimetric microbial sensor for real-time monitoring of the non-lytic M13 bacteriophage infection of Escherichia coli cells using a gold electrode covalently grafted with a monolayer of lipopolysaccharide specific antibody. After infection, damage to the lipopolysaccharide layer on the outer membrane of E. coli causes changes to its surface charge and morphology, resulting in the aggregation of redox probe, Fe(CN)6(3-/4-) at the electrode surface and thereby increases its electron-transfer rate. This consequent decrease of electron-transfer resistance in the presence of bacteriophage can be easily monitored using Faradaic impedance spectroscopy. Non-lytic bacterium-phage interaction which is hardly observable using conventional microscopic methods is detected within 3h using this impedimetric microbial sensor which demonstrates its excellent performance in terms of analysis time, ease and reduced reliance on labeling steps during in-situ monitoring of the phage infection process.

Cellular and molecular investigations of the adhesion and mechanics of Listeria monocytogenes lineages' I and II environmental and epidemic strains

Atomic force microscopy (AFM) was used to probe the mechanical and adherence properties of eight Listeria monocytogenes' strains representative of the species' two phylogenetic lineages I and II. From a functional perspective, lineage' I strains were characterized by lower overall adhesion forces and higher specific and nonspecific forces compared to lineage' II strains. From a structural perspective, lineage' II strains were characterized by higher Young's moduli and longer and stiffer biopolymers compared to lineage' I strains. Both lineages' I and II strains were similar in their grafting densities. Finally, our results indicated that epidemic and environmental strains of L. monocytogenes and irrespective of their lineage group were characterized by similar Young's moduli of elasticities and adhesion forces at the cellular level. However, at the molecular level, epidemic strains were characterized by higher specific and nonspecific forces, shorter, denser, and more flexible biopolymers compared to environmental strains.

Characterization of curli A production on living bacterial surfaces by scanning probe microscopy

Curli are adhesive surface fibers produced by many Enterobacteriaceae, such as Escherichia coli and Salmonella enterica. They are implicated in bacterial attachment and invasion to epithelial cells. In this study, atomic force microscopy was used to determine the effects of curli on topology and mechanical properties of live E. coli cells. Young's moduli of both curli-deficient and curli-overproducing mutants were significantly lower than that of their wild-type (WT) strain, while decay lengths of the former strains were higher than that of the latter strain. Surprisingly, topological images showed that, unlike the WT and curli-overproducing mutant, the curli-deficient mutant produced a large number of flagella-like fibers, which may explain why the strain had a lower Young's modulus than the WT. These results suggest that the mechanical properties of bacterial surfaces are greatly affected by the presence of filamentous structures such as curli and flagella.

Atomic force microscopy analysis of the Acinetobacter baumannii bacteriophage AP22 lytic cycle

Background

Acinetobacter baumannii is known for its ability to develop resistance to the major groups of antibiotics, form biofilms, and survive for long periods in hospital environments. The prevalence of infections caused by multidrug-resistant A. baumannii is a significant problem for the modern health care system, and application of lytic bacteriophages for controlling this pathogen may become a solution.

Methodology/Principal Findings

In this study, using atomic force microscopy (AFM) and microbiological assessment we have investigated A. baumannii bacteriophage AP22, which has been recently described. AFM has revealed the morphology of bacteriophage AP22, adsorbed on the surfaces of mica, graphite and host bacterial cells. Besides, morphological changes of bacteriophage AP22-infected A. baumannii cells were characterized at different stages of the lytic cycle, from phage adsorption to the cell lysis. The phage latent period, estimated from AFM was in good agreement with that obtained by microbiological methods (40 min). Bacteriophage AP22, whose head diameter is 62±1 nm and tail length is 88±9 nm, was shown to disperse A. baumannii aggregates and adsorb to the bacterial surface right from the first minute of their mutual incubation at 37°C.

Conclusions/Significance

High rate of bacteriophage AP22 specific adsorption and its ability to disperse bacterial aggregates make this phage very promising for biomedical antimicrobial applications. Complementing microbiological results with AFM data, we demonstrate an effective approach, which allows not only comparing independently obtained characteristics of the lytic cycle but also visualizing the infection process.

Measuring the stiffness of bacterial cells from growth rates in hydrogels of tunable elasticity

Although bacterial cells are known to experience large forces from osmotic pressure differences and their local microenvironment, quantitative measurements of the mechanical properties of growing bacterial cells have been limited. We provide an experimental approach and theoretical framework for measuring the mechanical properties of live bacteria. We encapsulated bacteria in agarose with a user-defined stiffness, measured the growth rate of individual cells and fit data to a thin-shell mechanical model to extract the effective longitudinal Young's modulus of the cell envelope of Escherichia coli (50-150 MPa), Bacillus subtilis (100-200 MPa) and Pseudomonas aeruginosa (100-200 MPa). Our data provide estimates of cell wall stiffness similar to values obtained via the more labour-intensive technique of atomic force microscopy. To address physiological perturbations that produce changes in cellular mechanical properties, we tested the effect of A22-induced MreB depolymerization on the stiffness of E. coli. The effective longitudinal Young's modulus was not significantly affected by A22 treatment at short time scales, supporting a model in which the interactions between MreB and the cell wall persist on the same time scale as growth. Our technique therefore enables the rapid determination of how changes in genotype and biochemistry affect the mechanical properties of the bacterial envelope.

Effect of colistin exposure and growth phase on the surface properties of live Acinetobacter baumannii cells examined by atomic force microscopy

The diminishing antimicrobial development pipeline has forced the revival of colistin as a last line of defence against infections caused by multidrug-resistant Gram-negative 'superbugs' such as Acinetobacter baumannii. The complete loss of lipopolysaccharide (LPS) mediates colistin resistance in some A. baumannii strains. Atomic force microscopy was used to examine the surface properties of colistin-susceptible and -resistant A. baumannii strains at mid-logarithmic and stationary growth phases in liquid and in response to colistin treatment. The contribution of LPS to surface properties was investigated using A. baumannii strains constructed with and without the lpxA gene. Bacterial spring constant measurements revealed that colistin-susceptible cells were significantly stiffer than colistin-resistant cells at both growth phases (P<0.01), whilst colistin treatment at high concentrations (32 mg/L) resulted in more rigid surfaces for both phenotypes. Multiple, large adhesive peaks frequently noted in force curves captured on colistin-susceptible cells were not evident for colistin-resistant cells. Adhesion events were markedly reduced following colistin exposure. The cell membranes of strains of both phenotypes remained intact following colistin treatment, although fine topographical details were illustrated. These studies, conducted for the first time on live A. baumannii cells in liquid, have contributed to our understanding of the action of colistin in this problematic pathogen.

Study of Antibacterial Efficacy of Hybrid Chitosan-Silver Nanoparticles for Prevention of Specific Biofilm and Water Purification

Antibacterial efficacy of silver nanoparticles (Ag NPs) deposited alternatively layer by layer (LBL) on chitosan polymer in the form of a thin film over a quartz plate and stainless steel strip has been studied. An eight-bilayer chitosan/silver (Cs/Ag)8 hybrid was prepared having a known concentration of silver. Techniques such as UV-visible spectroscopy, inductively coupled plasma optical emission spectroscopy (ICP-OES), and atomic force microscopy (AFM) were carried out to understand and elucidate the physical nature of the film. Gram-negative bacteria, Escherichia coli (E. coli), were used as a test sample in saline solution for antibacterial studies. The growth inhibition at different intervals of contact time and, more importantly, the antibacterial properties of the hybrid film on repeated cycling in saline solution have been demonstrated. AFM studies are carried out for the first time on the microbe to know the morphological changes affected by the hybrid film. The hybrid films on aging (3 months) are found to be as bioactive as before. Cytotoxicity experiments indicated good biocompatibility. The hybrid can be a promising bioactive material for the prevention of biofilms specific to E. coli and in purification of water for safe drinking.

Antibacterial action of dispersed single-walled carbon nanotubes on Escherichia coli and Bacillus subtilis investigated by atomic force microscopy

Single-walled carbon nanotubes (SWCNTs) exhibit strong antibacterial activities. Direct contact between bacterial cells and SWCNTs may likely induce cell damages. Therefore, the understanding of SWCNT-bacteria interactions is essential in order to develop novel SWCNT-based materials for their potential environmental, imaging, therapeutic, and military applications. In this preliminary study, we utilized atomic force microscopy (AFM) to monitor dynamic changes in cell morphology and mechanical properties of two typical bacterial models (gram-negative Escherichia coli and gram-positive Bacillus subtilis) upon incubation with SWCNTs. The results demonstrated that individually dispersed SWCNTs in solution develop nanotube networks on the cell surface, and then destroy the bacterial envelopes with leakage of the intracellular contents. The cell morphology changes observed on air dried samples are accompanied by an increase in cell surface roughness and a decrease in surface spring constant. To mimic the collision between SWCNTs and cells, a sharp AFM tip of 2 nm was chosen to introduce piercings on the cell surface. No clear physical damages were observed if the applied force was below 10 nN. Further analysis also indicates that a single collision between one nanotube and a bacterial cell is unlikely to introduce direct physical damage. Hence, the antibacterial activity of SWCNTs is the accumulation effect of large amount of nanotubes through interactions between SWCNT networks and bacterial cells.