Atomic force microscopy studies of bioprocess engineering surfaces - imaging, interactions and mechanical properties mediating bacterial adhesion

Diffusion-driven fabrication of calcium and phosphorous-doped zinc oxide heterostructures on titanium to achieve dual functions of osteogenesis and preventing bacterial infections

Conventional Ti-based implants are vulnerable to postsurgical infection and improving the antibacterial efficiency without compromising the osteogenic ability is one of the key issues in bone implant design. Although zinc oxide (ZnO) nanorods grown on Ti substrates hydrothermally can improve the antibacterial properties, but cannot meet the stringent requirements of bone implants, as rapid degradation of ZnO and uncontrolled leaching of Zn2+ are detrimental to peri-implant cells and tissues. To solve these problems, a lattice-damage-free method is adopted to modify the ZnO nanorods with thin calcium phosphate (CaP) shells. The Ca and P ions from the CaP shells diffuse thermally into the ZnO lattice to prevent the ZnO nanorods from rapid degradation and ensure the sustained release of Zn2+ ions as well. Furthermore, the designed heterostructural nanorods not only induce the osteogenic performances of MC3T3-E1 cells but also exhibit excellent antibacterial ability against S. aureus and E. coli bacteria via physical penetration. In vivo studies also reveal that hybrid Ti-ZnO@CaP5 can not only eradicates bacteria in contact, but also provides sufficient biocompatibility without causing excessive inflammation response. Our study provides insights into the design of multifunctional biomaterials for bone implants. STATEMENT OF SIGNIFICANCE: • A lattice-damage-free method is adopted to modify the ZnO nanorods with thin calcium phosphate (CaP) shells. • The dynamic process of Ca and P diffusion into the ZnO lattice is analyzed by experimental verification and theoretical calculation. • The degradation rate of ZnO nanorods is significantly decreased after CaP deposition. • The ZnO nanorods after CaP deposition can not only sterilize bacteria in contact via physical penetration, but also provide sufficient biocompatibility and osteogenic capability without causing excessive inflammation response..

Profiling to Probing: Atomic force microscopy to characterize nano-engineered implants

Surface modification of implants in the nanoscale or implant nano-engineering has been recognized as a strategy for augmenting implant bioactivity and achieving long-term implant success. Characterizing and optimizing implant characteristics is crucial to achieving desirable effects post-implantation. Modified implant enables tailored, guided and accelerated tissue integration; however, our understanding is limited to multicellular (bulk) interactions. Finding the nanoscale forces experienced by a single cell on nano-engineered implants will aid in predicting implants' bioactivity and engineering the next generation of bioactive implants. Atomic force microscope (AFM) is a unique tool that enables surface characterization and understanding of the interactions between implant surface and biological tissues. The characterization of surface topography using AFM to gauge nano-engineered implants' characteristics (topographical, mechanical, chemical, electrical and magnetic) and bioactivity (adhesion of cells) is presented. A special focus of the review is to discuss the use of single-cell force spectroscopy (SCFS) employing AFM to investigate the minute forces involved with the adhesion of a single cell (resident tissue cell or bacterium) to the surface of nano-engineered implants. Finally, the research gaps and future perspectives relating to AFM-characterized current and emerging nano-engineered implants are discussed towards achieving desirable bioactivity performances. This review highlights the use of advanced AFM-based characterization of nano-engineered implant surfaces via profiling (investigating implant topography) or probing (using a single cell as a probe to study precise adhesive forces with the implant surface). STATEMENT OF SIGNIFICANCE: Nano-engineering is emerging as a surface modification platform for implants to augment their bioactivity and achieve favourable treatment outcomes. In this extensive review, we closely examine the use of Atomic Force Microscopy (AFM) to characterize the properties of nano-engineered implant surfaces (topography, mechanical, chemical, electrical and magnetic). Next, we discuss Single-Cell Force Spectroscopy (SCFS) via AFM towards precise force quantification encompassing a single cell's interaction with the implant surface. This interdisciplinary review will appeal to researchers from the broader scientific community interested in implants and cell adhesion to implants and provide an improved understanding of the surface characterization of nano-engineered implants.

Evaluation of extracellular polymeric substances matrix volume, surface roughness and bacterial adhesion property of oral biofilm

Background/purpose

Oral biofilms are highly structured bacterial colonies embedded in a highly hydrated extracellular polymeric substances (EPS) matrix. This study aimed to investigate the characteristics of oral biofilm at different stages of maturation.

Materials and methods

Oral multispecies biofilms were grown anaerobically from plaque bacteria on collagen coated hydroxyapatite discs in brain heart infusion broth for one and three weeks. The volume of live bacteria and EPS matrix of the biofilms were determined by using corresponding fluorescent probes and confocal laser scanning microscopy. Atomic force microscopy (AFM) was used to quantitatively probe and correlate cell surface adhesion force of biofilms. The surface roughness was quantified in terms of the root mean square average of the height deviations. Adhesion was measured from force-distance data for the retraction of the cell from the surface.

Results

The volume of live bacteria and EPS of 3-week-old biofilms was higher than 1-week-old biofilms. The surface roughness value in 1-week-old biofilms was significantly higher than that in 3-week-old biofilms. AFM force-distance curve results showed that the adhesion force at the cell-cell interface was significantly more at-tractive than those at bacterial cells surface of both stages biofilms. Adhesion forces between the AFM tip and the surface of bacterial cell were fairly constant, whereas the cell-cell interface experienced greater adhesion forces in the biofilm's development.

Conclusion

As oral biofilms become mature, EPS volume and cell-cell adhesion forces increase while the surface roughness decreases.

Bacterial Adhesion Strength on Titanium Surfaces Quantified by Atomic Force Microscopy: A Systematic Review

Few studies have been able to elucidate the correlation of factors determining the strength of interaction between bacterial cells and substrate at the molecular level. The aim was to answer the following question: What biophysical factors should be considered when analyzing the bacterial adhesion strength on titanium surfaces and its alloys for implants quantified by atomic force microscopy? This review followed PRISMA. The search strategy was applied in four databases. The selection process was carried out in two stages. The risk of bias was analyzed. One thousand four hundred sixty-three articles were found. After removing the duplicates, 1126 were screened by title and abstract, of which 57 were selected for full reading and 5 were included; 3 had a low risk of bias and 2 moderated risks of bias. (1) The current literature shows the preference of bacteria to adhere to surfaces of the same hydrophilicity. However, this fact was contradicted by this systematic review, which demonstrated that hydrophobic bacteria developed hydrogen bonds and adhered to hydrophilic surfaces; (2) the application of surface treatments that induce the reduction of areas favorable for bacterial adhesion interfere more in the formation of biofilm than surface roughness; and (3) bacterial colonization should be evaluated in time-dependent studies as they develop adaptation mechanisms, related to time, which are obscure in this review.

Dual-action silver functionalized nanostructured titanium against drug resistant bacterial and fungal species

HYPOTHESIS

Titanium and its alloys are commonly used implant materials. Once inserted into the body, the interface of the biomaterials is the most likely site for the development of implant-associated infections. Imparting the titanium substrate with high-aspect-ratio nanostructures, which can be uniformly achieved using hydrothermal etching, enables a mechanical contact-killing (mechanoresponsive) mechanism of bacterial and fungal cells. Interaction between cells and the surface shows cellular inactivation via a physical mechanism meaning that careful engineering of the interface is needed to optimse the technology. This mechanism of action is only effective towards surface adsorbed microbes, thus any cells not directly in contact with the substrate will survive and limit the antimicrobial efficacy of the titanium nanostructures. Therefore, we propose that a dual-action mechanoresponsive and chemical-surface approach must be utilised to improve antimicrobial activity. The addition of antimicrobial silver nanoparticles will provide a secondary, chemical mechanism to escalate the microbial response in tandem with the physical puncture of the cells.

EXPERIMENTS

Hydrothermal etching is used as a facile method to impart variant nanostrucutres on the titanium substrate to increase the antimicrobial response. Increasing concentrations (0.25 M, 0.50 M, 1.0 M, 2.0 M) of sodium hydroxide etching solution were used to provide differing degrees of nanostructured morphology on the surface after 3 h of heating at 150 °C. This produced titanium nanospikes, nanoblades, and nanowires, respectively, as a function of etchant concentration. These substrates then provided an interface for the deposition of silver nanoparticles via a reduction pathway. Methicillin-resistant Staphylococcous aureus (MRSA) and Candida auris (C. auris) were used as model bacteria and fungi, respectively, to test the effectiveness of the nanostructured titanium with and without silver nanoparticles, and the bio-interactions at the interface.

FINDINGS

The presence of nanostructure increased the bactericidal response of titanium against MRSA from ∼ 10 % on commercially pure titanium to a maximum of ∼ 60 % and increased the fungicidal response from ∼ 10 % to ∼ 70 % in C. auris. Introducing silver nanoparticles increased the microbiocidal response to ∼ 99 % towards both bacteria and fungi. Importantly, this study highlights that nanostructure alone is not sufficient to develop a highly antimicrobial titanium substrate. A dual-action, physical and chemical antimicrobial approach is better suited to produce highly effective antibacterial and antifungal surface technologies.

Understanding the role of surface interactions in the antibacterial activity of layered double hydroxide nanoparticles by atomic force microscopy

Understanding the mechanisms of the interactions between zinc-based layered double hydroxides (LDHs) and bacterial surfaces is of great importance to improve the efficiency of these antibiotic-free antibacterial agents. In fact, the role of surface interactions in the antibacterial activity of zinc-based LDH nanoparticles compared to that of dissolution and generation of reactive oxygen species (ROS) is still not well documented. In this study, we show that ZnAl LDH nanoparticles exhibit a strong antibacterial effect against Staphylococcus aureus by inducing serious cell wall damages as revealed by the antibacterial activity tests and atomic force microscopy (AFM) imaging, respectively. The comparison of the antibacterial properties of ZnAl LDH nanoparticles and micron-sized ZnAl LDHs also demonstrated that the antibacterial activity of Zn-based LDHs goes beyond the simple dissolution into Zn²⁺ antibacterial ions. Furthermore, we developed an original approach to functionalize AFM tips with LDH films in order to probe their interactions with living S. aureus cells by means of AFM-based force spectroscopy (FS). The force spectroscopy analysis revealed that antibacterial ZnAl LDH nanoparticles show specific recognition of S. aureus cells with high adhesion frequency and remarkable force magnitudes. This finding provides a first insight into the antibacterial mechanism of Zn-based LDHs through direct surface interactions by which they are able to recognize and adhere to bacterial surfaces, thus damaging them and leading to subsequent growth inhibition.

A critical analysis of research methods and experimental models to study irrigants and irrigation systems

Irrigation plays an essential role in root canal treatment. The purpose of this narrative review was to critically appraise the experimental methods and models used to study irrigants and irrigation systems and to provide directions for future research. Studies on the antimicrobial effect of irrigants should use mature multispecies biofilms grown on dentine or inside root canals and should combine at least two complementary evaluation methods. Dissolution of pulp tissue remnants should be examined in the presence of dentine and, preferably, inside human root canals. Micro-omputed tomography is currently the method of choice for the assessment of accumulated dentine debris and their removal. A combination of experiments in transparent root canals and numerical modeling is needed to address irrigant penetration. Finally, models to evaluate irrigant extrusion through the apical foramen should simulate the periapical tissues and provide quantitative data on the amount of extruded irrigant. Mimicking the in vivo conditions as close as possible and standardization of the specimens and experimental protocols are universal requirements irrespective of the surrogate endpoint studied. Obsolete and unrealistic models must be abandoned in favour of more appropriate and valid ones that have more direct application and translation to clinical Endodontics.

Cold plasma surface treatments to prevent biofilm formation in food industries and medical sectors

Environmental conditions in food and medical fields enable the bacteria to attach and grow on surfaces leading to resistant bacterial biofilm formation. Indeed, the first step in biofilm formation is the bacterial irreversible adhesion. Controlling and inhibiting this adhesion is a passive approach to fight against biofilm development. This strategy is an interesting path in the inhibition of biofilm formation since it targets the first step of biofilm development. Those pathogenic structures are responsible for several foodborne diseases and nosocomial infections. Therefore, to face this public health threat, researchers employed cold plasma technologies in coating development. In this review, the different factors influencing the bacterial adhesion to a substrate are outlined. The goal is to present the passive coating strategies aiming to prevent biofilm formation via cold plasma treatments, highlighting antiadhesive elaborated surfaces. General aspects of surface treatment, including physico-chemical modification and application of cold plasma technologies, were also presented. KEY POINTS: • Factors surrounding pathogenic bacteria influence biofilm development. • Controlling bacterial adhesion prevents biofilm formation. • Materials can be coated via cold plasma to inhibit bacterial adhesion.

Nano- and Macroscale Imaging of Cholesterol Linoleate and Human Beta Defensin 2-Induced Changes in Pseudomonas aeruginosa Biofilms

The biofilm production of Pseudomonas aeruginosa (PA) is central to establishing chronic infection in the airways in cystic fibrosis. Epithelial cells secrete an array of innate immune factors, including antimicrobial proteins and lipids, such as human beta defensin 2 (HBD2) and cholesteryl lineolate (CL), respectively, to combat colonization by pathogens. We have recently shown that HBD2 inhibits biofilm production by PA, possibly linked to interference with the transport of biofilm precursors. Considering that both HBD2 and CL are increased in airway fluids during infection, we hypothesized that CL synergizes with HBD2 in biofilm inhibition. CL was formulated in phospholipid-based liposomes (CL-PL). As measured by atomic force microscopy of single bacteria, CL-PL alone and in combination with HBD2 significantly increased bacterial surface roughness. Additionally, extracellular structures emanated from untreated bacterial cells, but not from cells treated with CL-PL and HBD2 alone and in combination. Crystal violet staining of the biofilm revealed that CL-PL combined with HBD2 effected a significant decrease of biofilm mass and increased the number of larger biofilm particles consistent with altered cohesion of formed biofilms. These data suggest that CL and HBD2 affect PA biofilm formation at the single cell and community-wide level and that the community-wide effects of CL are enhanced by HBD2. This research may inform future novel treatments for recalcitrant infections in the airways of CF patients.

Bacterial adhesion to biomaterials: What regulates this attachment? A review

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Bacterial adhesion to the surface of dental materials play a significant role in infections.

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The factors that govern microbial attachment involves different types of physical-chemical interactions and biological processes.

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Studying bacterial adhesion makes it possible to understand the mechanisms involved in attachment and helps in the search for technologies that promote antibacterial surfaces.

Bacterial attachment to biomaterials is of great interest to the medical and dental field due to its impact on dental implants, dental prostheses, and others, leading to the need to introduce methods for biofilm control and mitigation of infections. Biofilm adhesion is a multifactorial process and involves characteristics relevant to the bacterial cell as well as biological, chemical, and physical properties relative to the surface of biomaterials. Bacteria encountered different environmental conditions during their growth and developed interspecies communication strategies, as well as various mechanisms to detect the environment and facilitate survival, such as chemical sensors or physical detection mechanisms. However, the factors that govern microbial attachment to surfaces are not yet fully understood. In order to understand how bacteria interact with surfaces, as well as to characterize the physical-chemical properties of bacteria adhesins, and to determine their interrelation with the adhesion to the substrate, in recent years new techniques of atomic force microscopy (AFM) have been developed and helped by providing quantitative results. Thus, the purpose of this review is to gather current studies about the factors that regulate microbial adhesion to surfaces in order to offer a guide to studies to obtain technologies that provide an antimicrobial surface.

Growth Phase- and Desiccation-Dependent Acinetobacter baumannii Morphology: An Atomic Force Microscopy Investigation

Acinetobacter baumannii has emerged as a major bacterial pathogen during the past three decades. The majority of the A. baumannii infections occur in hospitals and are caused by strains endowed with high desiccation tolerance, which represents an essential feature for the adaptation to the nosocomial environment. This work aims at investigating the desiccation response of the multidrug-resistant A. baumannii strain ACICU as a function of the bacterial growth phase and oxygen availability, by correlating bacterial survival with shape alterations. The three-dimensional morphological analysis of bacteria was carried out by atomic force microscopy (AFM), following the evolution of bacterial shape descriptors, such as the area, volume, roughness of individual cell membranes, and the cell cluster roughness, which exhibited peculiar and distinctive behavior as a function of the growth conditions. AFM images of A. baumannii ACICU cells revealed the prevalence of the coccoid morphology at all growth stages, with a tendency to reduce their size in the stationary phase, accompanied by a higher survival rate to air-drying. Moreover, cells harvested from the logarithmic phase featured a larger volume and resulted to be more sensitive to desiccation compared to the cells harvested at later growth stages. In addition, oxygen deprivation caused a significant decrease in cellular size and was associated with the formation of pores in the cell membrane, accompanied by a relative reduction in culturability after desiccation. Morphological plasticity and multidrug resistance may contribute to desiccation tolerance and therefore to persistence in the hospital setting.

Mechano-bactericidal actions of nanostructured surfaces

Antibiotic resistance is a global human health threat, causing routine treatments of bacterial infections to become increasingly difficult. The problem is exacerbated by biofilm formation by bacterial pathogens on the surfaces of indwelling medical and dental devices that facilitate high levels of tolerance to antibiotics. The development of new antibacterial nanostructured surfaces shows excellent prospects for application in medicine as next-generation biomaterials. The physico-mechanical interactions between these nanostructured surfaces and bacteria lead to bacterial killing or prevention of bacterial attachment and subsequent biofilm formation, and thus are promising in circumventing bacterial infections. This Review explores the impact of surface roughness on the nanoscale in preventing bacterial colonization of synthetic materials and categorizes the different mechanisms by which various surface nanopatterns exert the necessary physico-mechanical forces on the bacterial cell membrane that will ultimately result in cell death.

Resazurin assay for assessment of antimicrobial properties of electrospun nanofiber filtration membranes

We developed a simple and fast microplate assay for evaluation of the antimicrobial activity of electrospun nanofiber filtration membranes or similar porous materials for water treatment technologies. Resazurin (alamarBlue®) was used as an indicator of the amount of viable experimental microorganisms Gram-negative Escherichia coli, Gram-positive Enterococcus faecalis, and natural wastewater treatment plant effluent bacteria. A bacterial inoculum of concentration 1-3 × 105 CFU mL-1 was pipetted onto the surface of assessed both functionalized and respective control membranes and incubated in 12-well plates for 4 h at 37 °C. Kinetics of resazurin metabolization, i.e. its reduction to fluorescent resorufin, was evaluated fluorimetrically (λex520/λem590 nm). A number of viable bacteria on the membranes expressed as CFU mL-1 was calculated from the kinetic curves by using calibration curves that were constructed for both experimental bacterial species. Antimicrobial activities of the membranes were evaluated by either resazurin assay or modified ISO 20743 plate count assay. Results of both assays showed the significant antimicrobial activity of membranes functionalized with silver nanoparticles for both bacterial species and wastewater treatment plant effluent bacteria as well (log CFU reduction compared to control membrane > 4), while membranes containing specific quaternary ammonium salts were inefficient (log CFU reduction < 1). The suitability of resazurin microplate assay for testing nanofiber filtration membranes and analogous matrices has proven to be a faster and less demanding alternative to the traditionally used approach providing comparable results.

Layered Structure and Complex Mechanochemistry Underlie Strength and Versatility in a Bacterial Adhesive

While designing synthetic adhesives that perform in aqueous environments has proven challenging, microorganisms commonly produce bioadhesives that efficiently attach to a variety of substrates, including wet surfaces. The aquatic bacterium Caulobacter crescentus uses a discrete polysaccharide complex, the holdfast, to strongly attach to surfaces and resist flow. The holdfast is extremely versatile and has impressive adhesive strength. Here, we used atomic force microscopy in conjunction with superresolution microscopy and enzymatic assays to unravel the complex structure of the holdfast and to characterize its chemical constituents and their role in adhesion. Our data support a model whereby the holdfast is a heterogeneous material organized as two layers: a stiffer nanoscopic core layer wrapped into a sparse, far-reaching, flexible brush layer. Moreover, we found that the elastic response of the holdfast evolves after surface contact from initially heterogeneous to more homogeneous. From a composition point of view, besides N-acetyl-d-glucosamine (NAG), the only component that had been identified to date, our data show that the holdfast contains peptides and DNA. We hypothesize that, while polypeptides are the most important components for adhesive force, the presence of DNA mainly impacts the brush layer and the strength of initial adhesion, with NAG playing a primarily structural role within the core. The unanticipated complexity of both the structure and composition of the holdfast likely underlies its versatility as a wet adhesive and its distinctive strength. Continued improvements in understanding of the mechanochemistry of this bioadhesive could provide new insights into how bacteria attach to surfaces and could inform the development of new adhesives.IMPORTANCE There is an urgent need for strong, biocompatible bioadhesives that perform underwater. To strongly adhere to surfaces and resist flow underwater, the bacterium Caulobacter crescentus produces an adhesive called the holdfast, the mechanochemistry of which remains undefined. We show that the holdfast is a layered structure with a stiff core layer and a polymeric brush layer and consists of polysaccharides, polypeptides, and DNA. The DNA appears to play a role in the structure of the brush layer and initial adhesion, the peptides in adhesive strength, and the polysaccharides in the structure of the core. The complex, multilayer organization and diverse chemistry described here underlie the distinctive adhesive properties of the holdfast and will provide important insights into the mechanisms of bacterial adhesion and bioadhesive applications.

Bio-mimicking nano and micro-structured surface fabrication for antibacterial properties in medical implants

Orthopaedic and dental implants have become a staple of the medical industry and with an ageing population and growing culture for active lifestyles, this trend is forecast to continue. In accordance with the increased demand for implants, failure rates, particularly those caused by bacterial infection, need to be reduced. The past two decades have led to developments in antibiotics and antibacterial coatings to reduce revision surgery and death rates caused by infection. The limited effectiveness of these approaches has spurred research into nano-textured surfaces, designed to mimic the bactericidal properties of some animal, plant and insect species, and their topographical features. This review discusses the surface structures of cicada, dragonfly and butterfly wings, shark skin, gecko feet, taro and lotus leaves, emphasising the relationship between nano-structures and high surface contact angles on self-cleaning and bactericidal properties. Comparison of these surfaces shows large variations in structure dimension and configuration, indicating that there is no one particular surface structure that exhibits bactericidal behaviour against all types of microorganisms. Recent bio-mimicking fabrication methods are explored, finding hydrothermal synthesis to be the most commonly used technique, due to its environmentally friendly nature and relative simplicity compared to other methods. In addition, current proposed bactericidal mechanisms between bacteria cells and nano-textured surfaces are presented and discussed. These models could be improved by including additional parameters such as biological cell membrane properties, adhesion forces, bacteria dynamics and nano-structure mechanical properties. This paper lastly reviews the mechanical stability and cytotoxicity of micro and nano-structures and materials. While the future of nano-biomaterials is promising, long-term effects of micro and nano-structures in the body must be established before nano-textures can be used on orthopaedic implant surfaces as way of inhibiting bacterial adhesion.

Editorial: The European Symposium on Biochemical Engineering Sciences, Dublin 2016

The European Symposium on Biochemical Engineering Sciences, Dublin 2016.