Surface Topography Hinders Bacterial Surface Motility

Microbes in porous environments: from active interactions to emergent feedback

Microbes thrive in diverse porous environments-from soil and riverbeds to human lungs and cancer tissues-spanning multiple scales and conditions. Short- to long-term fluctuations in local factors induce spatio-temporal heterogeneities, often leading to physiologically stressful settings. How microbes respond and adapt to such biophysical constraints is an active field of research where considerable insight has been gained over the last decades. With a focus on bacteria, here we review recent advances in self-organization and dispersal in inorganic and organic porous settings, highlighting the role of active interactions and feedback that mediates microbial survival and fitness. We discuss open questions and opportunities for using integrative approaches to advance our understanding of the biophysical strategies which microbes employ at various scales to make porous settings habitable.

External Stimuli-Responsive Strategies for Surface Modification of Orthopedic Implants: Killing Bacteria and Enhancing Osteogenesis

Bacterial infection and insufficient osteogenic activity are the main causes of orthopedic implant failure. Conventional surface modification methods are difficult to meet the requirements for long-term implant placement. In order to better regulate the function of implant surfaces, especially to improve both the antibacterial and osteogenic activity, external stimuli-responsive (ESR) strategies have been employed for the surface modification of orthopedic implants. External stimuli act as "smart switches" to regulate the surface interactions with bacteria and cells. The balance between antibacterial and osteogenic capabilities of implant surfaces can be achieved through these specific ESR manifestations, including temperature changes, reactive oxygen species production, controlled release of bioactive molecules, controlled release of functional ions, etc. This Review summarizes the recent progress on different ESR strategies (based on light, ultrasound, electric, and magnetic fields) that can effectively balance antibacterial performance and osteogenic capability of orthopedic implants. Furthermore, the current limitations and challenges of ESR strategies for surface modification of orthopedic implants as well as future development direction are also discussed.

A Temperature and pH Dual-Sensitive Multifunctional Polyurethane with Bacteria-Triggered Antibacterial Activity

An effective and practical antibacterial strategy is to design multifunctional and stimuli-responsive materials that exhibit antibacterial activity in response to bacterial triggers. In this study, because the metabolism of Staphylococcus aureus (S. aureus) can acidify the surrounding environment and pH level can affect the lower critical solution temperature of temperature/pH dual-sensitive polymers, a monomer containing a temperature-sensitive N-isopropyl amide derivative and pH-sensitive tertiary amine groups is first synthesized. Then, the monomer is copolymerized with a polyurethane chain, and partial tertiary amine groups are quaternized to obtain bactericidal activity. The modified polyurethane exhibits temperature/pH sensitivity, antibacterial adhesion activity, bactericidal activity, and good cytocompatibility. An in situ investigation of bacterial behavior and pH changes in the bacterial suspension during the process confirms that the temperature/pH dual-sensitive polyurethane successfully achieves antibacterial activity though the metabolic activity of S. aureus without external intervention. This design concept provides a new perspective for antibacterial material design.

Overview of strategies to improve the antibacterial property of dental implants

The increasing number of peri-implant diseases and the unsatisfactory results of conventional treatment are causing great concern to patients and medical staff. The effective removal of plaque which is one of the key causes of peri-implant disease from the surface of implants has become one of the main problems to be solved urgently in the field of peri-implant disease prevention and treatment. In recent years, with the advancement of materials science and pharmacology, a lot of research has been conducted to enhance the implant antimicrobial properties, including the addition of antimicrobial coatings on the implant surface, the adjustment of implant surface topography, and the development of new implant materials, and significant progress has been made in various aspects. Antimicrobial materials have shown promising applications in the prevention of peri-implant diseases, but meanwhile, there are some shortcomings, which leads to the lack of clinical widespread use of antimicrobial materials. This paper summarizes the research on antimicrobial materials applied to implants in recent years and presents an outlook on the future development.

Enhanced bio-affinity of magnetic QD-P(St-GMA)@Fe3O4 micro-particles via surface-quaternized modification

In this work, a kind of bio-carrier quaternized-polystyrene-polyglycidyl methacrylate@Fe₃O₄ (QD-P(St-GMA)@Fe₃O₄, QD-PSGF) micro-particles was successfully prepared by modifying PSGF micro-particles through a hydrothermal method. The quaternary ammonium group and surface structure of QD-PSGF were confirmed through several characterization methods. We directly verified the efficacy of the quaternary ammonium group in promoting microbial activity due to QD-PSGF being synthesized by a hydrothermal method without changing the surface topography and pore. The bio-affinity of QD-PSGF microspheres was evaluated by bacterial adhesion and anaerobic digestion experiments. The results showed that a little quaternary ammonium group can increase bacterial adhesion by about 2-3 times and methane production by 40%. The novel developed QD-PSGF micro-particles can be a promising material as a biofilm carrier for bio-application.

Surface Texturing and Combinatorial Approaches to Improve Biocompatibility of Implanted Biomaterials

Biomaterial associated microbial infection and blood thrombosis are two of the barriers that inhibit the successful use of implantable medical devices in modern healthcare. Modification of surface topography is a promising approach to combat microbial infection and thrombosis without altering bulk material properties necessary for device function and without contributing to bacterial antibiotic resistance. Similarly, the use of other antimicrobial techniques such as grafting poly(ethylene glycol) (PEG) and nitric oxide (NO) release also improve the biocompatibility of biomaterials. In this review, we discuss the development of surface texturing techniques utilizing ordered submicron-size pillars for controlling bacterial adhesion and biofilm formation, and we present combinatorial approaches utilizing surface texturing in combination with poly(ethylene glycol) (PEG) grafting and NO release to improve the biocompatibility of biomaterials. The manuscript also discusses efforts towards understanding the molecular mechanisms of bacterial adhesion responses to the surface texturing and NO releasing biomaterials, focusing on experimental aspects of the approach.

In situ real-time investigation of Staphylococcus aureus on hemisphere-patterned polyurethane films

Surface patterning is a promising approach to prevent bacterial adhesion and biofilm formation without the concerns of antimicrobial resistance. To determine the parameters of a patterned surface that can affect bacterial behavior, a sphere-like coccus (Staphylococcus aureus) was investigated on a series of polyurethane films with ordered hemisphere patterns. The bacterial retention data in a growth medium indicated that the surface patterns significantly decreased bacterial adhesion and proliferation. The most notable effects were observed with the 2 µm-pattern as well as the patterned polycaprolactone and polystyrene films, and the accessible contact area of the polyurethane films, surface wettability, and spatial confinement, did not show an influence. An optical microscope with a modified incubation cell was used for in situ real-time observations of bacterial colonization, proliferation, and migration. Based on appropriate statistical analyses, it was concluded that topographical geometry played a dominant role. In combination with the retention assessment in a nongrowth medium, it was found that pattern-mediated inhibition of biofilm formation was mainly achieved by affecting bacterial proliferation rather than adhesion. This study provides new insight for designing biofilm-resistant biomimetic materials.

Self-Patterned Nanoscale Topography of Thin Copolymer Films Prepared by Evaporative Assembly-Resist Early-Stage Bacterial Adhesion

Biofilm formation on the surfaces of indwelling medical devices has become a growing health threat due to the development of antimicrobial resistance to infection-causing bacteria. For example, ventilator-associated pneumonia caused by Pseudomonas and Staphylococci species has become a significant concern in treatment of patients during COVID-19 pandemic. Nanostructured surfaces with antifouling activity are of interest as a promising strategy to prevent bacterial adhesion without triggering drug resistance. In this study, we report a facile evaporative approach to prepare block copolymer film coatings with nanoscale topography that resist bacterial adhesion. The initial attachment of the target bacterium Pseudomonas aeruginosa PAO1 to copolymer films as well as homopolymer films was evaluated by fluorescence microscopy. Significant reduction in bacterial adhesion (93-99% less) and area coverage (>92% less) on the copolymer films was observed compared with that on the control and homopolymer films [poly(methacrylic acid) (PMAA)─only 40 and 23% less, respectively]. The surfaces of poly(styrene)-PMAA copolymer films with patterned nanoscale topography that contains sharp peaks ranging from 20 to 80 nm spaced at 30-50 nm were confirmed by atomic force microscopy and the corresponding surface morphology analysis. Investigation of the surface wettability and surface potential of polymer films assists in understanding the effect of surface properties on the bacterial attachment. Comparison of bacterial growth studies in polymer solutions with the growth studies on coatings highlights the importance of physical nanostructure in resisting bacterial adhesion, as opposed to chemical characteristics of the copolymers. Such self-patterned antifouling surface coatings, produced with a straightforward and energy-efficient approach, could provide a convenient and effective method to resist bacterial fouling on the surface of medical devices and reduce device-associated infections.

Motility Suppression and Trapping Bacteria by ZnO Nanostructures

Regulating the swimming motility of bacteria near surfaces is essential to suppress or avoid bacterial contamination and infection in catheters and medical devices with wall surfaces. However, the motility of bacteria near walls strongly depends on the combination of the local physicochemical properties of the surfaces. To unravel how nanostructures and their local chemical microenvironment dynamically affect the bacterial motility near surfaces, here, we directly visualize the bacterial swimming and systematically analyze the motility of Escherichia coli swimming on ZnO nanoparticle films and nanowire arrays with further ultraviolet irradiation. The results show that the ZnO nanowire arrays reduce the swimming motility, thus significantly enhancing the trapping ability for motile bacteria. Additionally, thanks to the wide bandgap nature of a ZnO semiconductor, the ultraviolet irradiation rapidly reduces the bacteria locomotion due to the hydroxyl and singlet oxygen produced by the photodynamic effects of ZnO nanowire arrays in an aqueous solution. The findings quantitatively reveal how the combination of geometrical nanostructured surfaces and local tuning of the steric microenvironment are able to regulate the motility of swimming bacteria and suggest the efficient inhibition of bacterial translocation and infection by nanostructured coatings.

Microbial Response to Micrometer-Scale Multiaxial Wrinkled Surfaces

We investigate the effect of micrometer-scale surface wrinkling on the attachment and proliferation of model bacteria (Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli K12) and fungi (Candida albicans). Specifically, sinusoidal (1D), checkerboard (C), and herringbone (H) patterns were fabricated by mechanical wrinkling of plasma-oxidized polydimethylsiloxane (PDMS) bilayers and contrasted with flat (F) surfaces. Microbial deformation and orientation were found to correlate with the aspect ratio and commensurably with surface pattern dimensions and local pattern order. Significantly, the proliferation of P. aeruginosa could be described by a linear scaling between bacterial area coverage and available surface area, defined as a fraction of the line integral along each profile with negative curvature. However, in the early stages of proliferation (up to 6 h examined), that C and H patterns disrupt the spatial arrangement of bacteria, impeding proliferation for several hours and reducing it (by ∼50%) thereafter. Our findings suggest a simple framework to rationalize the impact of micrometer-scale topography on microbial action and demonstrate that multiaxial patterning order provides an effective strategy to delay and frustrate the early stages of bacterial proliferation.

Manipulating Bacterial Biofilms Using Materiobiology and Synthetic Biology Approaches

Bacteria form biofilms on material surfaces within hours. Biofilms are often considered problematic substances in the fields such as biomedical devices and the food industry; however, they are beneficial in other fields such as fermentation, water remediation, and civil engineering. Biofilm properties depend on their genome and the extracellular environment, including pH, shear stress, and matrices topography, stiffness, wettability, and charges during biofilm formation. These surface properties have feedback effects on biofilm formation at different stages. Due to emerging technology such as synthetic biology and genome editing, many studies have focused on functionalizing biofilm for specific applications. Nevertheless, few studies combine these two approaches to produce or modify biofilms. This review summarizes up-to-date materials science and synthetic biology approaches to controlling biofilms. The review proposed a potential research direction in the future that can gain better control of bacteria and biofilms.

NIR-Responsive TiO2 Biometasurfaces: Toward In Situ Photodynamic Antibacterial Therapy for Biomedical Implants

Implant-related microbial infection is a challenging clinical problem, and its treatment requires efficient eradication of the biofilm from the implant surface. Near-infrared (NIR)-responsive strategies are proposed as an emerging efficient antibacterial therapy. However, the utilization of photosensitizers or photocatalytic/photothermal nanomaterials in the available approach likely induces high potential risks of interfacial deterioration and biosafety compromise. Herein, a TiO₂ /TiO_{2- x} metasurface with potent NIR-responsive antibacterial activity is produced on a Ti alloy implant by a newly invented topochemical conversion-based alkaline-acid bidirectional hydrothermal method (aaBH). Electromagnetic simulations prove that NIR absorption and near-field distribution of the metasurface can be tuned by the dimension and arrangement of the nanostructural unit. Promising antibacterial efficacy is proved by both in vitro and in vivo tests, with low-power NIR irradiation for 10 min. Besides, the designed nanostructure in the metasurface itself also shows excellence in enhancing the adhesion-related gene expression of human gingival fibroblasts that are exposed to 10 min of NIR irradiation, proving the potent nanostructure-induced biological effects. This work provides a biosafe and upscalable metasurfacing approach with extraordinary capacity of manipulating light adsorption, photocatalysis, and biological properties.

Antibacterial surfaces: Strategies and applications

Antibacterial surfaces are surfaces that can resist bacteria, relying on the nature of the material itself. It is significant for safe food and water, human health, and industrial equipment. Biofilm is the main form of bacterial contamination on the material surface. Preventing the formation of biofilm is an efficient way to develop antibacterial surfaces. The strategy for constructing the antibacterial surface is divided into bacteria repelling and bacteria killing based on the formation of the biofilm. Material surface wettability, adhesion, and steric hindrance determine bacteria repelling performance. Bacteria should be killed by surface chemistry or physical structures when they are attached to a material surface irreversibly. Killing approaches are usually in the light of the cell membrane of bacteria. This review summarizes the fabrication methods and applications of antibacterial surfaces from the view of the treatment of the material surfaces. We also present several crucial points for developing long-term stability, no drug resistance, broad-spectrum, and even programable antibacterial surfaces.

Ultra-Short Laser Surface Properties Optimization of Biocompatibility Characteristics of 3D Poly-ε-Caprolactone and Hydroxyapatite Composite Scaffolds

The use of laser processing for the creation of diverse morphological patterns onto the surface of polymer scaffolds represents a method for overcoming bacterial biofilm formation and inducing enhanced cellular dynamics. We have investigated the influence of ultra-short laser parameters on 3D-printed poly-ε-caprolactone (PCL) and poly-ε-caprolactone/hydroxyapatite (PCL/HA) scaffolds with the aim of creating submicron geometrical features to improve the matrix biocompatibility properties. Specifically, the present research was focused on monitoring the effect of the laser fluence (F) and the number of applied pulses (N) on the morphological, chemical and mechanical properties of the scaffolds. SEM analysis revealed that the femtosecond laser treatment of the scaffolds led to the formation of two distinct surface geometrical patterns, microchannels and single microprotrusions, without triggering collateral damage to the surrounding zones. We found that the microchannel structures favor the hydrophilicity properties. As demonstrated by the computer tomography results, surface roughness of the modified zones increases compared to the non-modified surface, without influencing the mechanical stability of the 3D matrices. The X-ray diffraction analysis confirmed that the laser structuring of the matrices did not lead to a change in the semi-crystalline phase of the PCL. The combinations of two types of geometrical designs-wood pile and snowflake-with laser-induced morphologies in the form of channels and columns are considered for optimizing the conditions for establishing an ideal scaffold, namely, precise dimensional form, mechanical stability, improved cytocompatibility and antibacterial behavior.

Tuning surface topographies on biomaterials to control bacterial infection

Microbial contamination and subsequent formation of biofilms frequently cause failure of surgical implants and a good understanding of the bacteria-surface interactions is vital to the design and safety of biomaterials. In this review, the physical and chemical factors that are involved in the various stages of implant-associated bacterial infection are described. In particular, topographical modification strategies that have been employed to mitigate bacterial adhesion via topographical mechanisms are summarized and discussed comprehensively. Recent advances have improved our understanding about bacteria-surface interactions and have enabled biomedical engineers and researchers to develop better and more effective antibacterial surfaces. The related interdisciplinary efforts are expected to continue in the quest for next-generation medical devices to attain the ultimate goal of improved clinical outcomes and reduced number of revision surgeries.

The Synergy of Topographical Micropatterning and Ta|TaCu Bilayered Thin Film on Titanium Implants Enables Dual-Functions of Enhanced Osteogenesis and Anti-Infection

Poor osteogenesis and implant-associated infection are the two leading causes of failure for dental and orthopedic implants. Surface design with enhanced osteogenesis often fails in antibacterial activity, or vice versa. Herein, a surface design strategy, which overcomes this trade-off via the synergistic effects of topographical micropatterning and a bilayered nanostructured metallic thin film is presented. A specific microgrooved pattern is fabricated on the titanium surface, followed by sequential deposition of a nanostructured copper (Cu)-containing tantalum (Ta) (TaCu) layer and a pure Ta cap layer. The microgrooved patterns coupled with the nanorough Ta cap layer shows strong contact guidance to preosteoblasts and significantly enhances the osteogenic differentiation in vitro, while the controlled local sustained release of Cu ions is responsible for high antibacterial activity. Importantly, rat calvarial defect models in vivo further confirm that the synergy of microgrooved patterns and the Ta|TaCu bilayered thin film on titanium surface could effectively promote bone regeneration. The present effective and versatile surface design strategy provides significant insight into intelligent surface engineering that can control biological response at the site of healing in dental and orthopedic implants.

Implication of Surface Properties, Bacterial Motility, and Hydrodynamic Conditions on Bacterial Surface Sensing and Their Initial Adhesion

Biofilms are structured microbial communities attached to surfaces, which play a significant role in the persistence of biofoulings in both medical and industrial settings. Bacteria in biofilms are mostly embedded in a complex matrix comprised of extracellular polymeric substances that provide mechanical stability and protection against environmental adversities. Once the biofilm is matured, it becomes extremely difficult to kill bacteria or mechanically remove biofilms from solid surfaces. Therefore, interrupting the bacterial surface sensing mechanism and subsequent initial binding process of bacteria to surfaces is essential to effectively prevent biofilm-associated problems. Noting that the process of bacterial adhesion is influenced by many factors, including material surface properties, this review summarizes recent works dedicated to understanding the influences of surface charge, surface wettability, roughness, topography, stiffness, and combination of properties on bacterial adhesion. This review also highlights other factors that are often neglected in bacterial adhesion studies such as bacterial motility and the effect of hydrodynamic flow. Lastly, the present review features recent innovations in nanotechnology-based antifouling systems to engineer new concepts of antibiofilm surfaces.

Escherichia coli Adhesion and Biofilm Formation on Polydimethylsiloxane are Independent of Substrate Stiffness

Bacterial adhesion and biofilm formation on the surface of biomedical devices are detrimental processes that compromise patient safety and material functionality. Several physicochemical factors are involved in biofilm growth, including the surface properties. Among these, material stiffness has recently been suggested to influence microbial adhesion and biofilm growth in a variety of polymers and hydrogels. However, no clear consensus exists about the role of material stiffness in biofilm initiation and whether very compliant substrates are deleterious to bacterial cell adhesion. Here, by systematically tuning substrate topography and stiffness while keeping the surface free energy of polydimethylsiloxane substrates constant, we show that topographical patterns at the micron and submicron scale impart unique properties to the surface which are independent of the material stiffness. The current work provides a better understanding of the role of material stiffness in bacterial physiology and may constitute a cost-effective and simple strategy to reduce bacterial attachment and biofilm growth even in very compliant and hydrophobic polymers.

Characterization of Biofilm Formation by Mycobacterium chimaera on Medical Device Materials

Non-tuberculous mycobacteria (NTM) are widespread in the environment and are a public health concern due to their resistance to antimicrobial agents. The colonization of surgical heater-cooler devices (HCDs) by the slow-growing NTM species Mycobacterium chimaera has recently been linked to multiple invasive infections in patients worldwide. The resistance of M. chimaera to antimicrobials may be aided by a protective biofilm matrix of extracellular polymeric substances (EPS). This study explored the hypothesis that M. chimaera can form biofilms on medically relevant materials. Several M. chimaera strains, including two HCD isolates, were used to inoculate a panel of medical device materials. M. chimaera colonization of the surfaces was monitored for 6 weeks. M. chimaera formed a robust biofilm at the air-liquid interface of borosilicate glass tubes, which increased in mass over time. M. chimaera was observed by 3D Laser Scanning Microscopy to have motility during colonization, and form biofilms on stainless steel, titanium, silicone and polystyrene surfaces during the first week of inoculation. Scanning electron microscopy (SEM) of M. chimaera biofilms after 4 weeks of inoculation showed that M. chimaera cells were enclosed entirely in extracellular material, while cryo-preserved SEM samples further revealed that an ultrastructural component of the EPS matrix was a tangled mesh of 3D fiber-like projections connecting cells. Considering that slow-growing M. chimaera typically has culture times on the order of weeks, the microscopically observed ability to rapidly colonize stainless steel and titanium surfaces in as little as 24 h after inoculation is uncharacteristic. The insights that this study provides into M. chimaera colonization and biofilm formation of medical device materials are a significant advance in our fundamental understanding of M. chimaera surface interactions and have important implications for research into novel antimicrobial materials, designs and other approaches to help reduce the risk of infection.

How microbes read the map: Effects of implant topography on bacterial adhesion and biofilm formation

Microbes have remarkable capabilities to attach to the surface of implanted medical devices and form biofilms that adversely impact device function and increase the risk of multidrug-resistant infections. The physicochemical properties of biomaterials have long been known to play an important role in biofilm formation. More recently, a series of discoveries in the natural world have stimulated great interest in the use of 3D surface topography to engineer antifouling materials that resist bacterial colonization. There is also increasing evidence that some medical device surface topographies, such as those designed for tissue integration, may unintentionally promote microbial attachment. Despite a number of reviews on surface topography and biofilm control, there is a missing link between how bacteria sense and respond to 3D surface topographies and the rational design of antifouling materials. Motivated by this gap, we present a review of how bacteria interact with surface topographies, and what can be learned from current laboratory studies of microbial adhesion and biofilm formation on specific topographic features and medical devices. We also address specific biocompatibility considerations and discuss how to improve the assessment of the anti-biofilm performance of topographic surfaces. We conclude that 3D surface topography, whether intended or unintended, is an important consideration in the rational design of safe medical devices. Future research on next-generation smart antifouling materials could benefit from a greater focus on translation to real-world applications.

Potential biofilm control strategies for extended spaceflight missions

Biofilms, surface-adherent microbial communities, are associated with microbial fouling and corrosion in terrestrial water-distribution systems. Biofilms are also present in human spaceflight, particularly in the Water Recovery System (WRS) on the International Space Station (ISS). The WRS is comprised of the Urine Processor Assembly (UPA) and the Water Processor Assembly (WPA) which together recycles wastewater from human urine and recovered humidity from the ISS atmosphere. These wastewaters and various process streams are continually inoculated with microorganisms primarily arising from the space crew microbiome. Biofilm-related fouling has been encountered and addressed in spacecraft in low Earth orbit, including ISS and the Russian Mir Space Station. However, planned future missions beyond low Earth orbit to the Moon and Mars present additional challenges, as resupplying spare parts or support materials would be impractical and the mission timeline would be in the order of years in the case of a mission to Mars. In addition, future missions are expected to include a period of dormancy in which the WRS would be unused for an extended duration. The concepts developed in this review arose from a workshop including NASA personnel and representatives with biofilm expertise from a wide range of industrial and academic backgrounds. Here, we address current strategies that are employed on Earth for biofilm control, including antifouling coatings and biocides and mechanisms for mitigating biofilm growth and damage. These ideas are presented in the context of their applicability to spaceflight and identify proposed new topics of biofilm control that need to be addressed in order to facilitate future extended, crewed, spaceflight missions.

The cell organization underlying structural colour is involved in Flavobacterium IR1 predation

Flavobacterium IR1 is a gliding bacterium with a high degree of colonial organization as a 2D photonic crystal, resulting in vivid structural coloration when illuminated. Enterobacter cloacae B12, an unrelated bacterium, was isolated from the brown macroalga Fucus vesiculosus from the same location as IR1. IR1 was found to be a predator of B12. A process of surrounding, infiltration, undercutting and killing of B12 supported improved growth of IR1. A combination of motility and capillarity facilitated the engulfment of B12 colonies by IR1. Predation was independent of illumination. Mutants of IR1 that formed photonic crystals less effectively than the wild type were reduced in predation. Conversely, formation of a photonic crystal was not advantageous in resisting predation by Rhodococcus spp. PIR4. These observations suggest that the organization required to create structural colour has a biological function (facilitating predation) but one that is not directly related to the photonic properties of the colony. This work is the first experimental evidence supporting a role for this widespread type of cell organization in the Flavobacteriia.

Magnetically driven active topography for long-term biofilm control

Microbial biofilm formation on indwelling medical devices causes persistent infections that cannot be cured with conventional antibiotics. To address this unmet challenge, we engineer tunable active surface topographies with micron-sized pillars that can beat at a programmable frequency and force level in an electromagnetic field. Compared to the flat and static controls, active topographies with the optimized design prevent biofilm formation and remove established biofilms of uropathogenic Escherichia coli (UPEC), Pseudomonas aeruginosa, and Staphylococcus aureus, with up to 3.7 logs of biomass reduction. In addition, the detached biofilm cells are found sensitized to bactericidal antibiotics to the level comparable to exponential-phase planktonic cells. Based on these findings, a prototype catheter is engineered and found to remain clean for at least 30 days under the flow of artificial urine medium, while the control catheters are blocked by UPEC biofilms within 5 days.

Hierarchical Rose Petal Surfaces Delay the Early-Stage Bacterial Biofilm Growth

A variety of natural surfaces exhibit antibacterial properties; as a result, significant efforts in the past decade have been dedicated toward fabrication of biomimetic surfaces that can help control biofilm growth. Examples of such surfaces include rose petals, which possess hierarchical structures like the micropapillae measuring tens of microns and nanofolds that range in the size of 700 ± 100 nm. We duplicated the natural structures on rose petal surfaces via a simple UV-curable nanocasting technique and tested the efficacy of these artificial surfaces in preventing biofilm growth using clinically relevant bacteria strains. The rose petal-structured surfaces exhibited hydrophobicity (contact angle (CA) ≈ 130.8° ± 4.3°) and high CA hysteresis (∼91.0° ± 4.9°). Water droplets on rose petal replicas evaporated following the constant contact line mode, indicating the likely coexistence of both Cassie and Wenzel states (Cassie-Baxter impregnating the wetting state). Fluorescence microscopy and image analysis revealed the significantly lower attachment of Staphylococcus epidermidis (86.1 ± 6.2% less) and Pseudomonas aeruginosa (85.9 ± 3.2% less) on the rose petal-structured surfaces, compared with flat surfaces over a period of 2 h. An extensive biofilm matrix was observed in biofilms formed by both species on flat surfaces after prolonged growth (several days), but was less apparent on rose petal-biomimetic surfaces. In addition, the biomass of S. epidermidis (63.2 ± 9.4% less) and P. aeruginosa (76.0 ± 10.0% less) biofilms were significantly reduced on the rose petal-structured surfaces, in comparison to the flat surfaces. By comparing P. aeruginosa growth on representative unitary nanopillars, we demonstrated that hierarchical structures are more effective in delaying biofilm growth. The mechanisms are two-fold: (1) the nanofolds across the hemispherical micropapillae restrict initial attachment of bacterial cells and delay the direct contact of cells via cell alignment and (2) the hemispherical micropapillae arrays isolate bacterial clusters and inhibit the formation of a fibrous network. The hierarchical features on rose petal surfaces may be useful for developing strategies to control biofilm formation in medical and industrial contexts.

Modelling bacterial twitching in fluid flows: a CFD-DEM approach

Bacterial habitats are often associated with fluid flow environments. Bacterial twitching is important for initial bacterial colonization and biofilm formation. The existing research about bacteria twitching is largely experimental orientated. There is a lack of models of twitching motility of bacteria in shear flows, which could provide fundamental understanding about how bacterial twitching would be affected by bacteria associated properties such as number of pili and their distribution on the cell body and environmental factors such as flow and surface patterns. In this work, a three-dimensional modelling approach of Computational Fluid Dynamics (CFD) coupled with the Discrete Element Method (DEM) proposed to study bacterial twitching on flat and groove surfaces under shear flow conditions. Rod-shaped bacteria are modelled as groups of spherical particles and Type IV pili attached to bacteria are modelled as dynamic springs which can elongate, retract, attach and detach. The CFD-DEM model of rod-shape bacteria is validated against orbiting of immotile bacteria in shear flows. The effects of fluid flow rate and surface topography on twitching motility are studied. The model can successfully predict upstream twitching motility of rod-shaped bacteria in shear flows. Our model can predict that there would be an optimal range of wall shear stress in which bacterial upstream twitching is most efficient. The results also indicate that when bacteria twitch on groove surfaces, they are likely to accumulate around the downstream side of the groove walls.

Removal of Bacteria from Solids by Bubbles: Effect of Solid Wettability, Interaction Geometry, and Liquid-Vapor Interface Velocity

Air bubbles are a promising means of controlling fouling for a range of applications, particularly delaying fouling in marine environments. This work investigates the mechanism by which the collision of an air bubble with a solid removes adsorbed bacteria. A key feature of the work is that the numbers of bacteria were monitored via video microscopy throughout the collision; so, we were able to explore the mechanism of bacteria removal. When a bubble collides with a solid, an air-liquid interface crosses the solid twice, and we were able to distinguish the effects of the first and second air-liquid interfaces. The bacterium Pseudomonas aeruginosa was allowed to adhere to smooth poly(dimethylsiloxane) and then a collision with a bubble was investigated for one of three different approach geometries: perpendicular, parallel, and oscillating parallel to the solid surface. Other factors examined were the speed of the bubble, the duration of bacterial adhesion on the solid surface, and the wettability of the solid. Surface wettability was identified as the most significant factor. When the solid dewet, almost all bacteria were removed from hydrophobic surfaces upon the passage of the first air-liquid interface. In contrast, when a thin liquid film remained between the solid and the bubble (a hydrophilic solid), variable amounts of bacteria remained. Although almost all bacteria were initially removed from hydrophobic solids, many bacteria were redeposited on hydrophobic surfaces upon the passage of the second air-liquid interface, especially when the first and second air-liquid interfaces moved in opposite directions. As described previously, a lower velocity of the bubble allows more time for the thin liquid film to drain and improved removal efficiency on hydrophilic solids. A rougher solid (8 μm diameter hemispherical protrusions) decreased the detachment efficiency because bacteria and liquid were able to shelter in concavities.

Antibiotic Susceptibility of Escherichia coli Cells during Early-Stage Biofilm Formation

Bacteria form complex multicellular structures on solid surfaces known as biofilms, which allow them to survive in harsh environments. A hallmark characteristic of mature biofilms is the high-level antibiotic tolerance (up to 1,000 times) compared with that of planktonic cells. Here, we report our new findings that biofilm cells are not always more tolerant to antibiotics than planktonic cells in the same culture. Specifically, Escherichia coli RP437 exhibited a dynamic change in antibiotic susceptibility during its early-stage biofilm formation. This phenomenon was not strain specific. Upon initial attachment, surface-associated cells became more sensitive to antibiotics than planktonic cells. By controlling the cell adhesion and cluster size using patterned E. coli biofilms, cells involved in the interaction between cell clusters during microcolony formation were found to be more susceptible to ampicillin than cells within clusters, suggesting a role of cell-cell interactions in biofilm-associated antibiotic tolerance. After this stage, biofilm cells became less susceptible to ampicillin and ofloxacin than planktonic cells. However, when the cells were detached by sonication, both antibiotics were more effective in killing the detached biofilm cells than the planktonic cells. Collectively, these results indicate that biofilm formation involves active cellular activities in adaption to the attached life form and interactions between cell clusters to build the complex structure of a biofilm, which can render these cells more susceptible to antibiotics. These findings shed new light on bacterial antibiotic susceptibility during biofilm formation and can guide the design of better antifouling surfaces, e.g., those with micron-scale topographic structures to interrupt cell-cell interactions.IMPORTANCE Mature biofilms are known for their high-level tolerance to antibiotics; however, antibiotic susceptibility of sessile cells during early-stage biofilm formation is not well understood. In this study, we aim to fill this knowledge gap by following bacterial antibiotic susceptibility during early-stage biofilm formation. We found that the attached cells have a dynamic change in antibiotic susceptibility, and during certain phases, they can be more sensitive to antibiotics than planktonic counterparts in the same culture. Using surface chemistry-controlled patterned biofilm formation, cell-surface and cell-cell interactions were found to affect the antibiotic susceptibility of attached cells. Collectively, these findings provide new insights into biofilm physiology and reveal how adaptation to the attached life form may influence antibiotic susceptibility of bacterial cells.

Nanopillared Surfaces Disrupt Pseudomonas aeruginosa Mechanoresponsive Upstream Motility

Pseudomonas aeruginosa is an opportunistic, multidrug-resistant, human pathogen that forms biofilms in environments with fluid flow, such as the lungs of cystic fibrosis patients, industrial pipelines, and medical devices. P. aeruginosa twitches upstream on surfaces by the cyclic extension and retraction of its mechanoresponsive type IV pili motility appendages. The prevention of upstream motility, host invasion, and infectious biofilm formation in fluid flow systems remains an unmet challenge. Here, we describe the design and application of scalable nanopillared surface structures fabricated using nanoimprint lithography that reduce upstream motility and colonization by P. aeruginosa. We used flow channels to induce shear stress typically found in catheter tubes and microscopy analysis to investigate the impact of nanopillared surfaces with different packing fractions on upstream motility trajectory, displacement, velocity, and surface attachment. We found that densely packed, subcellular nanopillared surfaces, with pillar periodicities ranging from 200 to 600 nm and widths ranging from 70 to 215 nm, inhibit the mechanoresponsive upstream motility and surface attachment. This bacteria-nanostructured surface interface effect allows us to tailor surfaces with specific nanopillared geometries for disrupting cell motility and attachment in fluid flow systems.

Micro- and Nanotopography Sensitive Bacterial Attachment Mechanisms: A Review

Bacterial attachment to material surfaces can lead to the development of biofilms that cause severe economic and health problems. The outcome of bacterial attachment is determined by a combination of bacterial sensing of material surfaces by the cell and the physicochemical factors in the near-surface environment. This paper offers a systematic review of the effects of surface topography on a range of antifouling mechanisms, with a focus on how topographical scale, from micro- to nanoscale, may influence bacterial sensing of and attachment to material surfaces. A good understanding of these mechanisms can facilitate the development of antifouling surfaces based on surface topography, with applications in various sectors of human life and activity including healthcare, food, and water treatment.

Antimicrobial strategies for urinary catheters

Over 75% of hospital-acquired or nosocomial urinary tract infections are initiated by urinary catheters, which are used during the treatment of 16% of hospitalized patients. Taking the United States as an example, the costs of catheter-associated urinary tract infections (CAUTI) are in excess of $451 million dollars/year. The biofilm formation by pathogenic microbes that protects pathogens from host immune defense and antimicrobial agents is the leading cause for CAUTI. Thus, tremendous efforts have been devoted to antimicrobial coating for urinary catheters in the past few decades, and it has been demonstrated to be one of the most direct and efficient strategies to reduce infections. In this article, we briefly summarize the current methods for preparation of antimicrobial coatings based on different stages in the biofilm formation, highlight recent progress in the urinary catheter coating material design and selection, discuss approaches to improving their long-term antimicrobial efficacy, biocompatibility, multidrug resistance and recurrent infections, and finally outline future requirements and prospects in antimicrobial coating material design. The scope of the works surveyed is confined to antimicrobial urinary catheters. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 445-467, 2019.

Impact of surface topography on biofilm formation by Candida albicans

Candida albicans is a fungal pathogen that causes serious biofilm-based infections. Here we have asked whether surface topography may affect C. albicans biofilm formation. We tested biofilm growth of the prototypical wild-type strain SC5314 on a series of polydimethylsiloxane (PDMS) solids. The surfaces were prepared with monolayer coatings of monodisperse spherical silica particles that were fused together into a film using silica menisci. The surface topography was varied by varying the diameter of the silica particles that were used to form the film. Biofilm formation was observed to be a strong function of particle size. In the particle size range 4.0-8.0 μm, there was much more biofilm than in the size range 0.5-2.0 μm. The behavior of a clinical isolate from a clade separate from SC5314, strain p76067, showed results similar to that of SC5314. Our results suggest that topographic coatings may be a promising approach to reduce C. albicans biofilm infections.