Materials and surface engineering to control bacterial adhesion and biofilm formation: A review of recent advances

Targeted insights into Aeromonas hydrophila biofilms: Surface preferences, resistance mechanisms, and gene expression

This study provides a comprehensive analysis of biofilm formation, antibiotic resistance, motility, and gene expression in four Aeromonas hydrophila strains-ATCC 15467, ATCC 7966, KCTC 2358, and KCTC 11533-on stainless steel (SS), silicon rubber (SR), polyethylene terephthalate (PET), and high-density polyethylene (HDPE) surfaces over 24, 48, 72, and 96 h. Biofilm formation peaked at 72 h, with ATCC 7966 demonstrating the highest biofilm density on PET (6.50 ± 0.08 log CFU/cm²), underscoring PET's role as a favorable substrate for biofilm development. In contrast, HDPE consistently exhibited the lowest biofilm levels, reflecting its potential as a biofilm-resistant material. Antibiotic susceptibility profiling revealed multidrug resistance (MDR) in ATCC 15467 and KCTC 11533 (MARI = 0.80), particularly against beta-lactams, aminoglycosides, and fluoroquinolones while ATCC 7966 and KCTC 2358 displayed moderate resistance. Motility assays highlighted strain-specific capabilities, with KCTC 11533 exhibiting the highest swimming motility (76.0 ± 6.6 mm) and KCTC 2358 excelling in swarming (47.7 ± 3.5 mm). Genetic analysis confirmed the presence of luxS and ahyR in all strains, while csgA was exclusive to ATCC 7966, correlating with its superior biofilm formation. Confocal microscopy revealed biofilm maturation dynamics, with red fluorescence indicating cell death and aging at 96 h, while SEM images captured intricate surface-specific biofilm architectures. These findings elucidate the critical interplay between strain characteristics, surface properties, and incubation time, providing a foundation for developing targeted strategies to control A. hydrophila biofilms in food processing environments.

Antimicrobial Biomaterials Based on Physical and Physicochemical Action

Developing effective antimicrobial biomaterials is a relevant and fast-growing field in advanced healthcare materials. Several well-known (e.g., traditional antibiotics, silver, copper etc.) and newer (e.g., nanostructured, chemical, biomimetic etc.) approaches have been researched and developed in recent years and valuable knowledge has been gained. However, biomaterials associated infections (BAIs) remain a largely unsolved problem and breakthroughs in this area are sparse. Hence, novel high risk and potential high gain approaches are needed to address the important challenge of BAIs. Antibiotic free antimicrobial biomaterials that are largely based on physical action are promising, since they reduce the risk of antibiotic resistance and tolerance. Here, selected examples are reviewed such antimicrobial biomaterials, namely switchable, protein-based, carbon-based and bioactive glass, considering microbiological aspects of BAIs. The review shows that antimicrobial biomaterials mainly based on physical action are powerful tools to control microbial growth at biomaterials interfaces. These biomaterials have major clinical and application potential for future antimicrobial healthcare materials without promoting microbial tolerance. It also shows that the antimicrobial action of these materials is based on different complex processes and mechanisms, often on the nanoscale. The review concludes with an outlook and highlights current important research questions in antimicrobial biomaterials.

Leveraging the Dielectric Barrier Discharge Plasma Process to Create Regenerative Biocidal ePTFE Membranes

The utilization of dielectric barrier discharge (DBD) plasma treatment for modifying substrate surfaces constitutes an easy and simple approach with a potential for diverse applications. This technique was used to modify the surface of a commercial porous expanded poly(tetrafluoroethylene) (ePTFE) film with either dimethylaminoethyl methacrylate (DMAEMA) or (trimethylamino)ethyl methacrylate chloride (TMAEMA) monomers, aiming to obtain antibacterial ePTFE. Physicochemical analyses of the membranes revealed that DBD successfully enhanced the surface energy and surface charge of the membranes while maintaining high porosity (>75%) and large pore size (>1.0 μm). Evaluation of the bacteria killing-releasing (K-R) function revealed that both DMAEMA and TMAEMA endowed ePTFE with the ability to kill Escherichia coli bacteria. However, only TMAEMA-grafted ePTFE allowed for the release of dead bacteria from the surface upon washing with sodium hexametaphosphate (SHMP) saline solution, owing to its cationic charge derived from the quaternary amine. Washing with SHMP disturbed the electrostatic force between the polymer brushes and dead bacteria, which caused the release of the dead bacteria. Lastly, dead-end bacteria filtration showed that the TMAEMA-grafted ePTFE was able to kill 99.78% of the bacteria, while approximately 61.55% of bacteria were killed upon contact. The present findings support the feasibility of using DBD plasma treatment for designing surfaces that target bacteria and aid in the containment of disease-causing pathogens.

Physiochemically Distinct Surface Properties of SU-8 Polymer Modulate Bacterial Cell-Surface Holdfast and Colonization

SU-8 polymer is an excellent platform for diverse applications due to its high aspect ratio of micro/nanostructure fabrication and exceptional physicochemical and biocompatible properties. Although SU-8 polymer has often been investigated for various biological applications, how its surface properties influence the interaction of bacterial cells with the substrate and its colonization is poorly understood. In this work, we tailor SU-8 nanoscale surface properties to investigate single-cell motility, adhesion, and successive colonization of phytopathogenic bacteria, Xylella fastidiosa. Different surface properties of SU-8 thin films have been prepared using photolithography processing and oxygen plasma treatment. A more significant density of carboxyl groups in hydrophilic plasma-treated SU-8 surfaces promotes faster cell motility in the earlier growth stage. The hydrophobic nature of pristine SU-8 surfaces shows no trackable bacterial motility and 5-10 times more single cells adhered to the surface than its plasma-treated counterpart. In addition, plasma-treated SU-8 samples suppressed bacterial adhesion, with surfaces showing less than 5% coverage. These results not only showcase that SU-8 surface properties can impact the spatiotemporal bacterial behavior but also provide insights into pathogens' prominent ability to evolve and adapt to different surface properties.

Antibacterial Adhesion Strategy for Dental Titanium Implant Surfaces: From Mechanisms to Application

Dental implants are widely used to restore missing teeth because of their stability and comfort characteristics. Peri-implant infection may lead to implant failure and other profound consequences. It is believed that peri-implantitis is closely related to the formation of biofilms, which are difficult to remove once formed. Therefore, endowing titanium implants with anti-adhesion properties is an effective method to prevent peri-implant infection. Moreover, anti-adhesion strategies for titanium implant surfaces are critical steps for resisting bacterial adherence. This article reviews the process of bacterial adhesion, the material properties that may affect the process, and the anti-adhesion strategies that have been proven effective and promising in practice. This article intends to be a reference for further improvement of the antibacterial adhesion strategy in clinical application and for related research on titanium implant surfaces.

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.

Preparation of Smart Surfaces Based on PNaSS@PEDOT Microspheres: Testing of E. coli Detection

The main task of the research is to acquire fundamental knowledge about the effect of polymer structure on the physicochemical properties of films. A novel meta-material that can be used in manufacturing sensor layers was developed as a model. At the first stage, poly(sodium 4-styrenesulfonate) (PNaSS) cross-linked microspheres are synthesized (which are based on strong polyelectrolytes containing sulfo groups in each monomer unit), and at the second stage, PNaSS@PEDOT microspheres are formed. The poly(3,4-ethylenedioxythiophene) (PEDOT) shell was obtained by the acid-assisted self-polymerization of the monomer; this process is biologically safe and thus suitable for biomedical applications. The suitability of electrochemical impedance spectroscopy for E. coli detection was tested; it was revealed that the attached bacterial wall was destroyed upon application of constant oxidation potential (higher than 0.5 V), which makes the PNaSS@PEDOT microsphere particles promising materials for the development of antifouling coatings. Furthermore, under open-circuit conditions, the walls of E. coli bacteria were not destroyed, which opens up the possibility of employing such meta-materials as sensor films. Scanning electron microscopy, X-ray photoelectron spectroscopy, water contact angle, and wide-angle X-ray diffraction methods were applied in order to characterize the PNaSS@PEDOT films.

Controlled spatial organization of bacterial growth reveals key role of cell filamentation preceding Xylella fastidiosa biofilm formation

The morphological plasticity of bacteria to form filamentous cells commonly represents an adaptive strategy induced by stresses. In contrast, for diverse human and plant pathogens, filamentous cells have been recently observed during biofilm formation, but their functions and triggering mechanisms remain unclear. To experimentally identify the underlying function and hypothesized cell communication triggers of such cell morphogenesis, spatially controlled cell patterning is pivotal. Here, we demonstrate highly selective cell adhesion of the biofilm-forming phytopathogen Xylella fastidiosa to gold-patterned SiO₂ substrates with well-defined geometries and dimensions. The consequent control of both cell density and distances between cell clusters demonstrated that filamentous cell formation depends on cell cluster density, and their ability to interconnect neighboring cell clusters is distance-dependent. This process allows the creation of large interconnected cell clusters that form the structural framework for macroscale biofilms. The addition of diffusible signaling molecules from supernatant extracts provides evidence that cell filamentation is induced by quorum sensing. These findings and our innovative platform could facilitate therapeutic developments targeting biofilm formation mechanisms of X. fastidiosa and other pathogens.

Hybrid "Kill and Release" Antibacterial Cellulose Papers Obtained via Surface-Initiated Atom Transfer Radical Polymerization

Infectious diseases triggered by bacteria cause a severe risk to human health. To counter this issue, surfaces coated with antibacterial materials have been widely used in daily life to kill these bacteria. The substrates enabled with a hybrid kill and release strategy can be employed not only to kill the bacteria but also to wash them using external stimuli (temperature, pH, etc.). Utilizing this concept, we develop thermoresponsive antibacterial-cellulose papers to exhibit hybrid kill and release properties. Thermoresponsive copolymers [p(NIPAAm-co-AEMA)] are grafted on cellulose papers using a surface-initiated atom transfer radical polymerization approach for bacterial debris release. Later for antibacterial properties, silver nanoparticles (AgNPs) are immobilized on thermoresponsive copolymer-grafted cellulose papers using electrostatic interactions. We confirm the thermoresponsive copolymer grafting and AgNP coating by attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. Thermoresponsiveness and reusability of the modified cellulose papers are confirmed through water contact angle measurements. The interaction potency between AgNPs and modified cellulose is validated by inductively coupled plasma atomic emission spectroscopy analysis. Gram-negative bacteria Escherichia coli (E. coli DH5-α) is used to demonstrate antibacterial hybrid kill and release performance. Agar-diffusion testing demonstrates the antibacterial nature of the modified cellulose papers. The fluorescence micrograph reveals that modified cellulose papers can effectively release almost all the dead bacterial debris from their surfaces after thermal stimulus wash. The modified cellulose paper surfaces are expected to have wide applications in the field of exploring more antibacterial and smart surfaces.

A versatile pH-responsive peptide based dynamic biointerface for tracking bacteria killing and infection resistance

Herein we reported a versatile dynamic biointerface based on pH-responsive peptide self-assembly and disassembly to capture the bacteria to avoid bacteria further infected tissue around that can release peptides from the surface in a slightly acidic environment to kill the bacteria with the specificity. The exposed biointerface still presented infection resistance.

Three-Dimensional Micropatterning Deters Early Bacterial Adherence and Can Eliminate Colonization

Developing strategies to prevent bacterial infections that do not rely on the use of drugs is regarded globally as an important means to stem the tide of antimicrobial resistance, as argued by the World Health Organization (WHO) (Mendelson, M.; Matsoso, M. P. The World Health Organization Global Action Plan for Antimicrobial Resistance. S. Afr. Med. J. 2015, 105 (5), 325-325. DOI: 10.7196/SAMJ.9644). Given that many antimicrobial-resistant infections are caused by the bacterial colonization of indwelling medical devices such as catheters and ventilators, the use of microengineered surfaces to prevent the initial attachment of microbes to these devices is a promising solution. In this work, it is demonstrated that 3D engineered surfaces can inhibit the initial phases of surface colonization for Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa, representing the three most common catheter-associated urinary tract bacterial infections, identified by the WHO as urgent threats. A variety of designs including 11 different topographies and configurations that exhibited random distributions, sharp protrusions, and/or curvilinear shapes with dimensions ranging between 500 nm and 2 μm were tested to better understand the initial stages of surface colonization and how to optimize the design of fabricated surfaces for improved inhibition. These topographies were fabricated in two configurations to obtain either a standard 2D cross section or a 3D engineered topography using a novel UV lithography process enabling cost-efficient high-throughput manufacturing. Evaluating both the number of adhered bacteria and microcolonies formed by all three bacterial pathogens on the different surfaces provides insight into the initial colonization phase of bacterial growth on the various surfaces. The results demonstrate that both initial attachment and subsequent colonization can be significantly reduced on concrete 3D engineered patterns when compared to flat substrates and standard 2D micropatterns. Thus, this technology has great potential to reduce the colonization of bacteria on surfaces in clinical settings without the need for chemical treatments that might enhance antimicrobial resistance.

High-Throughput Methods in the Discovery and Study of Biomaterials and Materiobiology

The complex interaction of cells with biomaterials (i.e., materiobiology) plays an increasingly pivotal role in the development of novel implants, biomedical devices, and tissue engineering scaffolds to treat diseases, aid in the restoration of bodily functions, construct healthy tissues, or regenerate diseased ones. However, the conventional approaches are incapable of screening the huge amount of potential material parameter combinations to identify the optimal cell responses and involve a combination of serendipity and many series of trial-and-error experiments. For advanced tissue engineering and regenerative medicine, highly efficient and complex bioanalysis platforms are expected to explore the complex interaction of cells with biomaterials using combinatorial approaches that offer desired complex microenvironments during healing, development, and homeostasis. In this review, we first introduce materiobiology and its high-throughput screening (HTS). Then we present an in-depth of the recent progress of 2D/3D HTS platforms (i.e., gradient and microarray) in the principle, preparation, screening for materiobiology, and combination with other advanced technologies. The Compendium for Biomaterial Transcriptomics and high content imaging, computational simulations, and their translation toward commercial and clinical uses are highlighted. In the final section, current challenges and future perspectives are discussed. High-throughput experimentation within the field of materiobiology enables the elucidation of the relationships between biomaterial properties and biological behavior and thereby serves as a potential tool for accelerating the development of high-performance biomaterials.

Engineering High-Yield Biopolymer Secretion Creates an Extracellular Protein Matrix for Living Materials

The bacterial extracellular matrix forms autonomously, giving rise to complex material properties and multicellular behaviors. Synthetic matrix analogues can replicate these functions but require exogenously added material or have limited programmability. Here, we design a two-strain bacterial system that self-synthesizes and structures a synthetic extracellular matrix of proteins. We engineered Caulobacter crescentus to secrete an extracellular matrix protein composed of an elastin-like polypeptide (ELP) hydrogel fused to supercharged SpyCatcher [SC^(-)]. This biopolymer was secreted at levels of 60 mg/liter, an unprecedented level of biomaterial secretion by a native type I secretion apparatus. The ELP domain was swapped with either a cross-linkable variant of ELP or a resilin-like polypeptide, demonstrating this system is flexible. The SC^(-)-ELP matrix protein bound specifically and covalently to the cell surface of a C. crescentus strain that displays a high-density array of SpyTag (ST) peptides via its engineered surface layer. Our work develops protein design guidelines for type I secretion in C. crescentus and demonstrates the autonomous secretion and assembly of programmable extracellular protein matrices, offering a path forward toward the formation of cohesive engineered living materials.IMPORTANCE Engineered living materials (ELM) aim to mimic characteristics of natural occurring systems, bringing the benefits of self-healing, synthesis, autonomous assembly, and responsiveness to traditional materials. Previous research has shown the potential of replicating the bacterial extracellular matrix (ECM) to mimic biofilms. However, these efforts require energy-intensive processing or have limited tunability. We propose a bacterially synthesized system that manipulates the protein content of the ECM, allowing for programmable interactions and autonomous material formation. To achieve this, we engineered a two-strain system to secrete a synthetic extracellular protein matrix (sEPM). This work is a step toward understanding the necessary parameters to engineering living cells to autonomously construct ELMs.

Biological Control and Mitigation of Aflatoxin Contamination in Commodities

Aflatoxins (AFs) are toxic secondary metabolites produced mostly by Aspergillus species. AF contamination entering the feed and food chain has been a crucial long-term issue for veterinarians, medicals, agroindustry experts, and researchers working in this field. Although different (physical, chemical, and biological) technologies have been developed, tested, and employed to mitigate the detrimental effects of mycotoxins, including AFs, universal methods are still not available to reduce AF levels in feed and food in the last decades. Possible biological control by bacteria, yeasts, and fungi, their excretes, the role of the ruminal degradation, pre-harvest biocontrol by competitive exclusion or biofungicides, and post-harvest technologies and practices based on biological agents currently used to alleviate the toxic effects of AFs are collected in this review. Pre-harvest biocontrol technologies can give us the greatest opportunity to reduce AF production on the spot. Together with post-harvest applications of bacteria or fungal cultures, these technologies can help us strictly reduce AF contamination without synthetic chemicals.

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.

Magneto-mechanically actuated microstructures to efficiently prevent bacterial biofilm formation

Biofilm colonisation of surfaces is of critical importance in various areas ranging from indwelling medical devices to industrial setups. Of particular importance is the reduced susceptibility of bacteria embedded in a biofilm to existing antimicrobial agents. In this paper, we demonstrate that remotely actuated magnetic cantilevers grafted on a substrate act efficiently in preventing bacterial biofilm formation. When exposed to an alternating magnetic field, the flexible magnetic cantilevers vertically deflect from their initial position periodically, with an extremely low frequency (0.16 Hz). The cantilevers’ beating prevents the initial stage of bacterial adhesion to the substrate surface and the subsequent biofilm growth. Our experimental data on E. coli liquid cultures demonstrate up to a 70% reduction in biofilm formation. A theoretical model has been developed to predict the amplitude of the cantilevers vertical deflection. Our results demonstrate proof-of-concept for a device that can magneto-mechanically prevent the first stage in bacterial biofilm formation, acting as on-demand fouling release active surfaces.

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.

Resistance to Long-Term Bacterial Biofilm Formation Based on Hydrolysis-Induced Zwitterion Material with Biodegradable and Self-Healing Properties

Long-term resistance of biomaterials to the bacterial biofilm formation without antibiotic or biocide is highly demanded for biomedical applications. In this work, a novel biodegradable biomaterial with excellent capability to prevent long-term bacterial biofilm formation is prepared by the following two steps. Ethylcarboxybetaine ester analogue methacrylate (ECBEMA), poly(ethylene glycol) monomethacrylate (PEGMA), and 3-methacryloxypropyletris(trimethylsiloxy)silane (TRIS) were copolymerized to obtain p(ECBEMA-PEGMA-TRIS) (PEPT). Then, PEPT was cross-linked by isocyanate-terminated polylactic acid (IPDI-PLA-IPDI) to obtain the final PEPTx-PLAy (x and y are the number-average molecular weights (M_n) of PEPT and PLA, respectively) with optimal mechanical strength and adjustable surface regeneration rate. Static contact angle measurement, protein adsorption measurement, and attenuated total reflectance infrared (ATR-IR) results show that the PEPT19800-PLA800 film surface can generate a zwitterionic layer to resist nonspecific protein adsorption after surface hydrolysis. Quartz crystal microbalance with dissipation (QCM-D) results indicates that the PEPT19800-PLA800 film can undergo gradual degradation of the surface layer at the lowest swelling rate. Particularly, this material can efficiently resist the bacterial biofilm formation of both Gram-positive bacteria and Gram-negative bacteria over 14 and 6 days, respectively. Moreover, the material also shows an ideal self-healing feature to adapt to harsh conditions. Thus, this nonfouling material shows great potential in biomedical applications and marine antifouling coatings without antibiotic or biocide.

A comparison between electrospinning and rotary-jet spinning to produce PCL fibers with low bacteria colonization

One of the important components in tissue engineering is material structure, providing a model for fixing and the development of cells and tissues, which allows for the transport of nutrients and regulatory molecules to and from cells. The community claims the need for new materials with better properties for use in the clinic. Poly (ε-caprolactone) (PCL) is a biodegradable polymer, semi crystalline, with superior mechanical properties and has attracted an increasing interest due to its usefulness in various biomedical applications. Herein, two different methods (electrospinning versus rotary jet spinning) with different concentrations of PCL produced ultra thin-fibers each with particular characteristics, verified and analyzed by morphology, wettability, thermal and cytotoxicity features and for bacteria colonization. Different PCL scaffold morphologies were found to be dependent on the fabrication method used. All PCL scaffolds showed greater mammalian cell interactions. Most impressively, rotary-jet spun fibers showed that a special rough surface decreased bacteria colonization, emphasizing that no nanoparticle or antibiotic was used; maybe this effect is related with physical (scaffold) and/or biological mechanisms. Thus, this study showed that rotary jet spun fibers possess a special topography compared to electrospun fibers to reduce bacteria colonization and present no cytotoxicity when in contact with mammalian cells.

Detecting Escherichia coli Biofilm Development Stages on Gold and Titanium by Quartz Crystal Microbalance

Bacterial biofilms are responsible for persistent infections and biofouling, raising serious concerns in both medical and industrial processes. These motivations underpin the need to develop methodologies to study the complex biological structures of biofilms and prevent their formation on medical implants, tools, and industrial apparatuses. Here, we report the detailed comparison of Escherichia coli biofilm development stages (adhesion, maturation, and dispersion) on gold and titanium surfaces by monitoring the changes in both frequency and dissipation of a quartz crystal microbalance (QCM) device, a cheap and reliable microgravimetric sensor which allows the real-time and label-free characterization of various stages of biofilm development. Although gold is the most common electrode material used for QCM sensors, the titanium electrode is also readily available for QCM sensors; thus, QCM sensors with different metal electrodes serve as a simple platform to probe how pathogens interact with different metal substrates. The QCM outcomes are further confirmed by atomic force microscopy and crystal violet staining, thus validating the effectiveness of this surface sensitive sensor for microbial biofilm research. Moreover, because QCM technology can easily modify the substrate types and coatings, QCM sensors also provide well-controlled experimental conditions to study antimicrobial surface treatments and eradication procedures, even on mature biofilms.

Fabrication of PMPC/PTM/PEGDA micropatterns onto polypropylene films behaving with dual functions of antifouling and antimicrobial activities

Polymer materials with high biocompatibility and versatile functions are urgently required in the biomedical field. The hydrophobic surface and inert traits of polymer materials usually encounter severe biofouling and bacterial infection which hinder the potential application of polymers as biomedical materials. Although many antifouling or antimicrobial coatings have been developed for modification of biomedical devices/implants, few can simultaneously fulfill the requirements for antimicrobial and antifouling activities. Herein, we constructed bifunctional micropatterns with antifouling and antimicrobial properties onto polypropylene (PP) films using argon plasma activation treatment, photomask technique and UV-initiated graft polymerization method. Different sizes of PMPC/PTM/PEGDA micropatterns were fabricated on PP films to yield patterned PP-PMPC/PTM/PEGDA as evidenced by infrared (IR) spectroscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), where PMPC is poly(2-methacryloyloxyethyl phosphorylcholine) for enhancement of hydrophilicity and biocompatibility, PTM is poly(methacryloyloxyethyltrimethylammonium chloride) for contribution to antimicrobial activity and PEGDA is poly(ethylene glycol diacrylate) as the crosslinker. The surface hydrophilicity of patterned PP-PMPC/PTM/PEGDA was characterized by the static water contact angle test. The results showed that the PP sample with a micropattern with the size of 5 μm exhibited the best hydrophilicity. For biological assays of patterned PP-PMPC/PTM/PEGDA, the micropattern size at 5 μm performed the best for both antiplatelet adhesion and antimicrobial activities. We anticipate that this work could provide a new method for building bifunctional biomedical materials to promote the application of PP in biomedical fields.

Antifouling and Antibacterial Properties Constructed by Quaternary Ammonium and Benzyl Ester Derived from Lysine Methacrylamide

Hemocompatibility and antibacterial property are essential for blood contact devices and medical intervention materials. In this study, positively charged quaternary ammonium (QAC) and hydrophobic benzyl group (OBzl) were introduced onto hydrophilic lysine methacrylamide (LysAAm) to obtain two monomers LysAAm-QAC and LysAAm-OBzl, respectively. The structure characterizations of LysAAm-QAC and LysAAm-OBzl were determined by proton nuclear magnetic resonance, Fourier transform infrared spectroscopy, and time-of-flight secondary ion mass spectrometry. LysAAm-QAC and LysAAm-OBzl were cografted onto a silicon wafer with different feeding ratios to construct antifouling and antibacterial properties. The results of fibrinogen adsorption and platelet adhesion proved that the modified sample with the feeding ratio of 3:7 had superior antifouling property. Furthermore, an antimicrobial test with both 2 and 24 h indicated that the modified sample with the feeding ratio of 3:7 had antibacterial ability. The antifouling property was provided by the high surface coverage of LysAAm-QAC and LysAAm-OBzl (91.49%) and the hydrophilic main structure LysAAm on LysAAm-QAC and LysAAm-OBzl (water contact angle was 43.6°). The antibacterial property was improved with the proportion of LysAAm-OBzl (43.6-58.5%) because the increasing hydrophobic OBzl enhanced the ability to insert into the membrane of bacteria and raise the bactericidal efficiency. In application, LysAAm-QAC and LysAAm-OBzl with the feeding ratio of 3:7 were grafted onto the surface of poly(styrene-b-(ethylene-co-butylene)-b-styrene), and a bifunctional surface with antifouling and antibacterial properties was fabricated, which had promising applications in blood contact devices and medical intervention materials.

Boron Doped Diamond as a Low Biofouling Material in Aquatic Environments: Assessment of Pseudomonas aeruginosa Biofilm Formation

Boron doped diamond (BDD), given the robustness of the material, is becoming an electrode of choice for applications which require long-term electrochemical monitoring of analytes in aqueous environments. However, despite the extensive work in this area, there are no studies which directly assess the biofilm formation (biofouling) capabilities of the material, which is an essential consideration because biofouling often causes deterioration in the sensor performance. Pseudomonas aeruginosa is one of the most prevalent bacterial pathogens linked to water-related diseases, with a strong capacity for forming biofilms on surfaces that are exposed to aquatic environments. In this study, we comparatively evaluate the biofouling capabilities of oxygen-terminated (O-)BDD against materials commonly employed as either the packaging or sensing element in water quality sensors, with an aim to identify factors which control biofilm formation on BDD. We assess the monospecies biofilm formation of P. aeruginosa in two different growth media, Luria-Bertani, a high nutrient source and drinking water, a low nutrient source, at two different temperatures (20 and 37 °C). Multispecies biofilm formation is also investigated. The performance of O-BDD, when tested against all other materials, promotes the lowest extent of P. aeruginosa monospecies biofilm formation, even with corrections made for total surface area (roughness). Importantly, O-BDD shows the lowest water contact angle of all materials tested, that is, greatest hydrophilicity, strongly suggesting that for these bacterial species, the factors controlling the hydrophilicity of the surface are important in reducing bacterial adhesion. This was further proven by keeping the surface topography fixed and changing surface termination to hydrogen (H-), to produce a strongly hydrophobic surface. A noticeable increase in biofilm formation was found. Doping with boron also results in changes in hydrophobicity/hydrophilicity compared to the undoped counterpart, which in turn affects the bacterial growth. For practical electrochemical sensing applications in aquatic environments, this study highlights the extremely beneficial effects of employing smooth, O-terminated (hydrophilic) BDD electrodes.

Artificial Microbial Arenas: Materials for Observing and Manipulating Microbial Consortia

From the smallest ecological niche to global scale, communities of microbial life present a major factor in system regulation and stability. As long as laboratory studies remain restricted to single or few species assemblies, however, very little is known about the interaction patterns and exogenous factors controlling the dynamics of natural microbial communities. In combination with microfluidic technologies, progress in the manufacture of functional and stimuli-responsive materials makes artificial microbial arenas accessible. As habitats for natural or multispecies synthetic consortia, they are expected to not only enable detailed investigations, but also the training and the directed evolution of microbial communities in states of balance and disturbance, or under the effects of modulated stimuli and spontaneous response triggers. Here, a perspective on how materials research will play an essential role in generating answers to the most pertinent questions of microbial engineering is presented, and the concept of adaptive microbial arenas and possibilities for their construction from particulate microniches to 3D habitats is introduced. Materials as active and tunable components at the interface of living and nonliving matter offer exciting opportunities in this field. Beyond forming the physical horizon for microbial cultivates, they will enable dedicated intervention, training, and observation of microbial consortia.

Assorted Methods for Decontamination of Aflatoxin M1 in Milk Using Microbial Adsorbents

Aflatoxins (AF) are carcinogenic metabolites produced by different species of Aspergillus which readily colonize crops. AFM1 is secreted in the milk of lactating mammals through the ingestion of feedstuffs contaminated by aflatoxin B1 (AFB1). Therefore, its presence in milk, even in small amounts, presents a real concern for dairy industries and consumers of dairy products. Different strategies can lead to the reduction of AFM1 contamination levels in milk. They include adopting good agricultural practices, decreasing the AFB1 contamination of animal feeds, or using diverse types of adsorbent materials. One of the most effective types of adsorbents used for AFM1 decontamination are those of microbial origin. This review discusses current issues about AFM1 decontamination methods. These methods are based on the use of different bio-adsorbent agents such as bacteria and yeasts to complex AFM1 in milk. Moreover, this review answers some of the raised concerns about the binding stability of the formed AFM1-microbial complex. Thus, the efficiency of the decontamination methods was addressed, and plausible experimental variants were discussed.

"Command" surfaces with thermo-switchable antibacterial activity

In the presented work "smart" antibacterial surfaces based on silver nanoparticles (AgNPs) embedded in temperature-responsive poly(di(ethylene glycol)methyl ether methacrylate) - (POEGMA188) as well as poly(4-vinylpyridine) - (P4VP) coatings attached to a glass surface were successfully prepared. The composition, thickness, morphology and wettability of the resulting coatings were analyzed using ToF-SIMS, XPS, EDX, ellipsometry, AFM, SEM and CA measurements, respectively. Temperature-switched killing of the bacteria was tested against Escherichia coli ATCC 25922 (representative of Gram-negative bacteria) and Staphylococcus aureus ATCC 25923 (representative of Gram-positive bacteria) at 4 and 37 °C. In general at 4 °C no significant difference was observed between the amounts of bacteria accounted on the grafted brush coatings and within the control sample. In contrast, at 37 °C almost no bacteria were visible for temperature-responsive coating with AgNPs, whereas the growth of bacteria remains not disturbed for "pure" coating, indicating strong temperature-dependent antibacterial properties of AgNPs integrated into brushes.

Sensitizing bacterial cells to antibiotics by shape recovery triggered biofilm dispersion

Microbial biofilms are a leading cause of chronic infections in humans and persistent biofouling in industries due to extremely high-level tolerance of biofilm cells to antimicrobial agents. Eradicating mature biofilms is especially challenging because of the protection of the extracellular matrix and slow growth of biofilm cells. Recently, we reported that established biofilms can be effectively removed (e.g. 99.9% dispersion of 48 h Pseudomonas aeruginosa PAO1 biofilms) by shape memory polymer-based dynamic changes in surface topography. Here, we demonstrate that such biofilm dispersion also sensitizes biofilm cells to conventional antibiotics. For example, shape recovery in the presence of 50 µg/mL tobramycin reduced biofilm cell counts by more than 3 logs (2,479-fold) compared to the static flat control. The observed effects were attributed to the disruption of biofilm structure and increase in cellular activities as evidenced by an 11.8-fold increase in intracellular level of adenosine triphosphate (ATP), and 4.1-fold increase in expression of the rrnB gene in detached cells. These results can help guide the design of new control methods to better combat biofilm associated antibiotic-resistant infections. STATEMENT OF SIGNIFICANCE: Microbial infections are challenging due to high-level antibiotic resistance of biofilm cells. The protection of an extracellular matrix and slow growth of biofilm cells render conventional antibiotics ineffective. Thus, it is important to develop new technologies that can remove mature biofilms and sensitize biofilm cells to antibiotics. Recently, we demonstrated that dynamic change in surface topography can remove 48 h Pseudomonas aeruginosa PAO1 biofilms by 99.9%. In this study, we investigated how shape recovery triggered dispersion affect the physiology of biofilm cells and associated antibiotic susceptibility. These results are helpful for understanding biofilm dispersion and developing more effective control methods.

Eliminating the Need for Biocidal Agents in Anti-Biofouling Polymers by Applying Grafted Nanosilica Instead

A nondestructive one-step approach was applied for grafting biocide-free monodispersed silica nanoparticles (SNPs) with a diameter of 30 ± 10 nm on polystyrene, polyethylene, and polyvinyl chloride surfaces. The prepared surfaces were comprehensively characterized using spectroscopic (Fourier transform infrared attenuated total reflection, ultraviolet-visible, and X-ray photoelectron spectroscopy) and microscopic (high-resolution scanning electron microscopy and atomic force microscopy) methods. The modified polymers were found to maintain their original mechanical and physical properties, while their nanoroughness on the other hand had risen by 1.6-2.7 times because of SNP grafting. The SNP-grafted surfaces displayed anti-biofouling properties, resulting in a significant reduction in the attached Gram-positive Bacillus licheniformis or Gram-negative Pseudomonas aeruginosa bacteria compared to their nongrafted counterparts. Confocal laser scanning microscopy and scanning electron microscopy studies have confirmed that bacterial cells could not successfully adhere onto the SNP-grafted polymer films regardless of the polymer type, and their biofilm formation was therefore damaged. The presented facile and straightforward protocol allows eliminating the need for biocidal agents and resorts to grafted nanosilica instead. This strategy may serve as a feasible and safe platform for the development of sustainable anti-biofouling surfaces in biomedical devices; food, water, and air treatment systems; and industrial equipment.

Deconvoluting the effects of surface chemistry and nanoscale topography: Pseudomonas aeruginosa biofilm nucleation on Si-based substrates

HYPOTHESIS

The nucleation of biofilms is known to be affected by both the chemistry and topography of the underlying substrate, particularly when topography includes nanoscale (<100 nm) features. However, determining the role of topography vs. chemistry is complicated by concomitant variation in both as a result of typical surface modification techniques. Analyzing the behavior of biofilm-forming bacteria exposed to surfaces with systematic, independent variation of both topography and surface chemistry should allow differentiation of the two effects.

EXPERIMENTS

Silicon surfaces with reproducible nanotopography were created by anisotropic etching in deoxygenated water. Surface chemistry was varied independently to create hydrophilic (OH-terminated) and hydrophobic (alkyl-terminated) surfaces. The attachment and proliferation of Psuedomonas aeruginosa to these surfaces was characterized over a period of 12 h using fluorescence and confocal microscopy.

FINDINGS

The number of attached bacteria as well as the structural characteristics of the nucleating biofilm were influenced by both surface nanotopography and surface chemistry. In general terms, the presence of both nanoscale features and hydrophobic surface chemistry enhance bacterial attachment and colonization. However, the structural details of the resulting biofilms suggest that surface chemistry and topography interact differently on each of the four surface types we studied.

Nanoimprinting of biomedical polymers reduces candidal physical adhesion

Management of fungal biofilms represents a significant challenge to healthcare. As a preventive approach, minimizing adhesion between indwelling medical devices and microorganisms would be an important step forward. This study investigated the anti-fouling capacity of engineered nanoscale topographies to the pathogenic yeast Candida albicans. Highly ordered arrays of nano-pit topographies were shown to significantly reduce the physical adherence capacity of C. albicans. This study shows a potential of nanoscale patterns to inhibit and prevent pathogenic biofilm formation on biomedical substrates.

Microbial Biofilms in Pulmonary and Critical Care Diseases

Microbial biofilms can colonize medical devices and human tissues, and their role in microbial pathogenesis is now well established. Not only are biofilms ubiquitous in natural and human-made environments, but they are also estimated to be associated with approximately two-thirds of nosocomial infections. This multicellular aggregated form of microbial growth confers a remarkable resistance to killing by antimicrobials and host defenses, leading biofilms to cause a wide range of subacute or chronic infections that are difficult to eradicate. We have gained tremendous knowledge on the molecular, genetic, microbiological, and biophysical processes involved in biofilm formation. These insights now shape our understanding, diagnosis, and management of many infectious diseases and direct the development of novel antimicrobial therapies that target biofilms. Bacterial and fungal biofilms play an important role in a range of diseases in pulmonary and critical care medicine, most importantly catheter-associated infections, ventilator-associated pneumonia, chronic Pseudomonas aeruginosa infections in cystic fibrosis lung disease, and Aspergillus fumigatus pulmonary infections.

Smart Antibacterial Surfaces with Switchable Bacteria-Killing and Bacteria-Releasing Capabilities

The attachment and subsequent colonization of bacteria on the surfaces of synthetic materials and devices lead to serious problems in both human healthcare and industrial applications. Therefore, antibacterial surfaces that can prevent bacterial attachment and biofilm formation have been a long-standing focus of considerable interest and research efforts. Recently, a promising "kill-release" strategy has been proposed and applied to construct so-called smart antibacterial surfaces, which can kill bacteria attached to their surface and then undergo on-demand release of the dead bacteria and other debris to reveal a clean surface under an appropriate stimulus, thereby maintaining effective long-term antibacterial activity. This Review focuses on the recent progress (particularly over the past 5 years) on such smart antibacterial surfaces. According to the different design strategies, these surfaces can be divided into three categories: (i) "K + R"-type surfaces, which have both a killing unit and a releasing unit; (ii) "K → R"-type surfaces, which have a surface-immobilized killing unit that can be switched to perform a releasing function; and (iii) "K + (R)"-type surfaces, which have only a killing unit but can release dead bacteria upon the addition of a release solution. In the end, a brief perspective on future research directions and the major challenges in this promising field is also presented.

Optimization of electrospun poly(N-isopropyl acrylamide) mats for the rapid reversible adhesion of mammalian cells

Poly(N-isopropyl acrylamide) (pNIPAM) is a "smart" polymer that responds to changes in altering temperature near physiologically relevant temperatures, changing its relative hydrophobicity. Mammalian cells attach to pNIPAM at 37 °C and detach spontaneously as a confluent sheet when the temperature is shifted below the lower critical solution temperature (∼32 °C). A variety of methods have been used to create pNIPAM films, including plasma polymerization, self-assembled monolayers, and electron beam ionization. However, detachment of confluent cell sheets from these pNIPAM films can take well over an hour to achieve potentially impacting cellular behavior. In this work, pNIPAM mats were prepared via electrospinning (i.e., espNIPAM) by a previously described technique that the authors optimized for cell attachment and rapid cell detachment. Several electrospinning parameters were varied (needle gauge, collection time, and molecular weight of the polymer) to determine the optimum parameters. The espNIPAM mats were then characterized using Fourier-transform infrared, x-ray photoelectron spectroscopy, and scanning electron microscopy. The espNIPAM mats showing the most promise were seeded with mammalian cells from standard cell lines (MC3T3-E1) as well as cancerous tumor (EMT6) cells. Once confluent, the temperature of the cells and mats was changed to ∼25 °C, resulting in the extremely rapid swelling of the mats. The authors find that espNIPAM mats fabricated using small, dense fibers made of high molecular weight pNIPAM are extremely well-suited as a rapid release method for cell sheet harvesting.

On-Demand Removal of Bacterial Biofilms via Shape Memory Activation

Bacterial biofilms are a major cause of chronic infections and biofouling; however, effective removal of established biofilms remains challenging. Here we report a new strategy for biofilm control using biocompatible shape memory polymers with defined surface topography. These surfaces can both prevent bacterial adhesion and remove established biofilms upon rapid shape change with moderate increase of temperature, thereby offering more prolonged antifouling properties. We demonstrate that this strategy can achieve a total reduction of Pseudomonas aeruginosa biofilms by 99.9% compared to the static flat control. It was also found effective against biofilms of Staphylococcus aureus and an uropathogenic strain of Escherichia coli.

How Escherichia coli lands and forms cell clusters on a surface: a new role of surface topography

Bacterial response to surface topography during biofilm formation was studied using 5 μm tall line patterns of poly (dimethylsiloxane) (PDMS). Escherichia coli cells attached on top of protruding line patterns were found to align more perpendicularly to the orientation of line patterns when the pattern narrowed. Consistently, cell cluster formation per unit area on 5 μm wide line patterns was reduced by 14-fold compared to flat PDMS. Contrasting the reduced colony formation, cells attached on narrow patterns were longer and had higher transcriptional activities, suggesting that such unfavorable topography may present a stress to attached cells. Results of mutant studies indicate that flagellar motility is involved in the observed preference in cell orientation on narrow patterns, which was corroborated by the changes in cell rotation pattern before settling on different surface topographies. These findings led to a set of new design principles for creating antifouling topographies, which was validated using 10 μm tall hexagonal patterns.

Bacterial viability on chemically modified silicon nanowire arrays

The global threat of antimicrobial resistance is driving an urgent need for novel antimicrobial strategies. Functional surfaces are essential to prevent spreading of infection and reduce surface contamination. In this study we have fabricated and characterized multiscale-functional nanotopographies with three levels of functionalization: (1) nanostructure topography in the form of silicon nanowires, (2) covalent chemical modification with (3-aminopropyl)triethoxysilane, and (3) incorporation of chlorhexidine digluconate. Cell viability assays were carried out on two model microorganisms E. coli and S. aureus over these nanotopographic surfaces. Using SEM we have identified two growth modes producing distinctive multicellular structures, i.e. in plane growth for E. coli and out of plane growth for S. aureus. We have also shown that these chemically modified SiNWs arrays are effective in reducing the number of planktonic and surface-attached microorganisms.

Bioinspired Hierarchical Surface Structures with Tunable Wettability for Regulating Bacteria Adhesion

To circumvent the influence from varied topographies, the systematic study of wettability regulated Gram-positive bacteria adhesion is carried out on bioinspired hierarchical structures duplicated from rose petal structures. With the process of tuning the interfacial chemical composition of the self-assembled films from supramolecular gelators, the varied wettable surfaces from superhydrophilicity to superhydrophobicity can be obtained. The investigation of Gram-positive bacteria adhesion on the hierarchical surfaces reveals that Gram-positive bacteria adhesion is crucially mediated by peptidoglycan due to its different interaction mechanisms with wettable surfaces. The study makes it possible to systematically study the influence mechanism of wettability regulated bacteria adhesion and provides a sight to make the bioinspired topographies in order to investigate wettability regulated bioadhesion.

Fewer Bacteria Adhere to Softer Hydrogels

Clinically, biofilm-associated infections commonly form on intravascular catheters and other hydrogel surfaces. The overuse of antibiotics to treat these infections has led to the spread of antibiotic resistance and underscores the importance of developing alternative strategies that delay the onset of biofilm formation. Previously, it has been reported that during surface contact, bacteria can detect surfaces through subtle changes in the function of their motors. However, how the stiffness of a polymer hydrogel influences the initial attachment of bacteria is unknown. Systematically, we investigated poly(ethylene glycol) dimethacrylate (PEGDMA) and agar hydrogels that were 20 times thicker than the cumulative size of bacterial cell appendages, as a function of Young's moduli. Soft (44.05-308.5 kPa), intermediate (1495-2877 kPa), and stiff (5152-6489 kPa) hydrogels were synthesized. Escherichia coli and Staphylococcus aureus attachment onto the hydrogels was analyzed using confocal microscopy after 2 and 24 h incubation periods. Independent of hydrogel chemistry and incubation time, E. coli and S. aureus attachment correlated positively to increasing hydrogel stiffness. For example, after a 24 h incubation period, there were 52 and 82% fewer E. coli adhered to soft PEGDMA hydrogels than to the intermediate and stiff PEGDMA hydrogels, respectively. A 62 and 79% reduction in the area coverage by the Gram-positive microbe S. aureus occurred after 24 h incubation on the soft versus intermediate and stiff PEGDMA hydrogels. We suggest that hydrogel stiffness is an easily tunable variable that could potentially be used synergistically with traditional antimicrobial strategies to reduce early bacterial adhesion and therefore the occurrence of biofilm-associated infections.

Effects of Material Properties on Bacterial Adhesion and Biofilm Formation

Adhesion of microbes, such as bacteria and fungi, to surfaces and the subsequent formation of biofilms cause multidrug-tolerant infections in humans and fouling of medical devices. To address these challenges, it is important to understand how material properties affect microbe-surface interactions and engineer better nonfouling materials. Here we review the recent progresses in this field and discuss the main challenges and opportunities. In particular, we focus on bacterial biofilms and review the effects of surface energy, charge, topography, and stiffness of substratum material on bacterial adhesion. We summarize how these surface properties influence oral biofilm formation, and we discuss the important findings from nondental systems that have potential applications in dental medicine.

Natural Green coating inhibits adhesion of clinically important bacteria

Despite many advances, biomaterial-associated infections continue to be a major clinical problem. In order to minimize bacterial adhesion, material surface modifications are currently being investigated and natural products possess large potential for the design of innovative surface coatings. We report the bioguided phytochemical investigation of Pityrocarpa moniliformis and the characterization of tannins by mass spectrometry. It was demonstrated that B-type linked proanthocyanidins-coated surfaces, here termed Green coatings, reduced Gram-positive bacterial adhesion and supported mammalian cell spreading. The proposed mechanism of bacterial attachment inhibition is based on electrostatic repulsion, high hydrophilicity and the steric hindrance provided by the coating that blocks bacterium-substratum interactions. This work shows the applicability of a prototype Green-coated surface that aims to promote necessary mammalian tissue compatibility, while reducing bacterial colonization.