Review of Antimicrobial Nanocoatings in Medicine and Dentistry: Mechanisms of Action, Biocompatibility Performance, Safety, and Benefits Compared to Antibiotics

This review discusses topics relevant to the development of antimicrobial nanocoatings and nanoscale surface modifications for medical and dental applications. Nanomaterials have unique properties compared to their micro- and macro-scale counterparts and can be used to reduce or inhibit bacterial growth, surface colonization and biofilm development. Generally, nanocoatings exert their antimicrobial effects through biochemical reactions, production of reactive oxygen species or ionic release, while modified nanotopographies create a physically hostile surface for bacteria, killing cells via biomechanical damage. Nanocoatings may consist of metal nanoparticles including silver, copper, gold, zinc, titanium, and aluminum, while nonmetallic compounds used in nanocoatings may be carbon-based in the form of graphene or carbon nanotubes, or composed of silica or chitosan. Surface nanotopography can be modified by the inclusion of nanoprotrusions or black silicon. Two or more nanomaterials can be combined to form nanocomposites with distinct chemical or physical characteristics, allowing combination of different properties such as antimicrobial activity, biocompatibility, strength, and durability. Despite their wide range of applications in medical engineering, questions have been raised regarding potential toxicity and hazards. Current legal frameworks do not effectively regulate antimicrobial nanocoatings in matters of safety, with open questions remaining about risk analysis and occupational exposure limits not considering coating-based approaches. Bacterial resistance to nanomaterials is also a concern, especially where it may affect wider antimicrobial resistance. Nanocoatings have excellent potential for future use, but safe development of antimicrobials requires careful consideration of the "One Health" agenda, appropriate legislation, and risk assessment.

Wetting and Imbibition Characteristics of Pseudomonas fluorescens Biofilms Grown on Stainless Steel

This study aims to provide insights into biofilm resistance associated with their structural properties acquired during formation and development. On this account, the wetting and imbibition behavior of dehydrated Pseudomonas fluorescens biofilms grown on stainless steel electropolished substrates is thoroughly examined at different biofilm ages. A polar liquid (water) and a non-polar liquid (diiodomethane) are employed as wetting agents in the form of sessile droplets. A mathematical model is applied to appraise the wetting and imbibition performance of biofilms incorporating the evaporation of sessile droplets. The present results show that the examined biofilms are hydrophilic. The progressive growth of biofilms leads to a gradual increase of substrate surface coverage─up to full coverage─accompanied by a gradual decrease of biofilm surface roughness. It is noteworthy that just after 24 h of biofilm growth, the surface roughness increases about 6.7 times the roughness of the clean stainless steel surface. It is further found that the imbibition of liquid in the biofilm matrix is restricted only to the biofilm region under the sessile droplet. The lack of further capillary imbibition into the biofilm structure, beyond the droplet deposition region, implies that the biofilm matrix is not in the form of an extended network of interconnected micro/nanopores. All in all, the present results indicate a resilient biofilm structure to biocide penetration despite its hydrophilic nature.

A rotating bioreactor for the production of biofilms at the solid-air interface

Conventional bioreactors are typically developed for the production of planktonic bacteria or submerged biofilms. In contrast, reactors for the continuous production of biofilms at the solid-air interface are scarce, and they require specific conditions since the bacteria need to attach firmly to the surface and require a permanent supply of moisture and nutrients from below. Recently, research from the field of civil engineering has pinpoint an increased need for the production of terrestrial biofilms: several variants of Bacillus subtilis biofilms have been shown to be useful additives to mortar that increase the water repellency and thus the lifetime of the cementitious material. The bioreactor introduced here allows for the continuous production of such bacterial biofilms at the solid/air interface, and they have virtually identical properties as biofilms cultivated via classical microbiological techniques. This is made possible by equipping a rotating cylinder with a porous membrane that acts as a solid growth substrate the bacterial biomass can form on. In this configuration, nutrient supply is enabled via diffusive transport of a suitable growth medium from the core volume of the cylindrical reactor to the membrane surface. In addition to cultivating bacterial biofilms, the versatile and adaptable setup introduced here also enables the growth of other microbial organisms including the yeast Saccharomyces cerevisiae and the fungus Penicillium chrysogenum. This article is protected by copyright. All rights reserved.

Topography quantifications allow for identifying the contribution of parental strains to physical properties of co-cultured biofilms

Most biofilm research has so far focused on investigating biofilms generated by single bacterial strains. However, such single-species biofilms are rare in nature where bacteria typically coexist with other microorganisms. Although, from a biological view, the possible interactions occurring between different bacteria are well studied, little is known about what determines the material properties of a multi-species biofilm. Here, we ask how the co-cultivation of two B. subtilis strains affects certain important biofilm properties such as surface topography and wetting behavior. We find that, even though each daughter colony typically resembles one of the parent colonies in terms of morphology and wetting, it nevertheless exhibits a significantly different surface topography. Yet, this difference is only detectable via a quantitative metrological analysis of the biofilm surface. Furthermore, we show that this difference is due to the presence of bacteria belonging to the 'other' parent strain, which does not dominate the biofilm features. The findings presented here may pinpoint new strategies for how biofilms with hybrid properties could be generated from two different bacterial strains. In such engineered biofilms, it might be possible to combine desired properties from two strains by co-cultivation.

Wetting properties of dehydrated biofilms under different growth conditions

Biofilms are resilient to environmental conditions and often resistant even to strong disinfectants. It is crucial to investigate their interfacial properties, which can be effectively characterized by wetting analysis. Wetting phenomena on biofilm surfaces have been poorly investigated in literature, in particular a systematic study of wetting on real biofilm-coated substrates including the application of external body forces (forced wetting, i.e.: centrifugal and gravitational forces) is missing. The aim of this work is to study the role of nutrient and shear flow conditions on wetting properties of Pseudomonas fluorescens dehydrated biofilms, grown on glass substrates. An innovative device (Kerberos®), capable to study spreading/sliding behavior under the application of external body forces, is used here for a systematic analysis of wetting/de-wetting liquid droplets on horizontal substrates under the action of tangential forces. Results prove that, under different growth conditions, (i.e., nutrients and imposed flow), biofilms exhibit different wetting properties. At lower nutrient/shear flow conditions, biofilms show spreading/sliding behavior close to that of pure glass. At higher nutrient and shear flow conditions, droplets on biofilms show spreading followed by imbibition soon after deposition, which leads to peculiar droplet depinning during the rotation test. Wetting properties are derived as a function of the rotation speed from both top and side views videoframes through a dedicated image analysis technique. A detailed analysis of biofilm formation and morphology/topography is also provided here.

Tuning Microbial Activity via Programmatic Alteration of Cell/Substrate Interfaces

A wide portfolio of advanced programmable materials and structures has been developed for biological applications in the last two decades. Particularly, due to their unique properties, semiconducting materials have been utilized in areas of biocomputing, implantable electronics, and healthcare. As a new concept of such programmable material design, biointerfaces based on inorganic semiconducting materials as substrates introduce unconventional paths for bioinformatics and biosensing. In particular, understanding how the properties of a substrate can alter microbial biofilm behavior enables researchers to better characterize and thus create programmable biointerfaces with necessary characteristics on demand. Herein, the current status of advanced microorganism-inorganic biointerfaces is summarized along with types of responses that can be observed in such hybrid systems. This work identifies promising inorganic material types along with target microorganisms that will be critical for future research on programmable biointerfacial structures.

Wetting of Dehydrated Hydrophilic Pseudomonas fluorescens Biofilms under the Action of External Body Forces

Wetting of dehydrated Pseudomonas fluorescens biofilms grown on glass substrates by an external liquid is employed as a means to investigate the complex morphology of these biofilms along with their capability to interact with external fluids. The porous structure left behind after dehydration induces interesting droplet spreading on the external surface and imbibition into pores upon wetting. Static contact angles and volume loss by imbibition measured right upon droplet deposition indicate that biofilms of higher incubation times show a higher porosity and effective hydrophilicity. Furthermore, during subsequent rotation tests, using Kerberos device, these properties dictate a peculiar forced wetting/spreading behavior. As rotation speed increases a long liquid tail forms progressively at the rear part of the droplet, which stays pinned at all times, while only the front part of the droplet depins and spreads. Interestingly, the experimentally determined retention force for the onset of droplet sliding on biofilm external surface is lower than that on pure glass. An effort is made to describe such complex forced wetting phenomena by presenting apparent contact angles, droplet length, droplet shape contours, and edges position as obtained from detailed image analysis.

Machine Learning Approach to Analyze the Surface Properties of Biological Materials

Similar to how CRISPR has revolutionized the field of molecular biology, machine learning may drastically boost research in the area of materials science. Machine learning is a fast-evolving method that allows for analyzing big data and unveiling correlations that otherwise would remain undiscovered. It may hold invaluable potential to engineer novel functional materials with desired properties, a field, which is currently limited by time-consuming trial and error approaches and our limited understanding of how different material properties depend on each other. Here, we apply machine learning algorithms to classify complex biological materials based on their microtopography. With this approach, the surfaces of different variants of biofilms and plant leaves can not only be distinguished but also correctly classified according to their wettability. Furthermore, an importance ranking provided by one of the algorithms allows us to identify those surface features that are critical for a successful sample classification. Our study exemplifies how machine learning can contribute to the analysis and categorization of complex surfaces, a tool, which can be highly useful for other areas of materials science, such as damage assessment as well as adhesion or friction studies.

Bacillus subtilis biofilm formation and social interactions

Biofilm formation is a process in which microbial cells aggregate to form collectives that are embedded in a self-produced extracellular matrix. Bacillus subtilis is a Gram-positive bacterium that is used to dissect the mechanisms controlling matrix production and the subsequent transition from a motile planktonic cell state to a sessile biofilm state. The collective nature of life in a biofilm allows emergent properties to manifest, and B. subtilis biofilms are linked with novel industrial uses as well as probiotic and biocontrol processes. In this Review, we outline the molecular details of the biofilm matrix and the regulatory pathways and external factors that control its production. We explore the beneficial outcomes associated with biofilms. Finally, we highlight major advances in our understanding of concepts of microbial evolution and community behaviour that have resulted from studies of the innate heterogeneity of biofilms.

Hexyl-aminolevulinate ethosome-mediated photodynamic therapy against acne: in vitro and in vivo analyses

Biofilm formation by Propionibacterium acnes is known to cause failure of anti-acne treatment. Conventional therapies for acne are typically inadequate. Accordingly, in this study, we evaluated the therapeutic potential of photodynamic therapy (PDT) using hexyl-aminolevulinate (HAL)-loaded ethosomes (ESs) against the biofilms of P. acnes in vitro and P. acnes-induced inflammatory acne model in vivo. The antibacterial effects of HAL ESs were evaluated using XTT colorimetric assays and scanning electron microscopic observations of morphological changes. P. acnes was intradermally injected into the ears of Sprague-Dawley rats, and the anti-inflammatory effects of HAL ESs were measured by determining changes in appearance, histology, and the antibacterial effects by P. acnes abundance in ear tissues compared with blank control ESs, HAL alone, and 5-aminolevulinic acid (ALA) alone. The highest reduction in viability in P. acnes biofilms was observed after treatment with 5 mg/mL HAL ESs. Notably, blank control ESs also showed significant inhibitory effects. Furthermore, HAL ESs had superior therapeutic effects in the rat model compared with HAL or ALA solutions. The observed therapeutic effects of HAL ESs against P. acnes biofilms and P. acnes-induced inflammation suggest that PDT with HAL-loaded ESs may have potential applications in the treatment of acne.

Topography and Expansion Patterns at the Biofilm-Agar Interface in Bacillus subtilis Biofilms

Bacterial biofilms are complex microbial communities that are formed on various natural and synthetic surfaces. In contrast to bacteria in their planktonic form, biofilms are characterized by their relatively low susceptibility to anti-microbial treatments, in part due to limited diffusion throughout the biofilm and the complex distribution of bacterial cells within. The virulence of biofilms is therefore a combination of the structural properties and patterns of adhesion that anchor them to their host surface. In this paper, we analyze the topographical properties of Bacillus subtilis' biofilm-agar interface across different growth conditions. B. subtilis colonies were grown to maturity on biofilm-promoting agar-based media (LBGM), under standard and stress-inducing growth conditions. The biofilm-agar interface of the colony-type biofilms was modeled using confocal microscopy and computational analysis. Profilometry data were obtained from the macrocolonies and used for the analysis of the surface topography as it relates to the adhesion modes present at the biofilm-agar interface. Fluorescent microspheres were utilized to monitor the expansion patterns present at the interface between the macrocolonies and the solid growth medium. Contact surface analysis revealed topographical changes that could have a direct effect on the adhesion strength of the biofilm to its host surface, thus affecting its potential susceptibility to anti-microbial agents. The topographical characteristics of the biofilm-agar interface partially define the macrocolony structure and may have significant effects on bacterial survival and virulence.

The role of flow in bacterial biofilm morphology and wetting properties

Biofilms are bacterial communities embedded in an extracellular matrix, able to adhere to surfaces. Different experimental set-ups are widely used for in vitro biofilm cultivation; however, a well-defined comparison among different culture conditions, especially suited to interfacial characterization, is still lacking in the literature. The main objective of this work is to study the role of flow on biofilm formation, morphology and interfacial properties. Three different in vitro setups, corresponding to stagnant, shaking, and laminar flow conditions (custom-made flow cell), are used in this work to grow single strain biofilms of Pseudomonas fluorescens AR 11 on glass coupons. Results show that flow conditions significantly influenced biofilm formation kinetics, affecting mass transfer and cell attachment/detachment processes. Distinct morphological patterns are found under different flow regimes. Static contact angle data do not depend significantly on biofilm growth conditions in the parametric range investigated in this work.

Metal ions weaken the hydrophobicity and antibiotic resistance of Bacillus subtilis NCIB 3610 biofilms

Surface superhydrophobicity makes bacterial biofilms very difficult to fight, and it is a combination of their matrix composition and complex surface roughness which synergistically protects these biomaterials from wetting. Although trying to eradicate biofilms with aqueous (antibiotic) solutions is common practice, this can be a futile approach if the biofilms have superhydrophobic properties. To date, there are not many options available to reduce the liquid repellency of biofilms or to prevent this material property from developing. Here, we present a solution to this challenge. We demonstrate how the addition of metal ions such as copper and zinc during or after biofilm formation can render the surface of otherwise superhydrophobic B. subtilis NCIB 3610 biofilms completely wettable. As a result of this procedure, these smoother, hydrophilic biofilms are more susceptible to aqueous antibiotics solutions. Our strategy proposes a scalable and widely applicable step in a multi-faceted approach to eradicate biofilms.

Effects of extracellular polymeric substances and microbial community on the anti-scouribility of sewer sediment

Sewer sediment is the main source of overflow pollution, and the anti-scouribility of sewer sediment directly determines the amount of the discharged contaminants. In this study, sewer sediments of different depths were collected from combined and storm sewers in Shanghai, China. The anti-scouribility, represented by the shear stress of each layer of sewer sediment, was detected in situ. The microbial community and extracellular polymeric substances (EPS), including carbohydrates and proteins present in the sewer sediments were characterized. The results indicated that the distribution of the anti-scouribility of sewer sediment is regulated. There were positive correlations between the content of EPS, proteins, and carbohydrates, and the anti-scouribility of sediments (Pearson Corr. = 0.604, sig. = 0.219; Pearson Corr. = 0.623, sig. = 0.234; Pearson Corr. = 0.727, sig. = 0.359, respectively). Furthermore, the microbial community had a positive influence on anti-scouribility. In particular, the gram-positive bacterial phyla of Bacteroidetes and Firmicutes may be important and influential for the improvement of anti-scouribility of sediment owing to their production of cellulose.

Dual bioresponsive antibiotic and quorum sensing inhibitor combination nanoparticles for treatment of Pseudomonas aeruginosa biofilms in vitro and ex vivo

Many debilitating infections result from persistent microbial biofilms that do not respond to conventional antibiotic regimens. A potential method to treat such chronic infections is to combine agents which interfere with bacterial biofilm development together with an antibiotic in a single formulation. Here, we explore the use of a new bioresponsive polymer formulation derived from specifically modified alginate nanoparticles (NPs) in order to deliver ciprofloxacin (CIP) in combination with the quorum sensing inhibitor (QSI) 3-amino-7-chloro-2-nonylquinazolin-4(3H)-one (ACNQ) to mature Pseudomonas aeruginosa biofilms. The alginate NPs were engineered to incorporate a pH-responsive linker between the polysaccharide backbone and the QSI, and to encapsulate CIP via charge-charge interactions of the positively-charged drug with the carboxyl residues of the alginate matrix. In this way, a dual-action release of antibiotic and QSI was designed for the low-pH regions of a biofilm, involving cleavage of the QSI-linker to the alginate matrix and reduced charge-charge interactions between CIP and the polysaccharide as the alginate carboxyl side-chains protonated. When tested in a biofilm model the concomitant release of CIP + QSI from the pH-responsive nanoparticles significantly reduced the viability of the biofilm compared with CIP treatment alone. In addition, the alginate NPs were shown to penetrate deeply into P. aeruginosa biofilms, which we attribute in part to the charges of the NPs and the release of the QSI agent. Finally, we tested the formulation in both a 2D keratinocyte and a 3D ex vivo skin infection model. The dual-action bio-responsive QSI and CIP release nanoparticles effectively cleared the infection in the latter, suggesting considerable promise for combination therapeutics which prevent biofilm formation as well as effectively killing mature P. aeruginosa biofilms.

Biopolymer-enriched B. subtilis NCIB 3610 biofilms exhibit increased erosion resistance

The erosion resistance of bacterial biofilms can be a double-edged sword: it hampers the removal of undesired biofilms in biomedical settings, but it is necessary for beneficial biofilms to be used in aqueous environments for biotechnological applications. Whether or not a bacterial biofilm exhibits this material property depends on the bacterial species and the detailed composition of the biofilm matrix. Here, we demonstrate how the erosion resistance of B. subtilis NCIB 3610 biofilms can be enhanced by integrating foreign (bio)polymers into the matrix during biofilm growth. As a result of this artificial macromolecule addition, the engineered biofilm colonies show changes in their surface topography which, in turn, cause an alteration in the mode of surface superhydrophobicity. Surprisingly, the viscoelastic properties and permeability of the biofilms towards antibiotics remain unaffected. The method introduced here may present a promising strategy for engineering beneficial biofilms such, that they become more stable towards shear forces caused by flowing water but, at the same time, remain permeable to nutrients or other molecules.

Evolved Biofilm: Review on the Experimental Evolution Studies of Bacillus subtilis Pellicles

For several decades, laboratory evolution has served as a powerful method to manipulate microorganisms and to explore long-term dynamics in microbial populations. Next to canonical Escherichia coli planktonic cultures, experimental evolution has expanded into alternative cultivation methods and species, opening the doors to new research questions. Bacillus subtilis, the spore-forming and root-colonizing bacterium, can easily develop in the laboratory as a liquid-air interface colonizing pellicle biofilm. Here, we summarize recent findings derived from this tractable experimental model. Clonal pellicle biofilms of B. subtilis can rapidly undergo morphological and genetic diversification creating new ecological interactions, for example, exploitation by biofilm non-producers. Moreover, long-term exposure to such matrix non-producers can modulate cooperation in biofilms, leading to different phenotypic heterogeneity pattern of matrix production with larger subpopulation of "ON" cells. Alternatively, complementary variants of biofilm non-producers, each lacking a distinct matrix component, can engage in a genetic division of labor, resulting in superior biofilm productivity compared to the "generalist" wild type. Nevertheless, inter-genetic cooperation appears to be evanescent and rapidly vanquished by individual biofilm formation strategies altering the amount or the properties of the remaining matrix component. Finally, fast-evolving mobile genetic elements can unpredictably shift intra-species interactions in B. subtilis biofilms. Understanding evolution in clonal biofilm populations will facilitate future studies in complex multispecies biofilms that are more representative of nature.

Importance of the biofilm matrix for the erosion stability of Bacillus subtilis NCIB 3610 biofilms

Erosion of bacterial biofilms is dependent on the composition of the biofilm matrix and the surrounding chemical environment.