Microfluidic assay to quantify the adhesion of marine bacteria

Effect of Gravity on Bacterial Adhesion to Heterogeneous Surfaces

Bacterial adhesion is the first step in the formation of surface biofilms. The number of bacteria that bind to a surface from the solution depends on how many bacteria can reach the surface (bacterial transport) and the strength of interactions between bacterial adhesins and surface receptors (adhesivity). By using microfluidic channels and video microscopy as well as computational simulations, we investigated how the interplay between bacterial transport and adhesivity affects the number of the common human pathogen Escherichia coli that bind to heterogeneous surfaces with different receptor densities. We determined that gravitational sedimentation causes bacteria to concentrate at the lower surface over time as fluid moves over a non-adhesive region, so bacteria preferentially adhere to adhesive regions on the lower, inflow-proximal areas that are downstream of non-adhesive regions within the entered compartments. Also, initial bacterial attachment to an adhesive region of a heterogeneous lower surface may be inhibited by shear due to mass transport effects alone rather than shear forces per se, because higher shear washes out the sedimented bacteria. We also provide a conceptual framework and theory that predict the impact of sedimentation on adhesion between and within adhesive regions in flow, where bacteria would likely bind both in vitro and in vivo, and how to normalize the bacterial binding level under experimental set-ups based on the flow compartment configuration.

Bacterial surface attachment and fouling assay on polymer and carbon surfaces using Rheinheimera sp. identified using bacteria community analysis of brackish water

Biofouling on surfaces in contact with sea- or brackish water can severely impact the function of devices like reverse osmosis modules. Single species laboratory assays are frequently used to test new low fouling materials. The choice of bacterial strain is guided by the natural population present in the application of interest and decides on the predictive power of the results. In this work, the analysis of the bacterial community present in brackish water from Mashabei Sadeh, Israel was performed and Rheinheimera sp. was detected as a prominent microorganism. A Rheinheimera strain was selected to establish a short-term accumulation assay to probe initial bacterial attachment as well as biofilm growth to determine the biofilm-inhibiting properties of coatings. Both assays were applied to model coatings, and technically relevant polymers including laser-induced graphene. This strategy might be applied to other water sources to better predict the fouling propensity of new coatings.

Early biofilm and streamer formation is mediated by wall shear stress and surface wettability: A multifactorial microfluidic study

Biofilms are intricate communities of microorganisms encapsulated within a self-produced matrix of extra-polymeric substances (EPS), creating complex three-dimensional structures allowing for liquid and nutrient transport through them. These aggregations offer constituent microorganisms enhanced protection from environmental stimuli-like fluid flow-and are also associated with higher resistance to antimicrobial compounds, providing a persistent cause of concern in numerous sectors like the marine (biofouling and aquaculture), medical (infections and antimicrobial resistance), dentistry (plaque on teeth), food safety, as well as causing energy loss and corrosion. Recent studies have demonstrated that biofilms interact with microplastics, often influencing their pathway to higher trophic levels. Previous research has shown that initial bacterial attachment is affected by surface properties. Using a microfluidic flow cell, we have investigated the relationship between both wall shear stress (τw ) and surface properties (surface wettability) upon biofilm formation of two species (Cobetia marina and Pseudomonas aeruginosa). We investigated biofilm development on low-density polyethylene (LDPE) membranes, Permanox® slides, and glass slides, using nucleic acid staining and end-point confocal laser scanning microscopy. The results show that flow conditions affect biomass, maximum thickness, and surface area of biofilms, with higher τw (5.6 Pa) resulting in thinner biofilms than lower τw (0.2 Pa). In addition, we observed differences in biofilm development across the surfaces tested, with LDPE typically demonstrating more overall biofilm in comparison to Permanox® and glass. Moreover, we demonstrate the formation of biofilm streamers under laminar flow conditions within straight micro-channels.

Fluid Flow Induces Differential Detachment of Live and Dead Bacterial Cells from Nanostructured Surfaces

Nanotopographic surfaces are proven to be successful in killing bacterial cells upon contact. This non-chemical bactericidal property has paved an alternative way of fighting bacterial colonization and associated problems, especially the issue of bacteria evolving resistance against antibiotic and antiseptic agents. Recent advancements in nanotopographic bactericidal surfaces have made them suitable for many applications in medical and industrial sectors. The bactericidal effect of nanotopographic surfaces is classically studied under static conditions, but the actual potential applications do have fluid flow in them. In this study, we have studied how fluid flow can affect the adherence of bacterial cells on nanotopographic surfaces. Gram-positive and Gram-negative bacterial species were tested under varying fluid flow rates for their retention and viability after flow exposure. The total number of adherent cells for both species was reduced in the presence of flow, but there was no flowrate dependency. There was a significant reduction in the number of live cells remaining on nanotopographic surfaces with an increasing flowrate for both species. Conversely, we observed a flowrate-independent increase in the number of adherent dead cells. Our results indicated that the presence of flow differentially affected the adherent live and dead bacterial cells on nanotopographic surfaces. This could be because dead bacterial cells were physically pierced by the nano-features, whereas live cells adhered via physiochemical interactions with the surface. Therefore, fluid shear was insufficient to overcome adhesion forces between the surface and dead cells. Furthermore, hydrodynamic forces due to the flow can cause more planktonic and detached live cells to collide with nano-features on the surface, causing more cells to lyse. These results show that nanotopographic surfaces do not have self-cleaning ability as opposed to natural bactericidal nanotopographic surfaces, and nanotopographic surfaces tend to perform better under flow conditions. These findings are highly useful for developing and optimizing nanotopographic surfaces for medical and industrial applications.

Reduction of biofilm accumulation by constant and alternating potentials in static and dynamic field experiments

The application of electric fields to conductive coatings is an environmentally friendly way to reduce biofilm formation. In particular alternating potentials (APs) have received increasing attention in recent studies. Here, an electrochemical rotating disk setup for dynamic field exposure experiments was developed to study how APs alter the attachment of fouling organisms in a multispecies ocean environment. A specific focus of the device design was proper integration of the potentiostat in the strongly corroding saltwater environment. The effect of APs on the accumulation of fouling organisms in short term field exposures was studied. Potentials on conductive gold surfaces were periodically switched between -0.3 V and 0.3 V or between -0.8 V and 0.6 V at a frequency of 0.5 Hz. APs were capable of significantly reducing the attachment of marine fouling organisms compared with the conductive samples immersed at open circuit potentials.

Zwitterionic Peptides Reduce Accumulation of Marine and Freshwater Biofilm Formers

Zwitterionic peptides are facile low-fouling compounds for environmental applications as they are biocompatible and fully biodegradable as their degradation products are just amino acids. Here, a set of histidine (H) and glutamic acid (E), as well as lysine (K) and glutamic acid (E) based peptide sequences with zwitterionic properties were synthesized. Both oligopeptides (KE)₄K and (HE)₄H were synthesized in d and l configurations to test their ability to resist the nonspecific adsorption of the proteins lysozyme and fibrinogen. The coatings were additionally tested against the attachment of the marine organisms Navicula perminuta and Cobetia marina as well as the freshwater bacterium Pseudomonas fluorescens on the developed coatings. While the peptides containing lysine performed better in protein resistance assays and against freshwater bacteria, the sequences containing histidine were generally more resistant against marine organisms. The contribution of amino acid-intrinsic properties such as side chain pK_a values and hydrophobicity, as well as external parameters such as pH and salinity of fresh water and seawater on the resistance of the coatings is discussed. In this way, a detailed picture emerges as to which zwitterionic sequences show advantages in future generations of biocompatible, sustainable, and nontoxic fouling release coatings.

Control of Marine Bacteria and Diatom Biofouling by Constant and Alternating Potentials

The application of electrochemical potentials to surfaces is an easy and direct way to alter surface charge density, the structure of the electrochemical double layer, and the presence of electrochemically activated species. On such electrified interfaces the formation of biofilms is reduced. Here we investigate how applied potentials alter the colonization of surfaces by the marine bacterium Cobetia marina and the marine diatom Navicula perminuta. Different constant potentials between -0.8 and 0.6 V as well as regular switching between two potentials were investigated, and their influence on the attachment of the two biofilm-forming microorganisms on gold-coated working electrodes was quantified. Reduced bacteria and diatom attachment were found when negative potentials and alternating potentials were applied. The results are discussed on the basis of the electrochemical processes occurring at the working electrode in artificial seawater as revealed by cyclic voltammetry.

Formation and Function of the Primary Tube During Settlement and Metamorphosis of the Marine Polychaete Hydroides elegans (Haswell, 1883) (Serpulidae)

AbstractThe serpulid polychaete Hydroides elegans has emerged as a major model organism for studies of marine invertebrate settlement and metamorphosis and for processes involved in marine biofouling. Rapid secretion of an enveloping, membranous, organic primary tube provides settling larvae of H. elegans firm adhesion to a surface and a refuge within which to complete metamorphosis. While this tube is never calcified, it forms the template from which the calcified tube is produced at its anterior end. Examination of scanning and transmission electron micrographs of competent and settling larvae revealed that the tube is secreted from epidermal cells of the three primary segments, with material possibly transported through the larval cuticle via abundant microvilli. The tube is composed of complexly layered fibrous material that has an abundance of the amino acids that characterize the collagenous cuticle of other polychaetes, plus associated carbohydrates. The significance of the dependence on surface bacterial biofilms for stimulating settlement in this species is revealed as a complex interaction between primary tube material, as it is secreted, and the extracellular polymeric substances abundantly produced by biofilm-residing bacteria. This association appears to provide the settling larvae with an adhesion strength similar to that of bacteria in a biofilm and significantly less when larvae settle on a clean surface.

Bactericidal efficiency of micro- and nanostructured surfaces: a critical perspective

Micro/nanostructured surfaces (MNSS) have shown the ability to inactivate bacterial cells by physical means. An enormous amount of research has been conducted in this area over the past decade. Here, we review the various surface factors that affect the bactericidal efficiency. For example, surface hydrophobicity of the substrate has been accepted to be influential on the bactericidal effect of the surface, but a review of the literature suggests that the influence of hydrophobicity differs with the bacterial species. Also, various bacterial viability quantification methods on MNSS are critically reviewed for their suitability for the purpose, and limitations of currently used protocols are discussed. Presently used static bacterial viability assays do not represent the conditions of which those surfaces could be applied. Such application conditions do have overlaying fluid flow, and bacterial behaviours are drastically different under flow conditions compared to under static conditions. Hence, it is proposed that the bactericidal effect should be assessed under relevant fluid flow conditions with factors such as shear stress and flowrate given due significance. This review will provide a range of opportunities for future research in design and engineering of micro/nanostructured surfaces with varying experimental conditions.

Single-Cell Tracking on Polymer Microarrays Reveals the Impact of Surface Chemistry on Pseudomonas aeruginosa Twitching Speed and Biofilm Development

Bacterial biofilms exhibit up to 1000 times greater resistance to antibiotic or host immune clearance than planktonic cells. Pseudomonas aeruginosa produces retractable type IV pili (T4P) that facilitate twitching motility on surfaces. The deployment of pili is one of the first responses of bacteria to surface interactions and because of their ability to contribute to cell surface adhesion and biofilm formation, this has relevance to medical device-associated infections. While polymer chemistry is known to influence biofilm development, its impact on twitching motility is not understood. Here, we combine a polymer microarray format with time-lapse automated microscopy to simultaneously assess P. aeruginosa twitching motility on 30 different methacrylate/acrylate polymers over 60 min post inoculation using a high-throughput system. During this critical initial period where the decision to form a biofilm is thought to occur, similar numbers of bacterial cells accumulate on each polymer. Twitching motility is observed on all polymers irrespective of their chemistry and physical surface properties, in contrast to the differential biofilm formation noted after 24 h of incubation. However, on the microarray polymers, P. aeruginosa cells twitch at significantly different speeds, ranging from 5 to ∼13 nm/s, associated with crawling or walking and are distinguishable from the different cell surface tilt angles observed. Chemometric analysis using partial least-squares (PLS) regression identifies correlations between surface chemistry, as measured by time-of-flight secondary ion mass spectrometry (ToF-SIMS), and both biofilm formation and single-cell twitching speed. The relationships between surface chemistry and these two responses are different for each process. There is no correlation between polymer surface stiffness and roughness as determined by atomic force measurement (AFM), or water contact angle (WCA), and twitching speed or biofilm formation. This reinforces the dominant and distinct contributions of material surface chemistry to twitching speed and biofilm formation.

Low Fouling Peptides with an All (d) Amino Acid Sequence Provide Enhanced Stability against Proteolytic Degradation While Maintaining Low Antifouling Properties

Peptide-functionalized surfaces, composed of optimized l-peptides, show a high resistance toward nonspecific adsorption of proteins. As l-peptides are known to be prone to proteolytic degradation, the aim of this work is to enhance the stability against enzymatic degradation by using the all d-peptide mirror image of the optimized l-peptides and to determine if the all d-enantiomer retains the protein-resistant and antifouling properties. Two l-peptides and their d-peptide mirror images, some of them containing the nonproteinogenic amino acid α-aminoisobutyric acid (Aib), were synthesized and tested against non-specific adsorption of the proteins lysozyme and fibrinogen and the settlement of marine diatom Navicula perminuta and marine bacteria Cobetia marina. Both the d-enantiomer and the insertion of Aib protected the peptides from proteolytic degradation. Protein resistance was enhanced with the d-enantiomers while maintaining the resistance toward diatoms.

Effect of ozone stress on the intracellular metabolites from Cobetia marina

A GCxGC-MS system was employed with a non-polar × mid-polar column set for the metabolic non-target analysis of Cobetia marina, the model bacteria for marine biofouling. C. marina was treated with ozone to investigate the intracellular metabolic state change under oxidative stress. A minimal inhibitory concentration test was involved to guarantee that the applied ozone dosages were not lethal for the cells. In this study, non-target analyses were performed to identify the metabolites according to the NIST database. As a result, over 170 signals were detected under normal living conditions including 35 potential metabolites. By the comparison of ozone-treated and non-treated samples, five compounds were selected to describe observed trends of signals in the contour plots. Oleic acid exhibited a slight growth by increasing ozone dosage. In contrast, other metabolites such as the amino acid L-proline showed less abundance after ozone treatment, which was more evident once ozone dosage was raised. Thus, this work could provide a hint for searching for up/downregulating factors in such environmental stress conditions for C. marina. Graphical abstract.

Parallelized microfluidic diatom accumulation assay to test fouling-release coatings

Assessing the efficiency of the next generation of protective marine coatings is highly relevant for their optimization. In this paper, a parallelized microfluidic testing device is presented to quantify the accumulation of a model organism (Navicula perminuta) under constant laminar flow. Using automated microscopy in conjunction with image analysis, the adhesion densities on the tested surfaces could be determined after exposure to a flow of suspended algae for 90 min. The optimized protocol for the assay is presented, and the reproducibility of the densities of attached diatoms was verified on four identical surfaces (self-assembled dodecanethiol monolayers). A set of well-characterized self-assembled monolayers with different chemical terminations was used to validate the performance of the assay and its capability to discriminate diatom accumulation on different surface chemistries under dynamic conditions. The observed trends are in good agreement with previously published results obtained in single channel accumulation and detachment assays. To demonstrate the practical relevance of the dynamic experiment, diatom attachment on four technically relevant silicone coatings with different fouling-release properties could clearly be distinguished.

Dynamic field testing of coating chemistry candidates by a rotating disk system

Quick and reliable testing is crucial for the development of new fouling release (FR) coatings. Exposure of these coatings to natural multispecies communities is essential in evaluating their efficacy. To this end, we present a rotating disk setup for dynamic field exposure. To achieve a well-defined flow on the surface of the disk, an easy to use sample mounting system was developed that provides a smooth and even surface. We related the angular velocity of the disk to the wall shear stress on the surface with a hydrodynamic model. The wall shear stress was adjusted to values previously found to be suitable to discriminate dynamic diatom attachment on different coating chemistries in the lab. The effect of the dynamic conditions was shown by comparing polystyrene slides under static and dynamic exposure. Using a set of self-assembled monolayers, the discrimination potential of the assay in a multispecies environment was demonstrated.

Measuring a critical stress for continuous prevention of marine biofouling accumulation with aeration

When cleaning the hull of a ship, significant shear stresses are needed to remove established biofouling organisms. Given that there exists a link between the amount of time that fouling accumulates and the stress required to remove it, it is not surprising that more frequent grooming requires less shear stress. Yet, it is unclear if there is a minimum stress needed to prevent the growth of macrofouling in the limit of continuous grooming. This manuscript shows that single bubble stream aeration provides continuous grooming and prevents biofouling accumulation in regions where the average wall stress exceeds ~0.01 Pa. This value was found by comparing observations of biofouling growth from field studies with complementary laboratory measurements that probe the associated flow fields. These results suggest that aeration and other continuous grooming systems must exceed a wall stress of 0.01 Pa to prevent macrofouling accumulation.

Microfluidic accumulation assay probes attachment of biofilm forming diatom cells

Testing of fouling release (FR) technologies is of great relevance for discovery of the next generation of protective marine coatings. In this paper, an accumulation assay to test diatom interaction under laminar flow with the model organism Navicula perminuta is introduced. Using time lapse microscopy with large area sampling allows determination of the accumulation kinetics of the diatom on three model surfaces with different surface properties at different wall shear stresses. The hydrodynamic conditions within the flow cell are described and a suitable shear stress range to perform accumulation experiments is identified at which statistically significant discrimination of surfaces is possible. The observed trends compare well to published adhesion preferences of N. perminuta. Also, previously determined trends of critical wall shear stresses required for cell removal from the same set of functionalized interfaces shows consistent trends. Initial attachment mediated by extracellular polymeric substances (EPS) present outside the diatoms leads to the conclusion that the FR potential of the tested coating candidates can be deducted from dynamic accumulation experiments under well-defined hydrodynamic conditions. As well as testing new coating candidates for their FR properties, monitoring of the adhesion process under flow provides additional information on the mechanism and geometry of attachment and the population kinetics.

Experimental and computational analysis of a novel flow channel to assess the adhesion strength of sessile marine organisms

Bioadhesives produced by marine macroalgae represent a potential source of inspiration for the development of water-resistant adhesives. Assessing their adhesion strength, however, remains difficult owing to low volumes of adhesive material produced, low solubility and rapid curing time. These difficulties can be circumvented by testing the adhesion strength of macroalgae propagules attached to a substrate. In this paper, we present a simple, novel flow channel used to test the adhesion strength of the germlings of the fucalean alga Hormosira banksii to four substrates of biomedical relevance (PMMA, agar, gelatin and gelatin + lipid). The adhesion strength of H. banksii germlings was found to increase in a time-dependent manner, with minimal adhesion success after a settlement period of 6 h and maximum adhesion strength achieved 24 h after initial settlement. Adhesion success increased most dramatically between 6 and 12 h settlement time, while no additional increase in adhesion strength was recorded for settlement times over 24 h. No significant difference in adhesion strength to the various substrates was observed. Computational fluid dynamics (CFD) was used to estimate the influence of fluid velocity and germling density on drag force acting on the settled organisms. CFD modelling showed that, on average, the drag force decreased with increasing germling number, suggesting that germlings would benefit from gregarious settlement behaviour. Collectively, our results contribute to a better understanding of the mechanisms allowing benthic marine organisms to thrive in hydrodynamically stressful environments and provide useful insights for further investigations.

Superhydrophobic resistance to dynamic freshwater biofouling inception

Superhydrophobic nanotextured surfaces have gained increased usage in various applications due to their non-wetting and self-cleaning abilities. The aim of this study was to investigate nanotextured surfaces with respect to their resistance to the inception of freshwater biofouling at transitional flow conditions. Several coatings were tested including industry standard polyurethane (PUR), polytetrafluoroethylene (PTFE), capstone mixed polyurethane (PUR + CAP) and nanocomposite infused polyurethane (PUR + NC). Each surface was exposed to freshwater conditions in a lake at 4 m s(-1) for a duration of 45 min. The polyurethane exhibited the greatest fouling elements, in terms of both height and number of elements, with the superhydrophobic nanocomposite based polyurethane (PUR + NC) showing very little to no fouling. A correlation between the surface characteristics and the degree of fouling inception was observed.

Microfluidic detachment assay to probe the adhesion strength of diatoms

Fouling release (FR) coatings are increasingly applied as an environmentally benign alternative for controlling marine biofouling. As the technology relies on removing fouling by water currents created by the motion of ships, weakening of adhesion of adherent organisms is the key design goal for improved coatings. In this paper, a microfluidic shear force assay is used to quantify how easily diatoms can be removed from surfaces. The experimental setup and the optimization of the experimental parameters to study the adhesion of the diatom Navicula perminuta are described. As examples of how varying the physico-chemical surface properties affects the ability of diatoms to bind to surfaces, a range of hydrophilic and hydrophobic self-assembled monolayers was compared. While the number of cells that attached (adhered) was barely affected by the coatings, the critical shear stress required for their removal from the surface varied significantly.

Adhesion of marine fouling organisms on hydrophilic and amphiphilic polysaccharides

Polysaccharides are a promising material for nonfouling surfaces because their chemical composition makes them highly hydrophilic and able to form water-storing hydrogels. Here we investigated the nonfouling properties of hyaluronic acid (HA) and chondroitin sulfate (CS) against marine fouling organisms. Additionally, the free carboxyl groups of HA and CS were postmodified with the hydrophobic trifluoroethylamine (TFEA) to block free carboxyl groups and render the surfaces amphiphilic. All coatings were tested with respect to their protein resistance and against settlement and adhesion of different marine fouling species. Both the settlement and adhesion strength of a marine bacterium (Cobetia marina), zoospores of the seaweed Ulva linza, and cells of a diatom (Navicula incerta) were reduced compared to glass control surfaces. In most cases, TFEA capping increased or maintained the performance of the HA coatings, whereas for the very well performing CS coatings the antifouling performance was reduced after capping.

A novel biofilm channel for evaluating the adhesion of diatoms to non-biocidal coatings

Laboratory assessment of the adhesion of diatoms to non-toxic fouling-release coatings has tended to focus on single cells rather than the more complex state of a biofilm. A novel culture system based on open channel flow with adjustable bed shear stress values (0-2.4 Pa) has been used to produce biofilms of Navicula incerta. Biofilm development on glass and polydimethylsiloxane elastomer (PDMSe) showed a biphasic relationship with bed shear stress, which was characterised by regions of biofilm stability and instability reflecting cohesion between cells relative to the adhesion to the substratum. On glass, a critical shear stress of 1.3-1.4 Pa prevented biofilm development, whereas on PDMS, biofilms continued to grow at 2.4 Pa. Studies of diatom biofilms cultured on zwitterionic coatings using a bed shear stress of 0.54 Pa showed lower biomass production and adhesion strength on poly(sulfobetaine methacrylate) compared to poly(carboxybetaine methacrylate). The dynamic biofilm approach provides additional information to supplement short duration laboratory evaluations.