Biofilm cohesiveness measurement using a novel atomic force microscopy methodology

Biofilm formation of the food spoiler Brochothrix thermosphacta on different industrial surface materials using a biofilm reactor

Brochothrix thermosphacta is considered as a major food spoiler bacteria. This study evaluates biofilm formation by B. thermosphacta CD337(2) - a strong biofilm producer strain - on three food industry materials (polycarbonate (PC), polystyrene (PS), and stainless steel (SS)). Biofilms were continuously grown under flow at 25 °C in BHI broth in a modified CDC biofilm reactor. Bacterial cells were enumerated by plate counting, and biofilm spatial organization was deciphered by combining confocal laser scanning microscopy and image analysis. The biofilms had the same growth kinetics on all three materials and reach 8log CFU/cm2 as maximal concentration. Highly structured biofilms were observed on PC and PS, but less structured ones on SS. This difference was confirmed by structural quantification analysis using the image analysis software tool BiofilmQ. Biofilm on SS show less roughness, density, thickness and volume. The biofilm 3D structure seemed to be related to the coupon topography and roughness. The materials used in this study do not affect biofilm growth. However, their roughness and topography affect the biofilm architecture, which could influence biofilm behaviour.

How to study biofilms: technological advancements in clinical biofilm research

Biofilm formation is an important survival strategy commonly used by bacteria and fungi, which are embedded in a protective extracellular matrix of organic polymers. They are ubiquitous in nature, including humans and other animals, and they can be surface- and non-surface-associated, making them capable of growing in and on many different parts of the body. Biofilms are also complex, forming polymicrobial communities that are difficult to eradicate due to their unique growth dynamics, and clinical infections associated with biofilms are a huge burden in the healthcare setting, as they are often difficult to diagnose and to treat. Our understanding of biofilm formation and development is a fast-paced and important research focus. This review aims to describe the advancements in clinical biofilm research, including both in vitro and in vivo biofilm models, imaging techniques and techniques to analyse the biological functions of the biofilm.

Challenges and current advances in in vitro biofilm characterization

Biofilms are structured communities of bacterial cells encased in a self-produced polymeric matrix, which develop over time and exhibit temporal responses to stimuli from internal biological processes or external environmental changes. They can be detrimental, threatening public health and causing economic loss, while they also play beneficial roles in ecosystem health, biotechnology processes, and industrial settings. Biofilms express extreme heterogeneity in their physical properties and structural composition, resulting in critical challenges in understanding them comprehensively. The lack of detailed knowledge of biofilms and their phenotypes has deterred significant progress in developing strategies to control their negative impacts and take advantage of their beneficial applications. A range of in vitro models and characterization tools have been developed and used to study biofilm growth and, specifically, to investigate the impact of environmental and growth factors on their development. This review article discusses the existing knowledge of biofilm properties and explains how external factors, such as flow condition, surface, interface, and host factor, may impact biofilm growth. The limitations of current tools, techniques, and in vitro models that are currently used for biofilms are also presented.

Factors Influencing Pseudomonas aeruginosa Initial Adhesion and Evolution at the Dodecane-Water Interface

Bacterial adhesion and evolution at the oil-water interface are important for a broad range of applications such as food manufacturing and microbial-enhanced oil recovery, etc. However, our understanding on bacterial interfacial adhesion and evolution, particularly at the single-cell level, is still far from complete. In this work, by employing Pseudomonas aeruginosa PAO1 at the dodecane-water interface as a model system, we have studied the effects of different factors on bacterial interfacial adhesion and the dynamic evolution of bacterial interfacial behavior at the single-cell level. The results show that PAO1 cells displayed a chemotactic behavior toward dodecane. Among the tested factors, bacterial initial interfacial attachment showed a negative correlation with the secreted cell-surface associated lipopolysaccharide and Psl while a positive correlation with type IV pili. Adding nonbiological surfactant Pluronic F-127, as expected, greatly reduced the cell interfacial adhesion. More importantly, the dynamics analysis of cell attachment/detachment at the dodecane-water interface over a long-time scale revealed a reversible to irreversible attachment transition of cells. This transition is accompanied with the interface aging resulting from bacterial activities, which led to an increase of the interfacial viscoelasticity with time and finally the formation of the gel-like interface. Further analysis demonstrated the important role of exopolysaccharides in the latter process. Our findings provide more details of bacterial oil-water interfacial behavior at the single-cell level and may shed light on developing new strategies for controlling bacterial colonization at the oil-water interface.

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

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

Mycobacterial biofilms as players in human infections: a review

The role of biofilms in pathogenicity and treatment strategies is often neglected in mycobacterial infections. In recent years, the emergence of nontuberculous mycobacterial infections has necessitated the development of novel prophylactic strategies and elucidation of the mechanisms underlying the establishment of chronic infections. More importantly, the question arises whether members of the Mycobacterium tuberculosis complex can form biofilms and contribute to latent tuberculosis and drug resistance because of the long-lasting and recalcitrant nature of its infections. This review discusses some of the molecular mechanisms by which biofilms could play a role in infection or pathological events in humans.

Spatial distribution of mechanical properties in Pseudomonas aeruginosa biofilms, and their potential impacts on biofilm deformation

The mechanical properties of biofilms can be used to predict biofilm deformation under external forces, for example, under fluid flow. We used magnetic tweezers to spatially map the compliance of Pseudomonas aeruginosa biofilms at the microscale, then applied modeling to assess its effects on biofilm deformation. Biofilms were grown in capillary flow cells with Reynolds numbers (Re) ranging from 0.28 to 13.9, bulk dissolved oxygen (DO) concentrations from 1 mg/L to 8 mg/L, and bulk calcium ion (Ca²⁺ ) concentrations of 0 and 100 mg CaCl₂ /L. Higher Re numbers resulted in more uniform biofilm morphologies. The biofilm was stiffer at the center of the flow cell than near the walls. Lower bulk DO led to more stratified biofilms. Higher Ca²⁺ concentrations led to increased stiffness and more uniform mechanical properties. Using the experimental mechanical properties, fluid-structure interaction models predicted up to 64% greater deformation for heterogeneous biofilms, compared with a homogeneous biofilms with the same average properties. However, the deviation depended on the biofilm morphology and flow regime. Our results show significant spatial mechanical variability exists at the microscale, and that this variability can potentially affect biofilm deformation. The average biofilm mechanical properties, provided in many studies, should be used with caution when predicting biofilm deformation.

CFD-DEM modelling of biofilm streamer oscillations and their cohesive failure in fluid flow

Biofilm streamer motion under different flow conditions is important for a wide range of industries. The existing work has largely focused on experimental characterisations of these streamers, which is often time-consuming and expensive. To better understand the physics of biofilm streamer oscillation and their interactions in fluid flow, a computational fluid dynamics-discrete element method model has been developed. The model was used to study the flow-induced oscillations and cohesive failure of single and multiple biofilm streamers. We have studied the effect of streamer length on the oscillation at varied flow rates. The predicted single biofilm streamer oscillations in various flow rates agreed well with experimental measurements. We have also investigated the effect of the spatial arrangement of streamers on interactions between two oscillating streamers in parallel and tandem arrangements. Furthermore, cohesive failure of streamers was studied in an accelerating fluid flow, which is important for slowing down biofilm-induced clogging.

Responses of Acinetobacter baumannii Bound and Loose Extracellular Polymeric Substances to Hyperosmotic Agents Combined with or without Tobramycin: An Atomic Force Microscopy Study

In this work, contributions of extracellular polymeric substances (EPS) to the nanoscale mechanisms through which the multidrug-resistant Acinetobacter baumannii responds to antimicrobial and hyperosmotic treatments were investigated by atomic force microscopy. Specifically, the adhesion strengths to a control surface of silicon nitride (Si₃N₄) and the lengths of bacterial surface biopolymers of bound and loose EPS extracted from A. baumannii biofilms were quantified after individual or synergistic treatments with hyperosmotic agents (NaCl and maltodextrin) and an antibiotic (tobramycin). In the absence of any treatment, the loose EPS were significantly longer in length and higher in adhesion to Si₃N₄ than the bound EPS. When used individually, the hyperosmotic agents and tobramycin collapsed the A. baumannii bound and loose EPS. The combined treatment of maltodextrin with tobramycin collapsed only the loose EPS and did not alter the adhesion of both bound and loose EPS to Si₃N₄. In addition, the combined treatment was not as effective in collapsing the EPS molecules as when tobramycin was applied alone. Finally, the effects of treatments were dose-dependent. Altogether, our findings suggest that a sequential treatment could be effective in treating A. baumannii biofilms, in which a hyperosmotic agent is used first to collapse the EPS and limit the diffusion of nutrients into the biofilm, followed by the use of an antibiotic to kill the bacterial cells that escape from the biofilm because of starvation.

Viscoelastic response of Escherichia coli biofilms to genetically altered expression of extracellular matrix components

How the viscoelastic properties of the extracellular matrix affect the various biological functions conferred by biofilms is an important question in microbiology. In this study, the viscoelastic response of Escherichia coli biofilms to the genetically altered expression of extracellular matrix components was studied. Biofilms of the wild type E. coli MG1655 and its mutant strains producing different amounts of extracellular matrix components (curli, colanic acid, and poly-β-1,6-N-acetyl-d-glucosamine) were used to examine the viscoelastic behavior of biofilms grown at the solid-atmosphere interface. The results suggest that the presence of curli proteins dominates biofilm mechanical behavior. The rheological data indicate that the cohesive energy of the biofilm was the highest in the wild type strain. The results demonstrate the importance of extracellular matrix composition for biofilm mechanical properties. We propose that by genetically altering the expression of extracellular matrix polymers, bacteria are able to modulate the mechanical properties of their local environment in accordance with bulk environmental conditions.

Acquisition of time-frequency localized mechanical properties of biofilms and single cells with high spatial resolution

Biofilms are a cluster of bacteria embedded in extracellular polymeric substances (EPS) that contain a complex composition of polysaccharides, proteins, and extracellular DNA (eDNA). Desirable mechanical properties of the biofilms are critical for their survival, propagation, and dispersal, and the response of mechanical properties to different treatment conditions also sheds light on biofilm control and eradication in vivo and on engineering surfaces. However, it is challenging yet important to investigate mechanical behaviors of biofilms with a high spatial resolution because biofilms are very heterogeneous. Moreover, biofilms are viscoelastic, and their time-dependent mechanical behavior is difficult to capture. Herein, we develop a powerful technique that combines the high spatial resolution of an atomic force microscope (AFM) with a rigorous history-dependent viscoelastic analysis to deliver highly spatial-localized biofilm properties within a wide time-frequency window. By exploiting the use of static force spectroscopy in combination with an appropriate viscoelastic framework, we highlight the intensive amount of time-dependent information experimentally available that has been largely overlooked. It is shown that this technique provides a detailed nanorheological signature of the biofilms even at the single-cell level. We share the computational routines that would allow any user to perform the analysis from experimental raw data. The detailed localization of mechanical properties in space and in time-frequency domain provides insights into the understanding of biofilm stability, cohesiveness, dispersal, and control.

Effects of the colonization sequence of Listeria monocytogenes and Pseudomonas fluorescens on survival of biofilm cells under food-related stresses and transfer to salmon

This study evaluated how the colonization sequence of Listeria monocytogenes and Pseudomonas fluorescens affects biofilm formation and biofilm cell response to food-related stress (desiccation or disinfection) as well as the transferability of L. monocytogenes to salmon products. The results showed that the colonization sequence did not affect the population of dual species biofilms. Furthermore, survival number of L. monocytogenes was 0.8 log CFU/cm² higher when P. fluorescens was the first colonizer during desiccation or disinfectant treatment in comparison with dual-species biofilms with other colonization sequences. A lower transfer rate of L. monocytogenes biofilm cells from dual-species biofilms was observed as compared to single species biofilms. In particular, L. monocytogenes cells detached at a slower rate during transfer to 10 slices of salmon from dual-species biofilms first established by P. fluorescens. Confocal images revealed more exopolysaccharide production in dual-speciesbiofilms first established by P. fluorescens than in biofilms generated via other sequences. These results indicate that preexisting P. fluorescens biofilms on stainless steel can enhance resistance of L. monocytogenes to desiccation and disinfection, although this setup decreased the transfer rate of L. monocytogenes to salmon slices. Thus, this study highlights the risk of L. monocytogenes contamination in pre-formed Pseudomonas biofilms at salmon processing facilities.

Towards standardized mechanical characterization of microbial biofilms: analysis and critical review

Developing reliable anti-biofilm strategies or efficient biofilm-based bioprocesses strongly depends on having a clear understanding of the mechanisms underlying biofilm development, and knowledge of the relevant mechanical parameters describing microbial biofilm behavior. Many varied mechanical testing methods are available to assess these parameters. The mechanical properties thus identified can then be used to compare protocols such as antibiotic screening. However, the lack of standardization in both mechanical testing and the associated identification methods for a given microbiological goal remains a blind spot in the biofilm community. The pursuit of standardization is problematic, as biofilms are living structures, i.e., both complex and dynamic. Here, we review the main available methods for characterizing the mechanical properties of biofilms through the lens of the relationship linking experimental testing to the identification of mechanical parameters. We propose guidelines for characterizing biofilms according to microbiological objectives that will help the reader choose an appropriate test and a relevant identification method for measuring any given mechanical parameter. The use of a common methodology for the mechanical characterization of biofilms will enable reliable analysis and comparison of microbiological protocols needed for improvement of engineering process and screening.

Effect of divalent ions and a polyphosphate on composition, structure, and stiffness of simulated drinking water biofilms

The biofilm chemical and physical properties in engineered systems play an important role in governing pathogen transmission, fouling facilities, and corroding metal surfaces. Here, we investigated how simulated drinking water biofilm chemical composition, structure, and stiffness responded to the common scale control practice of adjusting divalent ions and adding polyphosphate. Magnetomotive optical coherence elastography (MM-OCE), a tool developed for diagnosing diseased tissues, was used to determine biofilm stiffness in this study. MM-OCE, together with atomic force microscopy (AFM), revealed that the biofilms developed from a drinking water source with high divalent ions were stiffer compared to biofilms developed either from the drinking water source with low divalent ions or the water containing a scale inhibitor (a polyphosphate). The higher stiffness of biofilms developed from the water containing high divalent ions was attributed to the high content of calcium carbonate, suggested by biofilm composition examination. In addition, by examining the biofilm structure using optical coherence tomography (OCT), the highest biofilm thickness was found for biofilms developed from the water containing the polyphosphate. Compared to the stiff biofilms developed from the water containing high divalent ions, the soft and thick biofilms developed from the water containing polyphosphate will be expected to have higher detachment under drinking water flow. This study suggested that water chemistry could be used to predict the biofilm properties and subsequently design the microbial safety control strategies.

A variety of analytical techniques are revealing the complex influences of ions in drinking water supplies on the structure of biofilms. Such biofilms often contaminate water supply pipes and machinery. Yun Shen and colleagues at the University of Illinois at Urbana-Champaign in the USA investigated the effects of ions with a double positive charge – ‘divalent cations’ – and polyphosphate ions. Divalent cations, especially calcium and magnesium ions, are abundant in drinking water in many regions, promoting the formation of limescale deposits. Polyphosphates are commonly added to water supplies to reduce limescale formation, inhibit corrosion and discourage biofilm formation. The research revealed that divalent cations increase biofilm stiffness, while polyphosphates promote softer but thicker biofilms that are more easily removed. The results will help optimize water treatment procedures to control both microbial contamination and limescale problems.

Multiscale Frictional Properties of Cotton Fibers: A Review

This review discusses the important concept of cotton fiber friction at both the macro- and nanoscale. First, the technological importance of fiber friction and its role in fiber breakage during fiber processing is discussed. Next, previous studies on frictional properties of cotton fibers are reviewed and different experimental procedures to measure friction between fibers or against another surface are evaluated. Friction models developed to explain friction process during various experimental procedures are considered and their limitations are discussed. Since interpretation of friction processes at the macroscale can be challenging (mainly due to difficulties in analyzing the multiple asperities in contact), a separate section is devoted to surveying studies on the emerging field of single-asperity friction experiments with atomic force microscope (AFM). Special attention is given to studies on nanoscale frictional characteristics of rough viscoelastic surfaces (e.g., plant cuticular biopolymers and cotton fibers). Due to the close relationship between friction and adhesion hysteresis at the nanoscale, adhesion studies with AFM on viscoelastic surfaces are also reviewed. Lastly, recommendations are made for future research in the field of frictional properties of cotton fibers.

Study of the Relation between the Resonance Behavior of Thickness Shear Mode (TSM) Sensors and the Mechanical Characteristics of Biofilms

This work analyzes some key aspects of the behavior of sensors based on piezoelectric Thickness Shear Mode (TSM) resonators to study and monitor microbial biofilms. The operation of these sensors is based on the analysis of their resonance properties (both resonance frequency and dissipation factor) that vary in contact with the analyzed sample. This work shows that different variations during the microorganism growth can be detected by the sensors and highlights which of these changes are indicative of biofilm formation. TSM sensors have been used to monitor in real time the development of Staphylococcus epidermidis and Escherichia coli biofilms, formed on the gold electrode of the quartz crystal resonators, without any coating. Strains with different ability to produce biofilm have been tested. It was shown that, once a first homogeneous adhesion of bacteria was produced on the substrate, the biofilm can be considered as a semi-infinite layer and the quartz sensor reflects only the viscoelastic properties of the region immediately adjacent to the resonator, not being sensitive to upper layers of the biofilm. The experiments allow the microrheological evaluation of the complex shear modulus (G* = G′ + jG″) of the biofilm at 5 MHz and at 15 MHz, showing that the characteristic parameter that indicates the adhesion of a biofilm for the case of S. epidermidis and E. coli, is an increase in the resonance frequency shift of the quartz crystal sensor, which is connected with an increase of the real shear modulus, related to the elasticity or stiffness of the layer. In addition both the real and the imaginary shear modulus are frequency dependent at these high frequencies in biofilms.

Biofilm over teeth and restorations: What do we need to know?

OBJECTIVE

The goal of this manuscript is to provide an overview of biofilm attributes and measurement approaches in the context of studying biofilms on tooth and dental material surfaces to improve oral health.

METHODS

A historical perspective and terminology are presented, followed by a general description of the complexity of oral biofilms. Then, an approach to grouping measurable biofilm properties is presented and considered in relation to biofilm-material interactions and material design strategies to alter biofilms. Finally, the need for measurement assurance in biofilm and biofilm-materials research is discussed.

RESULTS

Biofilms are highly heterogeneous communities that are challenging to quantify. Their characteristics can be broadly categorized into constituents (identity), quantity, structure, and function. These attributes can be measured over time and in response to substrates and external stimuli. Selecting the biofilm attribute(s) of interest and appropriate measurement methods will depend on the application and, in the case of antimicrobial therapies, the strategic approach and expected mechanism of action. To provide measurement assurance, community accepted protocols and guidelines for minimum data and metadata should be established and broadly applied. Consensus standards may help to streamline testing and demonstration of product claims.

SIGNIFICANCE

Understanding oral biofilms and their interactions with tooth and dental material surfaces holds great promise for enabling improvements in oral and overall human health. Both substrate and biofilm properties should be considered to develop a more thorough understanding of the system.

Modulation of the mechanical properties of bacterial biofilms in response to environmental challenges

In this review, we highlight recent research on the relationship between biofilm matrix composition, biofilm mechanics and environmental stimuli.

Microrheology of growing Escherichia coli biofilms investigated by using magnetic force modulation atomic force microscopy

Microrheology of growing biofilms provides insightful information about its structural evolution and properties. In this study, the authors have investigated the microrheology of Escherichia coli (strain HCB1) biofilms at different indentation depth (δ) by using magnetic force modulation atomic force microscopy as a function of disturbing frequency (f). As δ increases, the dynamic stiffness (k_s) for the biofilms in the early stage significantly increases. However, it levels off when the biofilms are matured. The facts indicate that the biofilms change from inhomogeneous to homogeneous in structure. Moreover, k_s is scaled to f, which coincides with the rheology of soft glasses. The exponent increases with the incubation time, indicating the fluidization of biofilms. In contrast, the upper layer of the matured biofilms is solidlike in that the storage modulus is always larger than the loss modulus, and its viscoelasticity is slightly influenced by the shear stress.

Critical review on biofilm methods

Biofilms are widespread in nature and constitute an important strategy implemented by microorganisms to survive in sometimes harsh environmental conditions. They can be beneficial or have a negative impact particularly when formed in industrial settings or on medical devices. As such, research into the formation and elimination of biofilms is important for many disciplines. Several new methodologies have been recently developed for, or adapted to, biofilm studies that have contributed to deeper knowledge on biofilm physiology, structure and composition. In this review, traditional and cutting-edge methods to study biofilm biomass, viability, structure, composition and physiology are addressed. Moreover, as there is a lack of consensus among the diversity of techniques used to grow and study biofilms. This review intends to remedy this, by giving a critical perspective, highlighting the advantages and limitations of several methods. Accordingly, this review aims at helping scientists in finding the most appropriate and up-to-date methods to study their biofilms.

Graft polymerization of styryl bisphosphonate monomer onto polypropylene films for inhibition of biofilm formation

There has been increased concern during the past few decades over the role bacterial biofilms play in causing a variety of health problems, especially since they exhibit a high degree of resistance to antibiotics and are able to survive in hostile environments. Biofilms consist of bacterial aggregates enveloped by a self-produced matrix attached to the surface. Ca(2+) ions promote the formation of biofilms, and enhance their stability, viscosity, and strength. Bisphosphonates exhibit a high affinity for Ca(2+) ions, and may inhibit the formation of biofilms by acting as sequestering agents for Ca(2+) ions. Although the antibacterial activity of bisphosphonates is well known, research into their anti-biofilm behavior is still in its early stages. In this study, we describe the synthesis of a new thin coating composed of poly(styryl bisphosphonate) grafted onto oxidized polypropylene films for anti-biofilm applications. This grafting process was performed by graft polymerization of styryl bisphosphonate vinylic monomer onto O2 plasma-treated polypropylene films. The surface modification of the polypropylene films was confirmed using surface measurements, including X-ray photoelectron spectroscopy, atomic force microscopy, and water contact angle goniometry. Significant inhibition of biofilm formation was achieved for both Gram-negative and Gram-positive bacteria.

Mechanical properties of the superficial biofilm layer determine the architecture of biofilms

Cells in biofilms sense and interact with their environment through the extracellular matrix. The physicochemical properties of the matrix, particularly at the biofilm-environment interface, determine how cells respond to changing conditions. In this study we describe the application of atomic force microscopy and confocal imaging to probe in situ the mechanical properties of these interfacial regions and to elucidate how key matrix components can contribute to the physical sensing by the cells. We describe how the Young's modulus of microcolonies differs according to the size and morphology of microcolonies, as well as the flow rate. The Young's modulus increased as a function of microcolony diameter, which was correlated with the production of the polysaccharide Psl at later stages of maturation for hemispherical or mushroom shaped microcolonies. The Young's modulus of the periphery of the biofilm colony was however independent of the hydrodynamic shear. The morphology of the microcolonies also influenced interfacial or peripheral stiffness. Microcolonies with a diffuse morphology had a lower Young's modulus than isolated, circular ones and this phenomenon was due to a deficiency of Psl. In this way, changes in the specific polysaccharide components imbue the biofilm with distinct physical properties that may modulate the way in which bacteria perceive or respond to their environment. Further, the physical properties of the polysaccharides are closely linked to the specific architectures formed by the developing biofilm.

Response of Simulated Drinking Water Biofilm Mechanical and Structural Properties to Long-Term Disinfectant Exposure

Mechanical and structural properties of biofilms influence the accumulation and release of pathogens in drinking water distribution systems (DWDS). Thus, understanding how long-term residual disinfectants exposure affects biofilm mechanical and structural properties is a necessary aspect for pathogen risk assessment and control. In this study, elastic modulus and structure of groundwater biofilms was monitored by atomic force microscopy (AFM) and optical coherence tomography (OCT) during three months of exposure to monochloramine or free chlorine. After the first month of disinfectant exposure, the mean stiffness of monochloramine- or free-chlorine-treated biofilms was 4 to 9 times higher than those before treatment. Meanwhile, the biofilm thickness decreased from 120 ± 8 μm to 93 ± 6-107 ± 11 μm. The increased surface stiffness and decreased biofilm thickness within the first month of disinfectant exposure was presumably due to the consumption of biomass. However, by the second to third month during disinfectant exposure, the biofilm mean stiffness showed a 2- to 4-fold decrease, and the biofilm thickness increased to 110 ± 7-129 ± 8 μm, suggesting that the biofilms adapted to disinfectant exposure. After three months of the disinfectant exposure process, the disinfected biofilms showed 2-5 times higher mean stiffness (as determined by AFM) and 6-13-fold higher ratios of protein over polysaccharide, as determined by differential staining and confocal laser scanning microscopy (CLSM), than the nondisinfected groundwater biofilms. However, the disinfected biofilms and nondisinfected biofilms showed statistically similar thicknesses (t test, p > 0.05), suggesting that long-term disinfection may not significantly remove net biomass. This study showed how biofilm mechanical and structural properties vary in response to a complex DWDS environment, which will contribute to further research on the risk assessment and control of biofilm-associated-pathogens in DWDS.

AFM structural characterization of drinking water biofilm under physiological conditions

Insights into the complex morphology of multi-species drinking water biofilm using atomic force microscopy (AFM).

Urinary catheter capable of repeated on-demand removal of infectious biofilms via active deformation

Biofilm removal from biomaterials is of fundamental importance, and is especially relevant when considering the problematic and deleterious impact of biofilm infections on the inner surfaces of urinary catheters. Catheter-associated urinary tract infections are the most common cause of hospital-acquired infections and there are over 30 million Foley urinary catheters used annually in the USA. In this paper, we present the design and optimization of urinary catheter prototypes capable of on-demand removal of biofilms from the inner luminal surface of catheters. The urinary catheters utilize 4 intra-wall inflation lumens that are pressure-actuated to generate region-selective strains in the elastomeric urine lumen, and thereby remove overlying biofilms. A combination of finite-element modeling and prototype fabrication was used to optimize the catheter design to generate greater than 30% strain in the majority of the luminal surface when subjected to pressure. The catheter prototypes are able to remove greater than 80% of a mixed community biofilm of Proteus mirabilis and Escherichia coli on-demand, and furthermore are able to remove the biofilm repeatedly. Additionally, experiments with the prototypes demonstrate that biofilm debonding can be achieved upon application of both tensile and compressive strains in the inner surface of the catheter. The fouling-release catheter offers the potential for a non-biologic, non-antibiotic method to remove biofilms and thereby for impacting the thus far intractable problem of catheter-associated infections.

Interfacial separation of a mature biofilm from a glass surface - A combined experimental and cohesive zone modelling approach

A good understanding of the mechanical stability of biofilms is essential for biofouling management, particularly when mechanical forces are used. Previous biofilm studies lack a damage-based theoretical model to describe the biofilm separation from a surface. The purpose of the current study was to investigate the interfacial separation of a mature biofilm from a rigid glass substrate using a combined experimental and numerical modelling approach. In the current work, the biofilm-glass interfacial separation process was investigated under tensile and shear stresses at the macroscale level, known as modes I and II failure mechanisms respectively. The numerical simulations were performed using a Finite Volume (FV)-based simulation package (OpenFOAM®) to predict the separation initiation using the cohesive zone model (CZM). Atomic force microscopy (AFM)-based retraction curve was used to obtain the separation properties between the biofilm and glass colloid at microscale level, where the CZM parameters were estimated using the Johnson-Kendall-Roberts (JKR) model. In this study CZM is introduced as a reliable method for the investigation of interfacial separation between a biofilm and rigid substrate, in which a high local stress at the interface edge acts as an ultimate stress at the crack tip.This study demonstrated that the total interfacial failure energy measured at the macroscale, was significantly higher than the pure interfacial separation energy obtained by AFM at the microscale, indicating a highly ductile deformation behaviour within the bulk biofilm matrix. The results of this study can significantly contribute to the understanding of biofilm detachments.

Material properties of biofilms-a review of methods for understanding permeability and mechanics

Microorganisms can form biofilms, which are multicellular communities surrounded by a hydrated extracellular matrix of polymers. Central properties of the biofilm are governed by this extracellular matrix, which provides mechanical stability to the 3D biofilm structure, regulates the ability of the biofilm to adhere to surfaces, and determines the ability of the biofilm to adsorb gases, solutes, and foreign cells. Despite their critical relevance for understanding and eliminating of biofilms, the materials properties of the extracellular matrix are understudied. Here, we offer the reader a guide to current technologies that can be utilized to specifically assess the permeability and mechanical properties of the biofilm matrix and its interacting components. In particular, we highlight technological advances in instrumentation and interactions between multiple disciplines that have broadened the spectrum of methods available to conduct these studies. We review pioneering work that furthers our understanding of the material properties of biofilms.

Mechanical properties of a mature biofilm from a wastewater system: from microscale to macroscale level

A fundamental understanding of biofilm mechanical stability is critical in order to describe detachment and develop biofouling control strategies. It is thus important to characterise the elastic deformation and flow behaviour of the biofilm under different modes of applied force. In this study, the mechanical properties of a mature wastewater biofilm were investigated with methods including macroscale compression and microscale indentation using atomic force microscopy (AFM). The mature biofilm was found to be mechanically isotropic at the macroscale level as its mechanical properties did not depend on the scales and modes of loading. However, the biofilm showed a tendency for mechanical inhomogeneity at the microscale level as indentation progressed deeper into the matrix. Moreover, it was observed that the adhesion force had a significant influence on the elastic properties of the biofilm at the surface, subjected to microscale tensile loading. These results are expected to inform a damage-based model for biofilm detachment.

Mitigation of nitrous oxide (N2O) emissions from denitrifying fluidized bed bioreactors (DFBBRs) using calcium

Nitrous oxide (N2O) is a significant anthropogenic greenhouse gases (AnGHGs) emitted from biological nutrient removal (BNR) processes. In this study, N2O production from denitrifying fluidized bed bioreactors (DFBBR) was reduced using calcium (Ca2+) dosage. The DFBBRs were operated on a synthetic municipal wastewater at four different calcium concentrations ranging from the typical municipal wastewater Ca2+ concentration (60 mg Ca2+/L) to 240 mg Ca2+/L at two different COD/N ratios. N2O emission rates, extracellular polymeric substances (EPS), water quality parameters, and microscopic images were monitored regularly in both phases. Calcium concentrations played a significant role in biofilm morphology with the detachment rates for R120Ca, R180Ca, and R240Ca 75% lower than for R60Ca, respectively. The N2O conversion rate at the typical municipal wastewater Ca2+ concentration (R60Ca) was about 0.53% of the influent nitrogen loading as compared with 0.34%, 0.42%, and 0.41% for R120Ca, R180Ca, and R240Ca, respectively corresponding to 21-36% reduction.

Single particle tracking reveals spatial and dynamic organization of the E. coli biofilm matrix

Biofilms are communities of surface-adherent bacteria surrounded by secreted polymers known as the extracellular polymeric substance (EPS). Biofilms are harmful in many industries, and thus it is of great interest to understand their mechanical properties and structure to determine ways to destabilize them. By performing single particle tracking with beads of varying surface functionalization it was found that charge interactions play a key role in mediating mobility within biofilms. With a combination of single particle tracking and microrheological concepts, it was found that Escherichia coli biofilms display height dependent charge density that evolves over time. Statistical analyses of bead trajectories and confocal microscopy showed inter-connecting micron scale channels that penetrate throughout the biofilm, which may be important for nutrient transfer through the system. This methodology provides significant insight into a particular biofilm system and can be applied to many others to provide comparisons of biofilm structure. The elucidation of structure provides evidence for the permeability of biofilms to microscale objects, and the ability of a biofilm to mature and change properties over time.

Atomic force microscopy in biofilm study

Biofilms have been classically visualized by Scanning Electron Microscopy (SEM). The complex operating procedure of SEM restricts its use in routine practice. There is a need of newer visualizing techniques for examining surfaces of biofilms, in particular under ambient conditions. We have presented the unique advantages of atomic force microscopy (AFM) in studying surfaces of biofilms through analyses of the height images obtained on biofilms of two gram positive and one gram negative bacteria, namely Staphylococcus aureus, Nocardia brasiliensis and Pseudomonas aeruginosa, respectively. Biofilm quality of the three different bacteria, ageing effects on Nocardia spp. biofilm surface and effects of the antibiotic ciprofloxacin at different doses on Staphylococcus and Pseudomonas biofilm surfaces have been investigated under ambient conditions and distinctive features have been observed.

Application of bioflocculating property of Pseudomonas aeruginosa strain IASST201 in treatment of oil-field formation water

A bioflocculating activity of 89.8% was depicted by an activated sludge-borne bacteria Pseudomonas aeruginosa strain IASST201 with a yield of bioflocculant of 2.68 g L(-1) obtained from production media broth after optimization of different parameters. The highest bioflocculation efficiency was found at the pre-stationary phase of the bacterial growth period in the production media broth at 96th hour examined from a growth-flocculation kinetics study. 85.67% of bioflocculation was observed in oil-field formation water, with a separation of 68.7% of aliphatic hydrocarbon contents of the formation water after the application of the bacterial bioflocculant by entrapment mechanism with formation of flocs which was analyzed and examined comparatively through gas-chromatography. Extensive removal of heavy metal contents of the oil-field formation water due to bioflocculation was estimated by Atomic Absorption Spectrophotometer (AAS). The SEM and AFM studies declare the extracellular polymeric nature of the bioflocculant produced by this bacterium clumped within bacterial biofilm supported with FTIR study of the extracted bioflocculant.

The significance of calcium ions on Pseudomonas fluorescens biofilms - a structural and mechanical study

The purpose of this study was to investigate the effects of calcium ions on the structural and mechanical properties of Pseudomonas fluorescens biofilms grown for 48 h. Advanced investigative techniques such as confocal laser scanning microscopy and atomic force spectroscopy were employed to characterize biofilm structure as well as biofilm mechanical properties following growth at different calcium concentrations. The presence of calcium during biofilm development led to higher surface coverage with distinct structural phenotypes in the form of a granular and heterogeneous surface, compared with the smoother and homogeneous biofilm surface in the absence of calcium. The presence of calcium also increased the adhesive nature of the biofilm, while reducing its elastic properties. These results suggest that calcium ions could have a functional role in biofilm development and have practical implications, for example, in analysis of biofouling in membrane-based water-treatment processes such as nanofiltration or reverse osmosis where elevated calcium concentrations may occur at the solid-liquid interface.

In-situ quantification of the interfacial rheological response of bacterial biofilms to environmental stimuli

Understanding the numerous factors that can affect biofilm formation and stability remain poorly understood. One of the major limitations is the accurate measurement of biofilm stability and cohesiveness in real-time when exposed to changing environmental conditions. Here we present a novel method to measure biofilm strength: interfacial rheology. By culturing a range of bacterial biofilms on an air-liquid interface we were able to measure their viscoelastic growth profile during and after biofilm formation and subsequently alter growth conditions by adding surfactants or changing the nutrient composition of the growth medium. We found that different bacterial species had unique viscoelastic growth profiles, which was also highly dependent on the growth media used. We also found that we could reduce biofilm formation by the addition of surfactants or changing the pH, thereby altering the viscoelastic properties of the biofilm. Using this technique we were able to monitor changes in viscosity, elasticity and surface tension online, under constant and varying environmental conditions, thereby providing a complementary method to better understand the dynamics of both biofilm formation and dispersal.

Potential of calcium to scaffold an endodontic biofilm, thus protecting the micro-organisms from disinfection

Biofilms in the root canal of a tooth (endodontic biofilm) can induce and sustain apical periodontitis which is an oral inflammatory disease. Still, little is known about the composition of the endodontic biofilm. Studies on biofilms in root canals focus on the identification of the microbial species, but the majority of the biofilm consists of matrix material. Environmental aspects determine the structure of the biofilm and extracellular matrix. Calcium is involved in biofilm formation and activity at three levels. Firstly in cell-environment; calcium may 'condition' the surfaces of support and bacterial cells. Secondly, in cell-cell interaction; calcium plays a role in build up of biofilm structures. Typically, calcium ions act as 'cation bridges' between polysaccharides originating from different cells. Thirdly, within cells, calcium is required for certain biochemical reactions in bacteria and some bacterial physiological activities. Because calcium is present in the root canal, it could play a significant role in the organization of the biofilm. Chelators, already used in endodontics to remove the smear layer by disintegration of the structural cohesion calcium bonds, could weaken the biofilm matrix by removing calcium from the extracellular matrix thus disturbing its coherence. Subsequently, this disruption could increase the efficacy of disinfecting agents.

Cohesiveness and hydrodynamic properties of young drinking water biofilms

Drinking water biofilms are complex microbial systems mainly composed of clusters of different size and age. Atomic force microscopy (AFM) measurements were performed on 4, 8 and 12 weeks old biofilms in order to quantify the mechanical detachment shear stress of the clusters, to estimate the biofilm entanglement rate ξ. This AFM approach showed that the removal of the clusters occurred generally for mechanical shear stress of about 100 kPa only for clusters volumes greater than 200 μm3. This value appears 1000 times higher than hydrodynamic shear stress technically available meaning that the cleaning of pipe surfaces by water flushing remains always incomplete. To predict hydrodynamic detachment of biofilm clusters, a theoretical model has been developed regarding the averaging of elastic and viscous stresses in the cluster and by including the entanglement rate ξ. The results highlighted a slight increase of the detachment shear stress with age and also the dependence between the posting of clusters and their volume. Indeed, the experimental values of ξ allow predicting biofilm hydrodynamic detachment with same order of magnitude than was what reported in the literature. The apparent discrepancy between the mechanical and the hydrodynamic detachment is mainly due to the fact that AFM mechanical experiments are related to the clusters local properties whereas hydrodynamic measurements reflected the global properties of the whole biofilm.

Effect of strain rate on the mechanical properties of Staphylococcus epidermidis biofilms

The quantification of biofilm mechanical properties can serve as a basis for understanding biofilm resilience and for developing biofilm control strategies. One aspect of tensile testing that is likely to be important for a viscoelastic material such as bacterial biofilm, but unfortunately is often overlooked (i.e., not controlled or reported), is the strain rate used during testing. Thus, we performed tensile testing on intact S. epidermidis biofilms using the microcantilever method at 12 strain rate values ranging over approximately 3 orders of magnitude (0.013-9.07 s(-1)). Ultimate or cohesive strength, elastic modulus, and toughness increased with increasing strain rate and approached a plateau at approximately 1.3 s(-1). Failure strain, on the other hand, did not exhibit any trend with strain rate. Given that the mean values of some parameters increased by as much as 1 order of magnitude over the strain rate range used in this work, we suggest that the strain rate during tensile testing should be carefully controlled and reported to facilitate comparisons among different studies. Furthermore, the quantitative expressions developed in this work that relate mechanical property values with strain rate may be useful for modeling the deformation of bacterial biofilms under applied loads.

Quantifying the tensile strength of microbial mats grown over noncohesive sediments

Biofilms in marine and fluvial environments can comprise strong bacterial and diatom mats covering large areas of the bed and act to bind sediments. In this case the bed material becomes highly resistant to shear stresses applied by the overlying fluid motion and detachment, when it does occur, is manifest in patches of biofilm of the order cm(2) being entrained into the flow. This article is the first to report tensile test data specific to the centimeter scale using moist biofilm/sediment composite materials; the strain (ε)-stress (σ) relationships permit quantification of the elasticity (Young's modulus, E) and cohesive strength of each specimen. Specifically, we compare the mechanical strength of cyanobacterial biofilm-only samples to that of biofilm cultured over sediment samples (glass beads or natural sands of d ~ 1 mm) for up to 8 weeks. The range of tensile strength (1,288-3,283 Pa) for composite materials was up to three times higher than previous tensile tests conducted at smaller scale on mixed culture biofilm [Ohashi et al. (1999) Water Sci Technol 39:261-268], yet of similar range to cohesive strength values recorded on return activated sludge flocs [RAS; Poppele and Hozalski (2003) J Microbiol Methods 55:607-615]. Composite materials were 3-6 times weaker than biofilm-only samples, indicating that adhesion to sediment grains is weaker than cohesion within the biofilm. Furthermore, in order to relate the tensile test results to the more common in-situ failure of bio-mats due to shear flow, controlled erosion experiments were conducted in a hydraulic flume with live fluid flow. Here, the fluid shear stress causing erosion was 3 orders of magnitude lower than tensile stress; this highlights both the problem of interpreting material properties measured ex-situ and the need for a better mechanistic model of bio-mat detachment.

Relation between EPS adherence, viscoelastic properties, and MBR operation: Biofouling study with QCM-D

Membrane fouling is one of the main constraints of the wide use of membrane bioreactor (MBR) technology. The biomass in MBR systems includes extracellular polymeric substances (EPS), metabolic products of active microbial secretion that adversely affect the membrane performance. Solids retention time (SRT) in the MBR is one of the most important parameters affecting membrane fouling in MBR systems, where fouling is minimized at optimal SRT. Among the operating parameters in MBR systems, SRT is known to strongly influence the ratio of proteins to polysaccharides in the EPS matrix. In this study, we have direct evidence for changes in EPS adherence and viscoelastic properties due to changes in the sludge removal rate that strongly correlate with the membrane fouling rate and EPS composition. EPS were extracted from a UF membrane in a hybrid growth MBR operated at sludge removal rates of 59, 35.4, 17.7, and 5.9 L day(-1) (corresponding SRT of 3, 5, 10, and 30 days, respectively). The EPS adherence and adsorption kinetics were carried out in a quartz crystal microbalance with dissipation monitoring (QCM-D) technology in several adsorption measurements to a gold sensor coated with Polyvinylidene Fluoride (PVDF). EPS adsorption to the sensor surface is characterized by a decrease of the oscillation frequency and an increase in the dissipation energy of the sensor during parallel flow of aqueous media, supplemented with EPS, above the sensor surface. The results from these experiments were further modeled using the Voigt based model, in which the thickness, shear modulus, and shear viscosity values of the adsorbed EPS layers on the PVDF crystal were calculated. The observations in the QCM-D suggested that the elevated fouling of the UF membrane is due to higher adherence of the EPS as well as reduction in viscosity and elasticity of the EPS adsorbed layer and elevation of the EPS fluidity. These results corroborate with confocal laser scanning microscopy (CLSM) image analysis showing thicker EPS in close proximity to the membrane surface operated at reactor conditions which induced more fouling at elevated sludge removal rates.

Characterisation of spin coated engineered Escherichia coli biofilms using atomic force microscopy

The ability of biofilms to withstand chemical and physical extremes gives them the potential to be developed as robust biocatalysts. Critical to this issue is their capacity to withstand the physical environment within a bioreactor; in order to assess this capability knowledge of their surface properties and adhesive strength is required. Novel atomic force microscopy experiments conducted under growth conditions (30°C) were used to characterise Escherichia coli biofilms, which were generated by a recently developed spin-coating method onto a poly-l-lysine coated glass substrate. High-resolution topographical images were obtained throughout the course of biofilm development, quantifying the tip-cell interaction force during the 10 day maturation process. Strikingly, the adhesion force between the Si AFM tip and the biofilm surface increased from 0.8 nN to 40 nN within 3 days. This was most likely due to the production of extracellular polymer substance (EPS), over the maturation period, which was also observed by electron microscopy. At later stages of maturation, multiple retraction events were also identified corresponding to biofilm surface features thought to be EPS components. The spin coated biofilms were shown to have stronger surface adhesion than an equivalent conventionally grown biofilm on the same glass substrate.

Elasticity and physico-chemical properties during drinking water biofilm formation

Atomic force microscope techniques and multi-staining fluorescence microscopy were employed to study the steps in drinking water biofilm formation. During the formation of a conditioning layer, surface hydrophobic forces increased and the range of characteristic hydrophobic forces diversified with time, becoming progressively complex in macromolecular composition, which in return triggered irreversible cellular adhesion. AFM visualization of 1 to 8 week drinking water biofilms showed a spatially discontinuous and heterogeneous distribution comprising an extensive network of filamentous fungi in which biofilm aggregates were embedded. The elastic modulus of 40-day-old biofilms ranged from 200 to 9000 kPa, and the biofilm deposits with a height >0.5 μm had an elastic modulus <600 kPa, suggesting that the drinking water biofilms were composed of a soft top layer and a basal layer with significantly higher elastic modulus values falling in the range of fungal elasticity.

Hydrodynamic deformation and removal of Staphylococcus epidermidis biofilms treated with urea, chlorhexidine, iron chloride, or DispersinB

The force-deflection and removal characteristics of bacterial biofilm were measured by two different techniques before and after chemical, or enzymatic, treatment. The first technique involved time lapse imaging of a biofilm grown in a capillary flow cell and subjected to a brief shear stress challenge imparted through increased fluid flow. Biofilm removal was determined by calculating the reduction in biofilm area from quantitative analysis of transmission images. The second technique was based on micro-indentation using an atomic force microscope. In both cases, biofilms formed by Staphylococcus epidermidis were exposed to buffer (untreated control), urea, chlorhexidine, iron chloride, or DispersinB. In control experiments, the biofilm exhibited force-deflection responses that were similar before and after the same treatment. The biofilm structure was stable during the post-treatment shear challenge (1% loss). Biofilms treated with chlorhexidine became less deformable after treatment and no increase in biomass removal was seen during the post-treatment shear challenge (2% loss). In contrast, biofilms treated with urea or DispersinB became more deformable and exhibited significant biofilm loss during the post-treatment flow challenge (71% and 40%, respectively). During the treatment soak phase, biofilms exposed to urea swelled. Biofilms exposed to iron chloride showed little difference from the control other than slight contraction during the treatment soak. These observations suggest the following interpretations: (1) chemical or enzymatic treatments, including those that are not frankly antimicrobial, can alter the cohesion of bacterial biofilm; (2) biocidal treatments (e.g., chlorhexidine) do not necessarily weaken the biofilm; and (3) biofilm removal following treatment with agents that make the biofilm more deformable (e.g., urea, DispersinB) depend on interaction between the moving fluid and the biofilm structure. Measurements such as those reported here open the door to development of new technologies for controlling detrimental biofilms by targeting biofilm cohesion rather than killing microorganisms.

Imaging and determining friction forces of specific interactions between human IgG and rat anti-human IgG

Covalently immobilized rat anti-human immunoglobulin (IgG) monolayers on thiol-modified gold substrates and human IgG linked with the tips were fabricated using the self-assembled monolayer method, and interactions between these systems were studied by friction force microscopy (FFM). In addition to observation of distinct nanostructures of protein monolayers due to recognition events, FFM also quantified the friction force due to protein-protein-specific interactions. The average friction force due to interactions between the antigen functionalized tip and the antibody monolayer was determined as 200-250 pN, significantly greater than that between either the bare tip and the antibody monolayer (0-50 pN), or the blocked antigen tip and the antibody monolayer (50-100 pN), indicative of antigen/antibody-specific interactions. These results, taken together, suggest that FFM is not only capable of tracking recognition events, but also quantifying the friction force due to specific interactions between biological molecules, such as antigen and antibody.