Films of bacteria at interfaces

Biofilm formation of Pseudomonas aeruginosa in spaceflight is minimized on lubricant impregnated surfaces

The undesirable, yet inevitable, presence of bacterial biofilms in spacecraft poses a risk to the proper functioning of systems and to astronauts' health. To mitigate the risks that arise from them, it is important to understand biofilms' behavior in microgravity. As part of the Space Biofilms project, biofilms of Pseudomonas aeruginosa were grown in spaceflight over material surfaces. Stainless Steel 316 (SS316) and passivated SS316 were tested for their relevance as spaceflight hardware components, while a lubricant impregnated surface (LIS) was tested as potential biofilm control strategy. The morphology and gene expression of biofilms were characterized. Biofilms in microgravity are less robust than on Earth. LIS strongly inhibits biofilm formation compared to SS. Furthermore, this effect is even greater in spaceflight than on Earth, making LIS a promising option for spacecraft use. Transcriptomic profiles for the different conditions are presented, and potential mechanisms of biofilm reduction on LIS are discussed.

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.

Tannin extracted from Penthorum chinense Pursh, a potential drug with antimicrobial and antibiofilm effects against methicillin-sensitive Staphylococcus aureus and methicillin-resistant Staphylococcus aureus

Staphylococcus aureus is an opportunistic pathogen. Due to the widespread use and abuse of antibiotics, various drug-resistant strains of S. aureus have emerged, with methicillin-resistant Staphylococcus aureus (MRSA) being the most prevalent. Bacterial biofilm is a significant contributor to bacterial infection and drug resistance. Consequently, bacterial biofilm formation has emerged as a therapeutic strategy. In this study, the chemical constituents, antimicrobial and antibiofilm properties of tannins isolated from Penthorum chinense Pursh (TPCP) were investigated. In vitro, TPCP exhibited antimicrobial properties. The minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) for methicillin-sensitive Staphylococcus aureus (MSSA) and MRSA were 156.25 and 312.5 μg/mL, and 312.5 and 625 μg/mL, respectively. According to the growth curves, TPCP significantly inhibited the growth of MSSA and MRSA. The results of the crystal violet biofilm assay in conjunction with confocal laser scanning and scanning electron microscopy demonstrated that TPCP destroyed preformed MSSA and MRSA biofilms. TPCP significantly decreased the secretion of exopolysaccharides and extracellular DNA. Subsequently, the mechanism was investigated using RT-PCR. Examining the expression of icaA, cidA, sigB, agrA, and sarA genes in MRSA, we discovered that TPCP inhibited biofilm formation by affecting the quorum-sensing system in bacteria. Our study demonstrates that TPCP exerts antibacterial effects by disrupting the formation of bacterial biofilms, suggesting that TPCP has clinical potential as a novel antibacterial agent for the prevention and treatment of MSSA and MRSA infections.

Influence of Surface Roughness, Nanostructure, and Wetting on Bacterial Adhesion

Bacterial fouling is a persistent problem causing the deterioration and failure of functional surfaces for industrial equipment/components; numerous human, animal, and plant infections/diseases; and energy waste due to the inefficiencies at internal and external geometries of transport systems. This work gains new insights into the effect of surface roughness on bacterial fouling by systematically studying bacterial adhesion on model hydrophobic (methyl-terminated) surfaces with roughness scales spanning from ∼2 nm to ∼390 nm. Additionally, a surface energy integration framework is developed to elucidate the role of surface roughness on the energetics of bacteria and substrate interactions. For a given bacteria type and surface chemistry; the extent of bacterial fouling was found to demonstrate up to a 75-fold variation with surface roughness. For the cases showing hydrophobic wetting behavior, both increased effective surface area with increasing roughness and decreased activation energy with increased surface roughness was concluded to enhance the extent of bacterial adhesion. For the cases of superhydrophobic surfaces, the combination of factors including (i) the surpassing of Laplace pressure force of interstitial air over bacterial adhesive force, (ii) the reduced effective substrate area for bacteria wall due to air gaps to have direct/solid contact, and (iii) the reduction of attractive van der Waals force that holds adhering bacteria on the substrate were summarized to weaken the bacterial adhesion. Overall, this study is significant in the context of designing antifouling coatings and systems as well as explaining variations in bacterial contamination and biofilm formation processes on functional surfaces.

Non-deacetylated poly-N-acetylglucosamine-hyperproducing Staphylococcus aureus undergoes immediate autoaggregation upon vortexing

Biofilms are microbial communities of cells embedded in a matrix of extracellular polymeric substances generated and adhering to each other or to a surface. Cell aggregates formed in the absence of a surface and floating pellicles that form biofilms at the air-liquid interface are also considered to be a type of biofilm. Staphylococcus aureus is a well-known cause of biofilm infections and high-molecular-weight polysaccharides, poly-N-acetylglucosamine (PNAG) is a main constituent of the biofilm. An icaADBC operon comprises major machinery to synthesize and extracellularly secrete PNAG. Extracellular PNAG is partially deacetylated by IcaB deacetylase, and the positively charged PNAG hence interacts with negatively charged cell surface to form the major component of biofilm. We previously reported a new regulator of biofilm (Rob) and demonstrated that Rob binds to a unique 5-bp motif, TATTT, present in intergenic region between icaADBC operon and its repressor gene icaR in Yu et al. The deletion of the 5-bp motif induces excessive adherent biofilm formation. The real function of the 5-bp motif is still unknown. In an attempt to isolate the 5-bp motif deletion mutant, we isolated several non-adherent mutants. They grew normally in turbid broth shaking culture but immediately auto-aggregated upon weak vortexing and sedimented as a lump resulting in a clear supernatant. Whole genome sequencing of the mutants identified they all carried mutations in icaB in addition to deletion of the 5-bp motif. Purification and molecular characterization of auto-aggregating factor in the culture supernatant of the mutant identified that the factor was a massively produced non-deacetylated PNAG. Therefore, we created a double deficient strain of biofilm inhibitory factors (5-bp motif, icaR, rob) and icaB to confirm the aggregation phenomenon. This peculiar phenomenon was only observed in Δ5bpΔicaB double mutant but not in ΔicaR ΔicaB or ΔrobΔicaB mutant. This study explains large amount of extracellularly produced non-deacetylated PNAG by Δ5bpΔicaB double mutation induced rapid auto-aggregation of S. aureus cells by vortexing. This phenomenon indicated that Staphylococcus aureus may form biofilms that do not adhere to solid surfaces and we propose this as a new mechanism of non-adherent biofilm formation of S. aureus.

Density and temperature controlled fluid extraction in a bacterial biofilm is determined by poly-γ-glutamic acid production

A hallmark of microbial biofilms is the self-production of an extracellular molecular matrix that encases the resident cells. The matrix provides protection from the environment, while spatial heterogeneity of gene expression influences the structural morphology and colony spreading dynamics. Bacillus subtilis is a model bacterial system used to uncover the regulatory pathways and key building blocks required for biofilm growth and development. In this work, we report on the emergence of a highly active population of bacteria during the early stages of biofilm formation, facilitated by the extraction of fluid from the underlying agar substrate. We trace the origin of this fluid extraction to the production of poly-γ-glutamic acid (PGA). The flagella-dependent activity develops behind a moving front of fluid that propagates from the boundary of the biofilm towards the interior. The extent of fluid proliferation is controlled by the presence of extracellular polysaccharides (EPS). We also find that PGA production is positively correlated with higher temperatures, resulting in high-temperature mature biofilm morphologies that are distinct from the rugose colony biofilm architecture typically associated with B. subtilis. Although previous reports have suggested that PGA production does not play a major role in biofilm morphology in the undomesticated isolate NCIB 3610, our results suggest that this strain produces distinct biofilm matrices in response to environmental conditions.

Emergent collective dynamics of pusher and puller squirmer rods: swarming, clustering, and turbulence

We study the interplay of steric and hydrodynamic interactions in suspensions of elongated microswimmers by simulating the full hydrodynamics of squirmer rods in the quasi two-dimensional geometry of a Hele-Shaw cell. To create pusher or puller-type squirmer rods, we concentrate the surface slip-velocity field more to the back or to the front of the rod and thereby are able to tune the rod's force-dipole strength. We study a wide range of aspect ratios and area fractions and provide corresponding state diagrams. The flow field of pusher-type squirmer rods destabilizes ordered structures and favors the disordered state at small area fractions and aspect ratios. Only when steric interactions become relevant, we observe a turbulent and dynamic cluster state, while for large aspect ratios a single swarm and jammed cluster occurs. The power spectrum of the turbulent state shows two distinct energy cascades at small and large wave numbers with power-law scaling and non-universal exponents. Pullers show a strong tendency to form swarms instead of the disordered state found for neutral and pusher rods. At large area fractions a dynamic cluster is observed and at larger aspect ratio a single swarm or jammed cluster occurs.

Motility Suppression and Trapping Bacteria by ZnO Nanostructures

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

Live-Cell Synchrotron-Based FTIR Evaluation of Metabolic Compounds in Brain Glioblastoma Cell Lines after Riluzole Treatment

Glioblastoma multiforme (GBM) is the most aggressive brain tumor, characterized by short median survival and an almost 100% tumor-related mortality. The standard of care treatment for newly diagnosed GBM includes surgical resection followed by concomitant radiochemotherapy. The prevention of disease progression fails due to the poor therapeutic effect caused by the great molecular heterogeneity of this tumor. Previously, we exploited synchrotron radiation-based soft X-ray tomography and hard X-ray fluorescence for elemental microimaging of the shock-frozen GBM cells. The present study focuses instead on the biochemical profiling of live GBM cells and provides new insight into tumor heterogenicity. We studied bio-macromolecular changes by exploring the live-cell synchrotron-based Fourier transform infrared (SR-FTIR) microspectroscopy in a set of three GBM cell lines, including the patient-derived glioblastoma cell line, before and after riluzole treatment, a medicament with potential anticancer properties. SR-FTIR microspectroscopy shows that GBM live cells of different origins recruit different organic compounds. The riluzole treatment of all GBM cell lines mainly affected carbohydrate metabolism and the DNA structure. Lipid structures and protein secondary conformation are affected as well by the riluzole treatment: cellular proteins assumed cross β-sheet conformation while parallel β-sheet conformation was less represented for all GBM cells. Moreover, we hope that a new live-cell approach for GBM simultaneous treatment and examination can be devised to target cancer cells more specifically, i.e., future therapies can develop more specific treatments according to the specific bio-macromolecular signature of each tumor type.

Architecture and physico-chemical properties of Bacillus amyloliquefaciens L-17 pellicle formed at the air-liquid interface

Bacillus amyloliquefaciens is a ubiquitous soil and plant-associated bacterial species which shows structural and adaptative responses to the environment. This present paper explores the ability of the strain L-17 to form subaerial biofilms on a liquid surface. Hydrophobic and non-wetting properties were observed for the rough top biofilm layer in contact with the air, which are quite different to the hydrophilic properties which were observed for the smooth biofilm layer in contact with the liquid. Both pellicle interfaces were visualized by scanning electron microscopy revealing a complex three-dimensional architecture composed of exopolymers organized in stacked fibrous network or sheet-like structures in the vicinity of the subaerial surface. Disruption of the extracellular matrix by combining physical and chemical treatments indicated that both loosely and tightly bound polysaccharides were found as major components of this complex pellicle. Proteins were also involved in the aggregation and cohesion of the matrix as multi extraction steps were needed to recover some tightly bounded proteins. This was confirmed by applying protease treatment which was able to significantly disrupt the pellicle. Overall results underline the ability of B. amyloliquefaciens L-17 to survive on air-liquid interfaces. This feature offers an interesting strategy to escape aquatic environments and develop aerial biofilm in response to environmental changes involving wet-dry cycles.

Dynamic and mechanical evolution of an oil-water interface during bacterial biofilm formation

We present an experimental study combining particle tracking, active microrheology, and differential dynamic microscopy (DDM) to investigate the dynamics and rheology of an oil-water interface during biofilm formation by the bacteria Pseudomonas Aeruginosa PA14. The interface transitions from an active fluid dominated by the swimming motion of adsorbed bacteria at early age to an active viscoelastic system at late ages when the biofilm is established. The microrheology measurements using microscale magnetic rods indicate that the biofilm behaves as a viscoelastic solid at late age. The bacteria motility at the interface during the biofilm formation, which is characterized in the DDM measurements, evolves from diffusive motion at early age to constrained, quasi-localized motion at later age. Similarly, the mobility of passively moving colloidal spheres at the interface decreases significantly with increasing interface age and shows a dependence on sphere size after biofilm formation that is orders-of-magnitude larger than that expected in a homogeneous system in equilibrium. We attribute this anomalous size dependence to either length-scale-dependent rheology of the biofilm or widely differing effects of the bacteria activity on the motion of spheres of different sizes.

Rediscovering bacterial exopolysaccharides of terrestrial and marine origins: novel insights on their distribution, biosynthesis, biotechnological production, and future perspectives

Bacteria exist in colonies as aggregates or associated with surfaces forming biofilms rather than planktonic cells. Living in such a unique manner is always mediated via a matrix of extracellular polymeric substances, which are composed mainly of polysaccharides or specifically exopolysaccharides (EPS). Biofilm formation and hence EPS production are affected by biotic and abiotic factors inducing/inhibiting several involved genes and other molecules. In addition, various aspects of bacterial EPS regarding: physiological functions, molecular weight, and chemical composition were demonstrated. Recent investigations have revealed a wide spectrum of EPS chemical and physicochemical properties showing promising applications in different industrial sectors. For instance, lactic acid bacteria (LAB)- and marine-derived EPS exhibit: immunomodulatory, antioxidant, antitumor, bioremediation of heavy metals, as well as thickening and viscosity modifiers in the food industry. However, bacterial EPS have not yet been commercially implemented, in contrast to plant-derived analogues. The current review aims to rediscover the EPS structural and biosynthetic features derived from marine and terrestrial bacteria, and applications as well.

Bacterial streamers as colloidal systems: Five grand challenges

Bacteria can thrive in biofilms, which are intricately organized communities with cells encased in a self-secreted matrix of extracellular polymeric substances (EPS). Imposed hydrodynamic stresses can transform this active colloidal dispersion of bacteria and EPS into slender thread-like entities called streamers. In this perspective article, the reader is introduced to the world of such deformable 'bacteria-EPS' composites that are a subclass of the generic flow-induced colloidal structures. While bacterial streamers have been shown to form in a variety of hydrodynamic conditions (turbulent and creeping flows), its abiotic analogues have only been demonstrated in low Reynolds number (Re < 1) particle-laden polymeric flows. Streamers are relevant to a variety of situations ranging from natural formations in caves and river beds to clogging of biomedical devices and filtration membranes. A critical review of the relevant biophysical aspects of streamer formation phenomena and unique attributes of its material behavior are distilled to unveil five grand scientific challenges. The coupling between colloidal hydrodynamics, device geometry and streamer formation are highlighted.

A Facile High-Throughput Model of Surface-Independent Staphylococcus aureus Biofilms by Spontaneous Aggregation

The canonical model of biofilm formation begins with the attachment and growth of microbial cells on a surface. While these in vitro models reasonably mimic biofilms formed on foreign bodies such as catheters and implants, this is not the case for biofilms formed in cystic fibrosis and chronic wound infections, which appear to present as aggregates not attached to a surface.

Many microbes in their natural habitats are found in biofilm ecosystems attached to surfaces and not as free-floating (planktonic) organisms. Furthermore, it is estimated that nearly 80% of human infections are associated with biofilms. Biofilms are traditionally defined as three-dimensional, structured microbial communities that are attached to a surface and encased in a matrix of exopolymeric material. While this view of biofilm largely arises from in vitro studies under static or flow conditions, in vivo observations have indicated that this view of biofilms is essentially true only for foreign-body infections on catheters or implants where biofilms are attached to the biomaterial. In mucosal infections such as chronic wounds or cystic fibrosis or joint infections, biofilms can be found unattached to a surface and as three-dimensional aggregates. In this work, we describe a high-throughput model of aggregate biofilms of methicillin-resistant Staphylococcus aureus (MRSA) using 96-well plate hanging-drop technology. We show that MRSA forms surface-independent biofilms, distinct from surface-attached biofilms, that are rich in exopolymeric proteins, polysaccharides, and extracellular DNA (eDNA), express biofilm-related genes, and exhibit heightened antibiotic resistance. We also show that the surface-independent biofilms of clinical isolates of MRSA from cystic fibrosis and central catheter-related infections demonstrate morphological differences. Overall, our results show that biofilms can form by spontaneous aggregation without attachment to a surface, and this new in vitro system can model surface-independent biofilms that may more closely mimic the corresponding physiological niche during infection.

IMPORTANCE The canonical model of biofilm formation begins with the attachment and growth of microbial cells on a surface. While these in vitro models reasonably mimic biofilms formed on foreign bodies such as catheters and implants, this is not the case for biofilms formed in cystic fibrosis and chronic wound infections, which appear to present as aggregates not attached to a surface. The hanging-drop model of biofilms of methicillin-resistant Staphylococcus aureus (MRSA), the major causative organism of skin and soft tissue infections, shows that these biofilms display morphological and antibiotic response patterns that are distinct from those of their surface-attached counterparts, and biofilm growth is consistent with their in vivo location. The simplicity and throughput of this model enable adoption to investigate other single or polymicrobial biofilms in a physiologically relevant setting.

Biofilm and swarming emergent behaviours controlled through the aid of biophysical understanding and tools

Bacteria can organise themselves into communities in the forms of biofilms and swarms. Through chemical and physical interactions between cells, these communities exhibit emergent properties that individual cells alone do not have. While bacterial communities have been mainly studied in the context of biochemistry and molecular biology, recent years have seen rapid advancements in the biophysical understanding of emergent phenomena through physical interactions in biofilms and swarms. Moreover, new technologies to control bacterial emergent behaviours by physical means are emerging in synthetic biology. Such technologies are particularly promising for developing engineered living materials (ELM) and devices and controlling contamination and biofouling. In this minireview, we overview recent studies unveiling physical and mechanical cues that trigger and affect swarming and biofilm development. In particular, we focus on cell shape, motion and density as the key parameters for mechanical cell-cell interactions within a community. We then showcase recent studies that use physical stimuli for patterning bacterial communities, altering collective behaviours and preventing biofilm formation. Finally, we discuss the future potential extension of biophysical and bioengineering research on microbial communities through computational modelling and deeper investigation of mechano-electrophysiological coupling.

Material properties of interfacial films of mucoid and nonmucoid Pseudomonas aeruginosa isolates

Chronic lung infection with bacterial biofilms is a leading cause of death in cystic fibrosis (CF) patients. Pseudomonas aeruginosa, one of the many species colonizing the lung airways, can undergo pathoadaptation, leading to a mucoid phenotype with interesting material properties. We hypothesize that the surface properties and extracellular materials of mucoid P. aeruginosa cells greatly influence the mechanical behavior of their films at fluid interfaces. In this study, we investigate the interfacial properties of films formed by nonmucoid (PANT) and mucoid (PASL) strains of P. aeruginosa isolated from CF patients. We use pendant drop elastometry to analyze the interfacial response of the films formed by PANT and PASL at the hexadecane-water interface. The dynamic rheological analyses of the films highlight the distinctive signature of the mucoid strains at fluid interfaces. The mucoid films exhibit greater relaxation following a compressive strain than a tensile one, while a full hysteresis response is achieved by the nonmucoid films; this indicates that the material properties of the PANT films are conserved under both compression and tension. The wrinkling and shape analyses of the interfacial bacterial films elucidate that the mucoid strain exhibits remarkable viscoelastic properties, enabling the remodeling of the living films and dissipation of the compressive stress. The comparative analysis of the material properties of mucoid and nonmucoid P. aeruginosa cells indicates that mucoid switch can play an important role in protecting the bacteria from interfacial stresses. Further characterization of interfacial bacterial films will provide new insights into the development of methods for controlling interfacial films of bacteria.

Evaluation of non-traditional visualization methods to detect surface attachment of biofilms

In food safety and food quality, biofilm research is of great importance for mitigating food-borne pathogens in food processing environments. To supplement the traditional staining techniques for biofilm characterization, we introduce several non-traditional imaging methods for detecting biofilm attachment to the solid-liquid and air-liquid interfaces. For strains of Pseudomonas aeruginosa (the positive control), Acinetobacter baumanii, Listeria monocytogenes and Salmonella enterica, the traditional crystal violet assay showed evidence of biofilm attachment to the well plate base as well as inferred the presence of an air-liquid biofilm attached on the upper well walls where the meniscus was present. However, air-liquid biofilms and solid-surface-attached biofilms were not detected for all of these strains using the non-traditional imaging methods. For L. monocytogenes, we were unable to detect biofilms at a particle-laden, air-liquid interface as evidenced through microscopy, which contradicts the meniscus staining test and suggests that the coffee-ring effect may lead to false positives when using meniscus staining. Furthermore, when L. monocytogenes was cultivated in a pendant droplet in air, only microbial sediment at the droplet apex was observed without any apparent bacterial colonization of the droplet surface. All other strains showed clear evidence of air-liquid biofilms at the air-liquid interface of a pendant droplet. To non-invasively detect if and when air-liquid pellicles form in a well plate, we also present a novel in situ reflection assay that demonstrates the capacity to do this quantitatively.

Staphylococcal Biofilm Development: Structure, Regulation, and Treatment Strategies

In many natural and clinical settings, bacteria are associated with some type of biotic or abiotic surface that enables them to form biofilms, a multicellular lifestyle with bacteria embedded in an extracellular matrix. Staphylococcus aureus and Staphylococcus epidermidis, the most frequent causes of biofilm-associated infections on indwelling medical devices, can switch between an existence as single free-floating cells and multicellular biofilms. During biofilm formation, cells first attach to a surface and then multiply to form microcolonies. They subsequently produce the extracellular matrix, a hallmark of biofilm formation, which consists of polysaccharides, proteins, and extracellular DNA. After biofilm maturation into three-dimensional structures, the biofilm community undergoes a disassembly process that leads to the dissemination of staphylococcal cells. As biofilms are dynamic and complex biological systems, staphylococci have evolved a vast network of regulatory mechanisms to modify and fine-tune biofilm development upon changes in environmental conditions. Thus, biofilm formation is used as a strategy for survival and persistence in the human host and can serve as a reservoir for spreading to new infection sites. Moreover, staphylococcal biofilms provide enhanced resilience toward antibiotics and the immune response and impose remarkable therapeutic challenges in clinics worldwide. This review provides an overview and an updated perspective on staphylococcal biofilms, describing the characteristic features of biofilm formation, the structural and functional properties of the biofilm matrix, and the most important mechanisms involved in the regulation of staphylococcal biofilm formation. Finally, we highlight promising strategies and technologies, including multitargeted or combinational therapies, to eradicate staphylococcal biofilms.

Staphylococcal Biofilm Development: Structure, Regulation, and Treatment Strategies

In many natural and clinical settings, bacteria are associated with some type of biotic or abiotic surface that enables them to form biofilms, a multicellular lifestyle with bacteria embedded in an extracellular matrix. Staphylococcus aureus and Staphylococcus epidermidis, the most frequent causes of biofilm-associated infections on indwelling medical devices, can switch between an existence as single free-floating cells and multicellular biofilms. During biofilm formation, cells first attach to a surface and then multiply to form microcolonies. They subsequently produce the extracellular matrix, a hallmark of biofilm formation, which consists of polysaccharides, proteins, and extracellular DNA. After biofilm maturation into three-dimensional structures, the biofilm community undergoes a disassembly process that leads to the dissemination of staphylococcal cells. As biofilms are dynamic and complex biological systems, staphylococci have evolved a vast network of regulatory mechanisms to modify and fine-tune biofilm development upon changes in environmental conditions. Thus, biofilm formation is used as a strategy for survival and persistence in the human host and can serve as a reservoir for spreading to new infection sites. Moreover, staphylococcal biofilms provide enhanced resilience toward antibiotics and the immune response and impose remarkable therapeutic challenges in clinics worldwide. This review provides an overview and an updated perspective on staphylococcal biofilms, describing the characteristic features of biofilm formation, the structural and functional properties of the biofilm matrix, and the most important mechanisms involved in the regulation of staphylococcal biofilm formation. Finally, we highlight promising strategies and technologies, including multitargeted or combinational therapies, to eradicate staphylococcal biofilms.

Motile Bacteria at Oil-Water Interfaces: Pseudomonas aeruginosa

Bacteria are important examples of active or self-propelled colloids. Because of their directed motion, they accumulate near interfaces. There, they can become trapped and swim adjacent to the interface via hydrodynamic interactions, or they can adsorb directly and swim in an adhered state with complex trajectories that differ from those in bulk in both form and spatiotemporal implications. We have adopted the monotrichous bacterium Pseudomonas aeruginosa PA01 as a model species and have studied its motion at oil-aqueous interfaces. We have identified conditions in which bacteria swim persistently without restructuring the interface, allowing detailed and prolonged study of their motion. In addition to characterizing the ensemble behavior of the bacteria, we have observed a gallery of distinct trajectories of individual swimmers on and near fluid interfaces. We attribute these diverse swimming behaviors to differing trapped states for the bacteria in the fluid interface. These trajectory types include Brownian diffusive paths for passive adsorbed bacteria, curvilinear trajectories including curly paths with radii of curvature larger than the cell body length, and rapid pirouette motions with radii of curvature comparable to the cell body length. Finally, we see interfacial visitors that come and go from the interfacial plane. We characterize these individual swimmer motions. This work may impact nutrient cycles for bacteria on or near interfaces in nature. This work will also have implications in microrobotics, as active colloids in general and bacteria in particular are used to carry cargo in this burgeoning field. Finally, these results have implications in engineering of active surfaces that exploit interfacially trapped self-propelled colloids.

Bacteria-nanoparticle interactions in the context of nanofouling

The attachment of microbial communities to surfaces is a well-known problem recognized to be involved in a variety of critical issues in the sectors of food processing, chronic wounds, infection from implants, clogging of membranes and corrosion of equipment. Considering the importance of the detrimental impact of biofouling, it has received much attention in the scientific community and from concerned stakeholders. With the development of nanotechnology and the nowadays widespread use of engineered nanoparticles (ENPs), concerns have been raised regarding their fate in terrestrial and aquatic environments. Safety aspects and public health issues are critical in the management of handling nanomaterials and their nanowastes. The interactions of various types of nanoparticles (NPs) with planktonic bacteria have also received attention due to their antimicrobial properties. However, their behavior in regard to biofilms is not well understood although, in the environment, most of the bacteria prefer living in sessile communities. The question appears relevant considering the need to build knowledge on the fate of nanoparticles and the fact that no one can exclude the risk of accumulation of nanoparticles in biofilms and on surfaces leading to a form of nanofouling involving both engineered nanoparticles (ENPs) and nanoplastics. The present analysis of recent research accounts allows in identifying that (1) research activities related to water remediation systems have been mostly oriented on the impact of NPs on pre-existing biofilms, (2) experimental designs are restricted to few scenarios of exposure, usually limited to relative short-time periods although nanofouling could favour the development of multi-resistant bacterial species through sub-lethal exposures over prolong periods of time (3) nanofouling in other systems in which biofilms develop remains to be addressed, and (4) new research directions are required for investigating the mechanisms involved and the subsequent impact of nanofouling on bacterial consortium responses encountered in a variety of environments such as those prevailing in food production/processing settings. Finally, this review aims at providing recent information and insights on nanoparticle-bacterial interactions in the context of biofilms in order to supply an updated outlook of research perspectives that could help establish the framework for production, use and fate of nanomaterials as well as future research directions.

Biofilms at interfaces: microbial distribution in floating films

Cellular motility is a key function guiding microbial adhesion to interfaces, which is the first step in the formation of biofilms. The close association of biofilms and bioremediation has prompted extensive research aimed at comprehending the physics of microbial locomotion near interfaces. We study the dynamics and statistics of microorganisms in a 'floating biofilm', i.e., a confinement with an air-liquid interface on one side and a liquid-liquid interface on the other. We use a very general mathematical model, based on a multipole representation and probabilistic simulations, to ascertain the spatial distribution of microorganisms in films of different viscosities. Our results reveal that microorganisms can be distributed symmetrically or asymmetrically across the height of the film, depending on their morphology and the ratio of the film's viscosity to that of the fluid substrate. Long-flagellated, elongated bacteria exhibit stable swimming parallel to the liquid-liquid interface when the bacterial film is less viscous than the underlying fluid. Bacteria with shorter flagella on the other hand, swim away from the liquid-liquid interface and accumulate at the free surface. We also analyze microorganism dynamics in a flowing film and show how a microorganism's ability to resist 'flow-induced-erosion' from interfaces is affected by its elongation and mode of propulsion. Our study generalizes past efforts on understanding microorganism dynamics under confinement by interfaces and provides key insights on biofilm initiation at liquid-liquid interfaces.

Hydrodynamic attraction of bacteria to gas and liquid interfaces

Near an interface, the distribution of swimming microorganisms such as bacteria is distinguished from inert colloidal particles because of the interfacial hydrodynamics induced by swimming. In this work, we use nontumbling flagellated bacteria, Escherichia coli, to study cell distribution near gas and liquid interfaces and compare it to the case of a solid wall. For low-viscosity ratios such as gas interfaces, we observe a stronger cell accumulation compared to that near liquid and solid surfaces. This contradicts known theoretical predictions. Therefore, we develop a model based on Brownian dynamics, including hydrodynamic effects and short-range physiochemical interactions between bacteria and interfaces. This model explains our experimental findings and can predict cell distribution near clean and surfactant-contaminated interfaces. By considering higher order singularities, this study helps explain bacteria orientation, trajectories, and cell density.

Effects of Ultrafine Bubbles on Gram-Negative Bacteria: Inhibition or Selection?

Ultrafine bubbles exist in all liquids and are naturally stable. As their properties are not entirely known, it is unclear how they impact the surrounding solution and comparable-sized particles within it. It is essential to further investigate the properties of ultrafine bubbles in order to expand their industrial application. In this regard, the effect of ultrafine bubbles on bacterial development is of particular interest. Our current study, using optical density measurements and fluorescence microscopic images has demonstrated that ultrafine gas bubbles impact the morphology and phenotype of Escherichia coli and Pseudomonas aeruginosa. Specifically, Fourier transform infrared spectroscopic measurements indicated a thickening of bacterial membranes in samples exposed to ultrafine bubbles. The study also confirmed that ultrafine bubbles can inhibit bacterial cell growth. This study signifies the role of surface phenomena in bacterial culture, which is crucial in the upstream processes of recombinant DNA technology applications.

Role of Caulobacter Cell Surface Structures in Colonization of the Air-Liquid Interface

In aquatic environments, Caulobacter spp. can be found at the boundary between liquid and air known as the neuston. I report an approach to study temporal features of Caulobacter crescentus colonization and pellicle biofilm development at the air-liquid interface and have defined the role of cell surface structures in this process. At this interface, C. crescentus initially forms a monolayer of cells bearing a surface adhesin known as the holdfast. When excised from the liquid surface, this monolayer strongly adheres to glass. The monolayer subsequently develops into a three-dimensional structure that is highly enriched in clusters of stalked cells known as rosettes. As this pellicle film matures, it becomes more cohesive and less adherent to a glass surface. A mutant strain lacking a flagellum does not efficiently reach the surface, and strains lacking type IV pili exhibit defects in organization of the three-dimensional pellicle. Strains unable to synthesize the holdfast fail to accumulate at the boundary between air and liquid and do not form a pellicle. Phase-contrast images support a model whereby the holdfast functions to trap C. crescentus cells at the air-liquid boundary. Unlike the holdfast, neither the flagellum nor type IV pili are required for C. crescentus to partition to the air-liquid interface. While it is well established that the holdfast enables adherence to solid surfaces, this study provides evidence that the holdfast has physicochemical properties that allow partitioning of nonmotile mother cells to the air-liquid interface and facilitate colonization of this microenvironment.IMPORTANCE In aquatic environments, the boundary at the air interface is often highly enriched with nutrients and oxygen. Colonization of this niche likely confers a significant fitness advantage in many cases. This study provides evidence that the cell surface adhesin known as a holdfast enables Caulobacter crescentus to partition to and colonize the air-liquid interface. Additional surface structures, including the flagellum and type IV pili, are important determinants of colonization and biofilm formation at this boundary. Considering that holdfast-like adhesins are broadly conserved in Caulobacter spp. and other members of the diverse class Alphaproteobacteria, these surface structures may function broadly to facilitate colonization of air-liquid boundaries in a range of ecological contexts, including freshwater, marine, and soil ecosystems.

Cu nanoparticles constrain segmental dynamics of cross-linked polyethers: a trade-off between non-fouling and antibacterial properties

Copper has a strong bactericidal effect against multi-drug resistant pathogens and polyethers are known for their resistance to biofilm formation. Herein, we combined Cu nanoparticles (NPs) and a polyether plasma polymer in the form of nanocomposite thin films and studied whether both effects can be coupled. Cu NPs were produced by magnetron sputtering via the aggregation in a cool buffer gas whereas polyether layers were synthesized by Plasma-Assisted Vapor Phase Deposition with poly(ethylene oxide) (PEO) used as a precursor. In situ specific heat spectroscopy and XPS analysis revealed the formation of a modified polymer layer around the NPs which propagates on the scale of a few nanometers from the Cu NP/polymer interface and then transforms into a bulk polymer phase. The chemical composition of the modified layer is found to be ether-deficient due to the catalytic influence of copper whereas the bulk polymer phase exhibits the chemical composition close to the original PEO. Two cooperative glass transition phenomena are revealed that belong to the modified polymer layer and the bulk phase. The former is characterized by constrained mobility of polymer segments which manifests itself via a 30 K increase of dynamic glass transition temperature. Furthermore, the modified layer is characterized by the heterogeneous structure which results in higher fragility of this layer as compared to the bulk phase. The Cu NPs/polyether thin films exhibit reduced protein adsorption; however, the constrained segmental dynamics leads to the deterioration of the non-fouling properties for ultra-thin polyether coatings. The films are found to have a bactericidal effect against multi-drug resistant Gram-positive Methicillin-Resistant Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa.

Hydrodynamic Interaction Enhances Colonization of Sinking Nutrient Sources by Motile Microorganisms

In this study, we document hydrodynamics-mediated trapping of microorganisms around a moving spherical nutrient source such as a settling marine snow aggregate. There exists a range of size and excess density of the nutrient source, and motility and morphology of the microorganism under which hydrodynamic interactions enable the passive capture of approaching microorganisms onto a moving nutrient source. We simulate trajectories of chemotactic and non-chemotactic bacteria encountering a sinking marine snow particle effusing soluble nutrients. We calculate the average nutrient concentration to which the bacteria are exposed, under regimes of strong and weak hydrodynamic trapping. We find that hydrodynamic trapping can significantly amplify (by ≈40%) the nutrient exposure of bacteria, both chemotactic and non-chemotactic. The subtle interactions between hydrodynamic and chemotactic effects reveal non-trivial variations in this “hydrodynamic amplification,” as a function of relevant biophysical parameters. Our study provides a consistent description of how microorganism motility, fluid flow and nutrient distribution affect foraging by marine microbes, and the formation of biofilms on spherical nutrient sources under the influence of fluid flow.

A Novel Soft Contact Piezo-Controlled Liquid Cell for Probing Polymer Films under Confinement using Synchrotron FTIR Microspectroscopy

Soft polymer films, such as polyelectrolyte multilayers (PEMs), are useful coatings in materials science. The properties of PEMs often rely on the degree of hydration, and therefore the study of these films in a hydrated state is critical to allow links to be drawn between their characteristics and performance in a particular application. In this work, we detail the development of a novel soft contact cell for studying hydrated PEMs (poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride)) using FTIR microspectroscopy. FTIR spectroscopy can interrogate the nature of the polymer film and the hydration water contained therein. In addition to reporting spectra obtained for hydrated films confined at the solid-solid interface, we also report traditional ATR FTIR spectra of the multilayer. The spectra (microspectroscopy and ATR FTIR) reveal that the PEM film build-up proceeds as expected based on the layer-by-layer assembly methodology, with increasing signals from the polymer FTIR peaks with increasing bilayer number. In addition, the spectra obtained using the soft contact cell indicate that the PEM film hydration water has an environment/degree of hydrogen bonding that is affected by the chemistry of the multilayer polymers, based on differences in the spectra obtained for the hydration water within the film compared to that of bulk electrolyte.

Cargo carrying bacteria at interfaces

The displacements of ensembles of colloids at the interface between oil and suspensions of the bacterium Pseudomonas aeruginosa PA14ΔpelA indicate enhanced colloid mobilities and apparently diffusive motion driven by interactions with the bacteria. However, inspection of individual trajectories of ∼500 particles reveals prolonged, directed displacements inconsistent with purely hydrodynamic interactions between swimming bacteria and colloids. Analysis of the properties of colloid paths indicates trajectories can be sorted into four distinct categories, including diffusive, persistent, curly, and mixed trajectory types. Non-diffusive trajectories are the norm, comprising 2/3 of the observed trajectories. Imaging of colloids in the interface reveals anisotropic assemblies formed by colloids decorated with one or more adhered bacteria that drive the colloids along these paths. The trajectories and enhanced transport result from individual colloids being moved as cargo by these adhered bacteria. The implications of these structures and open questions for interfacial transport are discussed and related to the active colloid literature.

Bacterial autoaggregation

Many bacteria, both environmental and pathogenic, exhibit the property of autoaggregation. In autoaggregation (sometimes also called autoagglutination or flocculation), bacteria of the same type form multicellular clumps that eventually settle at the bottom of culture tubes. Autoaggregation is generally mediated by self-recognising surface structures, such as proteins and exopolysaccharides, which we term collectively as autoagglutinins. Although a widespread phenomenon, in most cases the function of autoaggregation is poorly understood, though there is evidence to show that aggregating bacteria are protected from environmental stresses or host responses. Autoaggregation is also often among the first steps in forming biofilms. Here, we review the current knowledge on autoaggregation, the role of autoaggregation in biofilm formation and pathogenesis, and molecular mechanisms leading to aggregation using specific examples.

Flagellated microswimmers: Hydrodynamics in thin liquid films

The hydrodynamics of a flagellated microswimmer moving in thin films is discussed. The fully resolved hydrodynamics is exploited by solving the Stokes equations for the actual geometry of the swimmer. Two different interfaces are used to confine the swimmer: a bottom solid wall and a top air-liquid interface, as appropriate for a thin film. The swimmer follows curved clockwise trajectories that can converge towards an asymptotically stable circular path or can result in a collision with one of the two interfaces. A bias towards the air-liquid interface emerges. Slight changes in the swimmer geometry and film thickness strongly affect the resulting dynamics suggesting that a very reach phenomenology occurs in the presence of confinement. Under specific conditions, the swimmer follows a "crown-like" path. Implications for the motion of bacteria close to an air bubble moving in a microchannel are discussed.

Hydrodynamics-mediated trapping of micro-swimmers near drops

In this paper, we investigate the swimming characteristics and dynamics of a model micro-swimmer in the vicinity of a clean drop, and of a surfactant covered drop. We model the swimmer as a force dipole and utilize the image-singularity system to study the dynamical behavior of the swimmer. Motivated by bacterial bio-remediation of insoluble hydrocarbons (HCs) released during oil spills, we report the 'trapping characteristics' - critical trapping radius, basin of attraction and trapping time distribution - of deterministic and stochastic swimmers, as a function of viscosity ratio, and dimensionless surface viscosity. We find that addition of surfactant reduces the critical trapping radius of a drop by ∼30%. The basin of attraction though, is not affected acutely for any combination in the parameter space of viscosity ratio and surface viscosity. We also carry out a dynamical system analysis of our problem, for deterministic swimmers, to clarify the aforementioned concepts. For hydrodynamics combined with diffusion based motion, we note increments ranging from ∼5-25% in the interface-retention times of surfactant-laden drops, as compared to clean drops. These differences occur for low values of surface viscosity, and saturate rapidly as the surface viscosity increases. With potential applications in bioremediation, our results highlight the importance of considering dispersant-addition in oil spills involving insoluble hydrocarbons.