Fluid-driven interfacial instabilities and turbulence in bacterial biofilms

Multispecies Bacterial Biofilms and Their Evaluation Using Bioreactors

Pathogenic biofilm formation within food processing industries raises a serious public health and safety concern, and places burdens on the economy. Biofilm formation on equipment surfaces is a rather complex phenomenon, wherein multiple steps are involved in bacterial biofilm formation. In this review we discuss the stages of biofilm formation, the existing literature on the impact of surface properties and shear stress on biofilms, types of bioreactors, and antimicrobial coatings. The review underscores the significance of prioritizing biofilm prevention strategies as a first line of defense, followed by control measures. Utilizing specific biofilm eradication strategies as opposed to a uniform approach is crucial because biofilms exhibit different behavioral outcomes even amongst the same species when the environmental conditions change. This review is geared towards biofilm researchers and food safety experts, and seeks to derive insights into the scope of biofilm formation, prevention, and control. The use of suitable bioreactors is paramount to understanding the mechanisms of biofilm formation. The findings provide useful information to researchers involved in bioreactor selection for biofilm investigation, and food processors in surfaces with novel antimicrobial coatings, which provide minimal bacterial attachment.

Magnetic Soft Robot for Minimally Invasive Urethral Catheter Biofilm Eradication

Catheter-related biofilm infection remains the main problem for millions of people annually, affecting morbidity, mortality, and quality of life. Despite the recent advances in the prevention of biofilm formation, alternative methods for biofilm prevention or eradication still should be found to avoid traumatic and expensive removal or catheter replacement. Soft magnetic robots have drawn significant interest in favor of remote control, fast response, and wide space for design. In this work, we demonstrated magnetic soft robots as a minimally invasive, safe, and effective approach to eliminate biofilm from urethral catheters (20 Fr or 5.1 mm in diameter). Seven designs of the robot were fabricated (size 4.5 × 15 mm), characterized, and tested in the presence of a rotating magnetic field. As a proof-of-concept, we demonstrated the superior efficiency of biofilm removal on the model of a urethral catheter using a magnetic robot, reaching full eradication for the octagram-shaped robot (velocity 2.88 ± 0.6 mm/s) at a 15 Hz frequency and a 10 mT amplitude. These findings are helpful for the treatment of biofilm-associated catheter contamination, which allows an increase in the catheter wearing time without frequent replacement and treatment of catheter-associated infections.

The biofilm matrix: multitasking in a shared space

The biofilm matrix can be considered to be a shared space for the encased microbial cells, comprising a wide variety of extracellular polymeric substances (EPS), such as polysaccharides, proteins, amyloids, lipids and extracellular DNA (eDNA), as well as membrane vesicles and humic-like microbially derived refractory substances. EPS are dynamic in space and time and their components interact in complex ways, fulfilling various functions: to stabilize the matrix, acquire nutrients, retain and protect eDNA or exoenzymes, or offer sorption sites for ions and hydrophobic substances. The retention of exoenzymes effectively renders the biofilm matrix an external digestion system influencing the global turnover of biopolymers, considering the ubiquitous relevance of biofilms. Physico-chemical and biological interactions and environmental conditions enable biofilm systems to morph into films, microcolonies and macrocolonies, films, ridges, ripples, columns, pellicles, bubbles, mushrooms and suspended aggregates - in response to the very diverse conditions confronting a particular biofilm community. Assembly and dynamics of the matrix are mostly coordinated by secondary messengers, signalling molecules or small RNAs, in both medically relevant and environmental biofilms. Fully deciphering how bacteria provide structure to the matrix, and thus facilitate and benefit from extracellular reactions, remains the challenge for future biofilm research.

The biofilm life cycle: expanding the conceptual model of biofilm formation

Bacterial biofilms are often defined as communities of surface-attached bacteria and are typically depicted with a classic mushroom-shaped structure characteristic of Pseudomonas aeruginosa. However, it has become evident that this is not how all biofilms develop, especially in vivo, in clinical and industrial settings, and in the environment, where biofilms often are observed as non-surface-attached aggregates. In this Review, we describe the origin of the current five-step biofilm development model and why it fails to capture many aspects of bacterial biofilm physiology. We aim to present a simplistic developmental model for biofilm formation that is flexible enough to include all the diverse scenarios and microenvironments where biofilms are formed. With this new expanded, inclusive model, we hereby introduce a common platform for developing an understanding of biofilms and anti-biofilm strategies that can be tailored to the microenvironment under investigation.

Fouling of mammalian hair fibres exposed to a titanium dioxide colloidal suspension

Fouling of surfaces in prolonged contact with liquid often leads to detrimental alteration of material properties and performance. A wide range of factors which include mass transport, surface properties and surface interactions dictate whether foulants are able to adhere to a surface. Passive means of foulant rejection, such as the microscopic patterns, have been known to develop in nature. In this work, we investigate the anti-fouling behaviour of animal fur and its apparent passive resistance to fouling. We compare the fouling performance of several categories of natural and manufactured fibres, and present correlations between contamination susceptibility and physio-mechanical properties of the fibre and its environment. Lastly, we present a correlation between the fouling intensity of a fibre and the cumulative impact of multiple interacting factors declared in the form of a dimensionless group. Artificial and natural hair strands exhibit comparable anti-fouling behaviour in flow, however, the absence of flow improves the performance of some artificial fibres. Among the plethora of factors affecting the fouling of fur hair, the dimensionless groups we present herein provide the best demarcation between fibres of different origin.

Effects of turbulence fluctuation intensity in bioreactor of sewage treatment on physical and chemical properties of biofilms

The flow velocity plays an important role in the growth and characteristics of biofilm in the bioreactor as well as its treatment efficiency, which has been a hot research topic. In a sewage treatment bioreactor, the type of flow is usually turbulence. According to the methods of Reynolds decomposition, the flow velocity of turbulence consists of the time-mean velocity and fluctuation velocity, which usually exist simultaneously in real flow. However, the current research on the influence of turbulence flow velocity mainly focuses on the time-mean velocity, while the fluctuation velocity has not been reported because of very difficult to control. To this end, in this paper, a laboratory oscillating-grid turbulence (OGT) bioreactor with zero time-mean velocity and only fluctuation velocity was designed. In this bioreactor, the fluctuation velocity could be easily manipulated by varying the operational parameters of the grid. Based on the numerical simulation of Gas-liquid two-phase flow, the distributions of fluctuation velocity and corresponding turbulence fluctuation intensity, gas holdup, and Reynolds stress were obtained. After that, the effects of the turbulent fluctuation intensity on the biofilm thickness, density, and composition of extracellular polymeric substances (EPS) were studied experimentally. The results showed that turbulent fluctuation had a significant effect on the physical and chemical properties of biofilms, and the fluctuation velocity promoted the increase in the biofilm density and the content of protein and carbohydrates in EPS. This study was intended to provide theoretical support for the design and operation optimization of bioreactors.

Hydrodynamics and surface properties influence biofilm proliferation

A biofilm is an interface-associated colloidal dispersion of bacterial cells and excreted polymers in which microorganisms find protection from their environment. Successful colonization of a surface by a bacterial community is typically a detriment to human health and property. Insight into the biofilm life-cycle provides clues on how their proliferation can be suppressed. In this review, we follow a cell through the cycle of attachment, growth, and departure from a colony. Among the abundance of factors that guide the three phases, we focus on hydrodynamics and stratum properties due to the synergistic effect such properties have on bacteria rejection and removal. Cell motion, whether facilitated by the environment via medium flow or self-actuated by use of an appendage, drastically improves the survivability of a bacterium. Once in the vicinity of a stratum, a single cell is exposed to near-surface interactions, such as van der Waals, electrostatic and specific interactions, similarly to any other colloidal particle. The success of the attachment and the potential for detachment is heavily influenced by surface properties such as material type and topography. The growth of the colony is similarly guided by mainstream flow and the convective transport throughout the biofilm. Beyond the growth phase, hydrodynamic traction forces on a biofilm can elicit strongly non-linear viscoelastic responses from the biofilm soft matter. As the colony exhausts the means of survival at a particular location, a set of trigger signals activates mechanisms of bacterial release, a life-cycle phase also facilitated by fluid flow. A review of biofilm-relevant hydrodynamics and startum properties provides insight into future research avenues.

Biofilm mechanics: Implications in infection and survival

It has long been recognized that biofilms are viscoelastic materials, however the importance of this attribute to the survival and persistence of these microbial communities is yet to be fully realized. Here we review work, which focuses on understanding biofilm mechanics and put this knowledge in the context of biofilm survival, particularly for biofilm-associated infections. We note that biofilm viscoelasticity may be an evolved property of these communities, and that the production of multiple extracellular polymeric slime components may be a way to ensure the development of biofilms with complex viscoelastic properties. We discuss viscoelasticity facilitating biofilm survival in the context of promoting the formation of larger and stronger biofilms when exposed to shear forces, promoting fluid-like behavior of the biofilm and subsequent biofilm expansion by viscous flow, and enabling resistance to both mechanical and chemical methods of clearance. We conclude that biofilm viscoelasticity contributes to the virulence of chronic biofilm infections.

Use of an oxygen planar optode to assess the effect of high velocity microsprays on oxygen penetration in a human dental biofilms in-vitro

BACKGROUND

Dental plaque biofilms are the causative agents of caries, gingivitis and periodontitis. Both mechanical and chemical strategies are used in routine oral hygiene strategies to reduce plaque build-up. If allowed to mature biofilms can create anoxic microenvironments leading to communities which harbor pathogenic Gram-negative anaerobes. When subjected to high velocity fluid jets and sprays biofilms can be fluidized which disrupts the biofilm structure and allows the more efficient delivery of antimicrobial agents.

METHODS

To investigate how such jets may disrupt anoxic niches in the biofilm, we used planar optodes to measure the dissolved oxygen (DO) concentration at the base of in-vitro biofilms grown from human saliva and dental plaque. These biofilms were subject to "shooting" treatments with a commercial high velocity microspray (HVM) device.

RESULTS

HVM treatment resulted in removal of much of the biofilm and a concurrent rapid shift from anoxic to oxic conditions at the base of the surrounding biofilm. We also assessed the impact of HVM treatment on the microbial community by tracking 7 target species by qPCR. There was a general reduction in copy numbers of the universal 16S RNA by approximately 95%, and changes of individual species in the target region ranged from approximately 1 to 4 log reductions.

CONCLUSION

We concluded that high velocity microsprays removed a sufficient amount of biofilm to disrupt the anoxic region at the biofilm-surface interface.

Computational and Experimental Investigation of Biofilm Disruption Dynamics Induced by High-Velocity Gas Jet Impingement

Knowledge of mechanisms promoting disruption though mechanical forces is essential in optimizing biofilm control strategies which rely on fluid shear. Our results provide insight into how biofilm disruption dynamics is governed by applied forces and fluid properties, revealing a mechanism for ripple formation and fluid-biofilm mixing. These findings have important implications for the rational design of new biofilm cleaning strategies with fluid jets, such as determining optimal parameters (e.g., jet velocity and position) to remove the biofilm from a certain zone (e.g., in dental hygiene or debridement of surgical site infections) or using antimicrobial agents which could increase the interfacial area available for exchange, as well as causing internal mixing within the biofilm matrix, thus disrupting the localized microenvironment which is associated with antimicrobial tolerance. The developed model also has potential application in predicting drag and pressure drop caused by biofilms on bioreactor, pipeline, and ship hull surfaces.

Experimental data showed that high-speed microsprays can effectively disrupt biofilms on their support substratum, producing a variety of dynamic reactions such as elongation, displacement, ripple formation, and fluidization. However, the mechanics underlying the impact of high-speed turbulent flows on biofilm structure is complex under such extreme conditions, since direct measurements of viscosity at these high shear rates are not possible using dynamic testing instruments. Here, we used computational fluid dynamics simulations to assess the complex fluid interactions of ripple patterning produced by high-speed turbulent air jets impacting perpendicular to the surface of Streptococcus mutans biofilms, a dental pathogen causing caries, captured by high-speed imaging. The numerical model involved a two-phase flow of air over a non-Newtonian biofilm, whose viscosity as a function of shear rate was estimated using the Herschel-Bulkley model. The simulation suggested that inertial, shear, and interfacial tension forces governed biofilm disruption by the air jet. Additionally, the high shear rates generated by the jet impacts coupled with shear-thinning biofilm property resulted in rapid liquefaction (within milliseconds) of the biofilm, followed by surface instability and traveling waves from the impact site. Our findings suggest that rapid shear thinning under very high shear flows causes the biofilm to behave like a fluid and elasticity can be neglected. A parametric sensitivity study confirmed that both applied force intensity (i.e., high jet nozzle air velocity) and biofilm properties (i.e., low viscosity and low air-biofilm surface tension and thickness) intensify biofilm disruption by generating large interfacial instabilities.

Impacts of a changing earth on microbial dynamics and human health risks in the continuum between beach water and sand

Although infectious disease risk from recreational exposure to waterborne pathogens has been an active area of research for decades, beach sand is a relatively unexplored habitat for the persistence of pathogens and fecal indicator bacteria (FIB). Beach sand, biofilms, and water all present unique advantages and challenges to pathogen introduction, growth, and persistence. These dynamics are further complicated by continuous exchange between sand and water habitats. Models of FIB and pathogen fate and transport at beaches can help predict the risk of infectious disease from beach use, but knowledge gaps with respect to decay and growth rates of pathogens in beach habitats impede robust modeling. Climatic variability adds further complexity to predictive modeling because extreme weather events, warming water, and sea level change may increase human exposure to waterborne pathogens and alter relationships between FIB and pathogens. In addition, population growth and urbanization will exacerbate contamination events and increase the potential for human exposure. The cumulative effects of anthropogenic changes will alter microbial population dynamics in beach habitats and the assumptions and relationships used in quantitative microbial risk assessment (QMRA) and process-based models. Here, we review our current understanding of microbial populations and transport dynamics across the sand-water continuum at beaches, how these dynamics can be modeled, and how global change factors (e.g., climate and land use) should be integrated into more accurate beachscape-based models.

Reduction in Pseudomonas aeruginosa and Staphylococcus aureus biofilms from implant materials in a diffusion dominated environment

Antibiotic-loaded calcium sulfate beads (CS-B) are used to treat biofilm-related periprosthetic joint infections (PJI). A previous study has shown that such beads are effective in reducing lawns biofilms grown on agar plates; however, the ability of CS-B to eradicate biofilms grown on solid orthopedic material surfaces has not been investigated. We grew biofilms of bioluminescent strains of Pseudomonas aeruginosa Xen41 and a USA300 MRSA Staphylococcus aureus SAP231 on ultra-high molecular weight polyethylene (PE), hydroxyapatite (HA), and 316L stainless steel (SS) coupons for three days under static growth conditions, with daily nutrient exchange. The coupons were rinsed with sterile phosphate buffered saline (PBS) to remove planktonic bacteria and placed in a petri dish, surrounded by four either antibiotic vancomycin and tobramycin loaded (CS-B_V+T ) or unloaded beads (CS-B_U ). A thin layer of agar was overlaid to simulate a periprosthetic infection where an implant abuts soft tissue and then incubated for 72 h. The amount of biofilm was measured by bioluminescence imaging (BLI) for activity and viable cell count (CFUs). Coupons exposed to CS-B_V+T showed a significant reduction in the amount of biofilm within 24 h, regardless of the bacterial strain or material type. The coupons exposed to control CS-B_U had no effect on bacteria over 72 h. Statement of Clinical Significance: Antibiotic-loaded calcium sulfate beads (CS-B) were effective in significantly reducing mature biofilms of P. aeruginosa and S. aureus from orthopedic relevant surfaces in our in vitro agar model. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3081-3085, 2018.

Computational Investigation of Ripple Dynamics in Biofilms in Flowing Systems

Biofilms are collections of microorganisms that aggregate using a self-produced matrix of extracellular polymeric substance. It has been broadly demonstrated that many microbial infections in the body, including dental plaque, involve biofilms. While studying experimental models of biofilms relevant to mechanical removal of oral biofilms, distinct ripple patterns have been observed. In this work, we describe a multiphase model used to approximate the dynamics of the biofilm removal process. We show that the fully nonlinear model provides a better representation of the experimental data than the linear stability analysis. In particular, we show that the full model more accurately reflects the relationship between the apparent wavelength and the external forcing velocities, especially at mid-to-low velocities at which the linear theory neglects important interactions. Finally, the model provides a framework by which the removal process (presumably governed by highly nonlinear behavior) can be studied.

Antibiotic loaded calcium sulfate bead and pulse lavage eradicates biofilms on metal implant materials in vitro

Pulse lavage (PL) debridement and antibiotic loaded calcium sulfate beads (CS-B) are both used for the treatment of biofilm related periprosthetic joint infection (PJI). However, the efficacy of these alone and in combination for eradicating biofilm from orthopaedic metal implant surfaces is unclear. The purpose of the study was to understand the efficacy of PL and antibiotic loaded CS-B in eradicating bacterial biofilms on 316L stainless steel (SS) alone and in combination in vitro. Biofilms of bioluminescent strains of Pseudomonas aeruginosa Xen41 and a USA300 MRSA Staphylococcus aureus SAP231 were grown on SS coupons for 3 days. The coupons were either, (i) debrided for 3 s with PL, (ii) exposed to tobramycin (TOB) and vancomycin (VAN) loaded CS-B for 24 h, or (iii) exposed to both. An untreated biofilm served as a control. The amount of biofilm was measured by bioluminescence, viable plate count and confocal microscopy using live/dead staining. PL alone reduced the CFU count of both strains of biofilms by approximately 2 orders of magnitude, from an initial cell count on metal surface of approximately 10⁹ CFU/cm² . The antibiotic loaded CS-B caused an approximate six log reduction and the combination completely eradicated viable biofilm bacteria. Bioluminescence and confocal imaging corroborated the CFU data. While PL and antibiotic loaded CS-B both significantly reduced biofilm, the combination of two was more effective than alone in removing biofilms from SS implant surfaces. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2349-2354, 2018.