Now showing 1 - 10 of 17
  • Publication
    A physical impact of organic fouling layers on bacterial adhesion during nanofiltration
    Organic conditioning films have been shown to alter properties of surfaces, such as hydrophobicity and surface free energy. Furthermore, initial bacterial adhesion has been shown to depend on the conditioning film surface properties as opposed to the properties of the virgin surface. For the particular case of nanofiltration membranes under permeate flux conditions, however, the conditioning film thickens to form a thin fouling layer. This study hence sought to determine if a thin fouling layer deposited on a nanofiltration membrane under permeate flux conditions governed bacterial adhesion in the same manner as a conditioning film on a surface. Thin fouling layers (less than 50 μm thick) of humic acid or alginic acid were formed on Dow Filmtec NF90 membranes and analysed using Atomic Force Microscopy (AFM), confocal microscopy and surface energy techniques. Fluorescent microscopy was then used to quantify adhesion of Pseudomonas fluorescens bacterial cells onto virgin or fouled membranes under filtration conditions.It was found that instead of adhering on or into the organic fouling layer, the bacterial cells penetrated the thin fouling layer and adhered directly to the membrane surface underneath. Contrary to what surface energy measurements of the fouling layer would indicate, bacteria adhered to a greater extent onto clean membranes (24 ± 3% surface coverage) than onto those fouled with humic acid (9.8 ± 4%) or alginic acid (7.5 ± 4%). These results were confirmed by AFM measurements which indicated that a considerable amount of energy (10−7 J/μm) was dissipated when attempting to penetrate the fouling layers compared to adhering onto clean NF90 membranes (10−15 J/μm). The added resistance of this fouling layer was thusly seen to reduce the number of bacterial cells which could reach the membrane surface under permeate conditions. This research has highlighted an important difference between fouling layers for the particular case of nanofiltration membranes under permeate flux conditions and surface conditioning films which should be considered when conducting adhesion experiments under filtration conditions. It has also shown AFM to be an integral tool for such experiments.
    Scopus© Citations 22  431
  • Publication
    The role of cell-surface interactions in bacterial initial adhesion and consequent biofilm formation on nanofiltration/reverse osmosis membranes
    Until recently, the realization that membrane biofouling during nanofiltration (NF) and reverse osmosis (RO) processes is an unavoidable occurrence, has led to a paradigm shift in which biofouling management approaches rather than biofouling prevention are now being considered. To implement this new concept, it is crucial to understand the fundamentals of cell-surface interactions during bacterial adhesion, a prerequisite to biofouling of membranes. As such, with membrane biofouling already being widely studied and documented, greater attention should be given to the factors involved in the initial bioadhesion onto membranes during NF/RO processes. This review focuses on the interactions between bacterial cells and NF/RO membranes, emphasizing the mechanisms of bacterial adhesion to NF/RO membranes with particular reference to the effects of micro-environmental conditions experienced at the membrane interface, such as feed-water composition, hydrodynamics, permeate flux and conditioning layers. This review also discusses membrane surface properties and how it relates to bacterial adhesion as well as latest advancements in antibacterial membranes, identifying areas that need further investigation.
    Scopus© Citations 214  1371
  • Publication
    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.
    Scopus© Citations 34  488
  • Publication
    Antifouling activity of enzyme-functionalized silica nanobeads
    The amelioration of biofouling in industrial processing equipment is critical for performance and reliability. While conventional biocides are effective in biofouling control, they are potentially hazardous to the environment and in some cases corrosive to materials. Enzymatic approaches have been shown to be effective and can overcome the disadvantages of traditional biocides, however they are typically uneconomic for routine biofouling control. The aim of this study was to design a robust and reusable enzyme-functionalized nano-bead system having biofilm dispersion properties. This work describes the biochemical covalent functionalization of silica-based nanobeads (hereafter referred to as Si-NanoB) with Proteinase K (PK). Results showed that PK-functionalized Si-NanoB are effective in dispersing both protein-based model biofilms and structurally altering Pseudomonas fluorescens biofilms, with significant decreases in surface coverage and thickness of 30.1% and 38.85%, respectively, while increasing surface roughness by 19 % following 24 h treatments on bacterial biofilms. This study shows that enzyme-functionalized nanobeads may potentially be an environmentally friendly and cost effective alternative to pure enzyme and chemical treatments.
    Scopus© Citations 18  395
  • Publication
    The effects of extrinsic factors on the structural and mechanical properties of Pseudomonas fluorescens biofilms: A combined study of nutrient concentrations and shear conditions
    The growth of biofilms on surfaces is a complicated process influenced by several environmental factors such as nutrient availability and fluid shear. In this study, combinations of growth conditions were selected for the study of Pseudomonas fluorescens biofilms including as cultivation time (24- or 48 h), nutrient levels (1:1 or 1:10 King B medium), and shear conditions (75 RPM shaking, 0.4 mL min −1 or 0.7 mL min −1). The use of Confocal Laser Scanning Microscopy (CLSM) determined biofilm structure, while liquid-phase Atomic Force Microscopy (AFM) techniques resolved the mechanical properties of biofilms. Under semi-static conditions, high nutrient environments led to more abundant biofilms with three times higher EPS content compared to biofilms grown under low nutrient conditions. AFM results revealed that biofilms formed under these conditions were less stiff, as shown by their Young's modulus values of 2.35 ± 0.08 kPa, compared to 4.98 ± 0.02 kPa for that of biofilms formed under low nutrient conditions. Under dynamic conditions, however, biofilms exposed to low nutrient conditions and high shear rates led to more developed biofilms compared to other tested dynamic conditions. These biofilms were also found to be significantly more adhesive compared to their counterparts grown at higher nutrient conditions.
    Scopus© Citations 30  34
  • Publication
    Biofilm recruitment under nanofiltration conditions: the influence of resident biofilm structural parameters on planktonic cell invasion
    (Wiley, 2017-12-01) ;
    It is now generally accepted that biofouling is inevitable in pressure-driven membrane processes for water purification. A large number of published articles describe the development of novel membranes in an effort to address biofouling in such systems. It is reasonable to assume that such membranes, even those with antimicrobial properties, when applied in industrial-scale systems will experience some degree of biofouling. In such a scenario, an understanding of the fate of planktonic cells, such as those entering with the feed water, has important implications with respect to contact killing particularly for membranes with antimicrobial properties. This study thus sought to investigate the fate of planktonic cells in a model nanofiltration biofouling system. Here, the interaction between auto-fluorescent Pseudomonas putida planktonic cells and 7-day-old Pseudomonas fluorescens resident biofilms was studied under permeate flux conditions in a nanofiltration cross flow system. We demonstrate that biofilm cell recruitment during nanofiltration is affected by distinctive biofilm structural parameters such as biofilm depth.
    Scopus© Citations 1  256
  • Publication
    Comparison of biomass detachment from two different Pseudomonas spp. biofilms under constant shear conditions
    In the context of biofilm development, detachment is of practical importance when placed in a biofilm management perspective. The objective of the present study was to examine biofilm structure and biofilm detachment under controlled conditions for two distinct microorganisms grown under constant shear conditions. Detached biofilm biomass was regularly collected and analysed over the course of 72 h biofilm growth by Pseudomonas putida and Pseudomonas fluorescens cells, and biofilm structural development assessed using confocal microscopy. The two Pseudomonas spp., which had very similar specific growth rates in planktonic culture, presented notably different characteristics in terms of biofilm morphology but their detachment behaviours over time were very similar. These findings underline the intrinsic complexity of the detachment phenomenon.
    Scopus© Citations 4  353
  • Publication
    Cicada Wing Surface Topography: An Investigation into the Bactericidal Properties of Nanostructural Features
    (American Chemical Society, 2015-11-09) ; ; ; ;
    Recently, the surface of the wings of the Psaltoda claripennis cicada species has been shown to possess bactericidal properties and it has been suggested that the nanostructure present on the wings was responsible for the bacterial death. We have studied the surface-based nanostructure and bactericidal activity of the wings of three different cicadas (Megapomponia intermedia, Ayuthia spectabile and Cryptotympana aguila) in order to correlate the relationship between the observed surface topographical features and their bactericidal properties. Atomic force microscopy and scanning electron microscopy performed in this study revealed that the tested wing species contained a highly uniform, nanopillar structure on the surface. The bactericidal properties of the cicada wings were investigated by assessing the viability of autofluorescent Pseudomonas fluorescens cells following static adhesion assays and targeted dead/live fluorescence staining through direct microscopic counting methods. These experiments revealed a 20-25% bacterial surface coverage on all tested wing species; however, significant bactericidal properties were observed in the M. intermedia and C. aguila species as revealed by the high dead:live cell ratio on their surfaces. The combined results suggest a strong correlation between the bactericidal properties of the wings and the scale of the nanotopography present on the different wing surfaces.
    Scopus© Citations 250  1203
  • Publication
    Revealing region-specific biofilm viscoelastic properties by means of a microrheological approach
    Particle-tracking microrheology is an in situ technique that allows quantification of biofilm material properties. It overcomes the limitations of alternative techniques such as bulk rheology or force spectroscopy by providing data on region specific material properties at any required biofilm location and can be combined with confocal microscopy and associated structural analysis.  This article describes single particle tracking microrheology combined with confocal laser scanning microscopy to resolve the biofilm structure in 3 dimensions and calculate the creep compliances locally. Samples were analysed from Pseudomonas fluorescens biofilms that were cultivated over two timescales (24hr and 48hr) and alternate ionic conditions (with and without calcium chloride supplementation).  The region-based creep compliance analysis showed that the creep compliance of biofilm void zones is the primary contributor to biofilm mechanical properties, contributing to the overall viscoelastic character. 
    Scopus© Citations 26  318
  • Publication
    Upon impact: the fate of adhering Pseudomonas fluorescens cells during Nanofiltration
    Nanofiltration (NF) is a high-pressure membrane filtration process increasingly applied in drinking water treatment and water reuse processes. NF typically rejects divalent salts, organic matter, and micropollutants. However, the efficiency of NF is adversely affected by membrane biofouling, during which microorganisms adhere to the membrane and proliferate to create a biofilm. Here we show that adhered Pseudomonas fluorescens cells under high permeate flux conditions are met with high fluid shear and convective fluxes at the membrane-liquid interface, resulting in their structural damage and collapse. These results were confirmed by fluorescent staining, flow cytometry, and scanning electron microscopy. This present study offers a 'first-glimpse' of cell damage and death during the initial phases of bacterial adhesion to NF membranes and raises a key question about the role of this observed phenomena during early-stage biofilm formation under permeate flux and cross-flow conditions.
      397Scopus© Citations 11