Bacteria in nature frequently grow as biofilms, yet little is known regarding how biofilm bacteria morphologically adapt to low nutrient availability, which is common in unsaturated environments such as the terrestrial subsurface or on plant leaves. For unsaturated biofilms, in which the substratum may provide all nutrients, what are the relationships between nutrition and cell size and shape-the simplest metrics of cellular morphology? To address this question, we cultured Pseudomonas aeruginosa, a ubiquitous gram-negative bacterium that is environmentally and medically important, on membranes overlaying solid media, and then measured cellular dimensions using atomic force microscopy (AFM). Nutrition was controlled chemically by media composition and physically by stacking membranes to increase the path length for nutrient diffusion. Under conditions of carbon-nitrogen imbalance, low carbon bioavailability, or increased nutrient diffusional path length, cells elongated while maintaining constant width. A mathematical relationship suggests that, by elongating, biofilm bacteria strategically enlarge their nutrient collection surface without substantially changing the ratio of surface area to volume (SA/V). We conclude that P. aeruginosa growing as unsaturated biofilm with a planar nutrient source morphologically adapt to starvation by elongating. This adaptation, if generalizable, differs from a better-understood starvation response (i.e., cell size decreases; thus SA/V in-creases) for planktonic bacteria in well-mixed environments.

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The role of the physicochemical and surface properties of NF/RO membranes influencing bacterial adhesion has been widely studied. However, there exists a poor understanding of the potential role membrane topographical heterogeneities can have on bacterial adhesion. Heterogeneities on material surfaces have been shown to influence bacterial adhesion and biofilm development. The purpose of this study was therefore to investigate whether the presence of membrane topographical heterogeneities had a significant role during bacterial adhesion as this could significantly impact on how biofouling develops on membranes during NF/RO operation. An extensive study was devised in which surface topographical heterogeneities from two commercial membranes, NF270 and BW30, were assessed for their role in the adhesion of two model organisms of different geometrical shapes, Pseudomonas fluorescens and Staphylococcus epidermidis. The influence of cross-flow velocity and permeate flux was also tested, as well as the angle to which bacteria adhered compared to the flow direction. Bacterial adhesion onto the membranes and in their surface topographical heterogeneities was assessed using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), fluorescence microscopy and image analysis. Results showed that up to 30% of total adhered cells were found in membrane defect areas when defect areas only covered up to 13% of the membrane surface area. This suggests that topographical heterogeneities may play a significant role in establishing environmental niches during the early stages of biofilm development. Furthermore, no noticeable difference between the angle of cell attachment in defect areas compared to the rest of the membrane surface was found.

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Fouling remains a prevalent and serious problem in industries using membrane processes.Efforts to mitigate fouling are improving, however, membrane fouling cannot be completely eliminated. Therefore fouling control via development of sustainable cleaning methods are crucial. Despite osmotic backwashing showing promise, little is understood about this cleaning method for removal of fouling from reverse osmosis (RO) membranes. This paper systematically examines how organic fouling characteristics and osmotic backwashing parameters influence cleaning efficiency. Alginic acid was used as a model foulant and numerous microscopy techniques, including confocal microscopy, scanning electron microscopy and atomic force microscopy were used to examine the membrane fouling before and after cleaning to gain a clearer understanding of the mechanisms involved. Increasing CaCl2 concentration in the fouling solution resulted in an increase in fouling layer thickness from 37 to 179 µm, due to the complexation of Ca 2+ and the carboxyl groups in the alginate.Osmotic backwashing efficiency with 0.7 M NaCl decreased as the fouling layer became thicker and the pure water flux (PWF) recovery decreased from 92% to 81%. Osmotic backwashing efficiency also decreased with increasing initial permeate flux, as less fouling was removed: the fouling generated at higher initial fluxes is largely irreversible, resulting in a denser and more compact fouling layer. In an effort to increase osmotic backwashing flux, a CaCl2 draw solution was used, however, the Ca 2+ ions were found to interact with the alginate in the fouling layer, rendering this method inefficient, when compared to NaCl draw solutions which originated similar osmotic backwashing fluxes. Interestingly, the fouling layer was found to swell from 16 µm to 141 µm, when osmotic backwashing was carried out with a NaCl draw solution, followed by contact with a low ionic strength solution used for PWF testing. This phenomenon does not occur to the same extent after backwashing with CaCl2. The same trends were obtained for bovine serum albumin (BSA) fouling, whilst humic acid (HA) did not display any swelling phenomena. However, it showed the same cleaning inefficiency when using CaCl2 as a draw solution.

The significance of nanofiltration membrane surface properties when interacting with microbeads with and without permeate flux was investigated. This was achieved by characterising the surface tension and zeta potential of micro beads and NF90 membranes to determine the colloid-membrane interaction forces. Dynamic adhesion assays under different ionic strengths (0.1M and 0.01M) and pH (5, 7, 9) were conducted. Experimental results showed that at high ionic strength, pH does not have a significant effect on adhesion rates, while at low ionic strength the adhesion rate increased at pH 7 (4.56 s -1 cm -2 ) compared to pH 5 and pH 9, with rates of 2.69 and 3.

In an effort to mitigate biofouling on thin film composite membranes such as nanofiltration and reverse osmosis, a myriad of different surface modification strategies has been published. The use of silver nanoparticles (Ag-NPs) has emerged as being particularly promising. Nevertheless, the stability of these surface modifications is still poorly understood, particularly under permeate flux conditions. Leaching or elution of Ag-NPs from the membrane surface can not only affect the antimicrobial characteristics of the membrane, but could also potentially present an environmental liability when applied in industrial-scale systems. This study sought to investigate the dynamics of silver elution and the bactericidal effect of an Ag-NP functionalised NF270 membrane. Inductively coupled plasma-atomic emission spectroscopy was used to show that the bulk of leached silver occurred at the start of experimental runs, and was found to be independent of salt or permeate conditions used. Cumulative amounts of leached silver did, however, stabilise following the initial release, and were shown to have maintained the biocidal characteristics of the modified membrane, as observed by a higher fraction of structurally damaged Pseudomonas fluorescens cells. These results highlight the need to comprehensively assess the time-dependent nature of bactericidal membranes.