The impact of material properties, nutrient load and shear stress on biofouling in food industries

Biofouling in Membrane Bioreactors-Mitigation and Current Status: a Review

Biological fouling as termed biofouling is caused by varied living organisms and is difficult to eliminate from the environment thus becoming a major issue during membrane bioreactors. Biofouling in membrane bioreactors (MBRs) is a crucial problem in increasing liquid pressure due to reduced pore diameter, clogging of the membrane pores, and alteration of the chemical composition of the water which greatly limits the growth of MBRs. Thus, membrane biofouling and/or microbial biofilms is a hot research topic to improve the market competitiveness of the MBR technology. Though several antibiofouling strategies (addition of bioflocculant or sponge into MBRs) came to light, biological approaches are sustainable and more practicable. Among the biological approaches, quorum sensing-based biofouling control (so-called quorum quenching) is an interesting and promising tool in combating biofouling issues in the MBRs. Several review articles have been published in the area of membrane biofouling and mitigation approaches. However, there is no single source of information about biofouling and/or biofilm formation in different environmental settings and respective problems, antibiofilm strategies and current status, quorum quenching, and its futurity. Thus, the objectives of the present review were to provide latest insights on mechanism of membrane biofouling, quorum sensing molecules, biofilm-associated problems in different environmental setting and antibiofilm strategies, special emphasis on quorum quenching, and its futurity in the biofilm/biofouling control. We believe that these insights greatly help in the better understanding of biofouling and aid in the development of sustainable antibiofouling strategies.

A high-throughput integrated biofilm-on-a-chip platform for the investigation of combinatory physicochemical responses to chemical and fluid shear stress

Physicochemical conditions play a key role in the development of biofilm removal strategies. This study presents an integrated, double-layer, high-throughput microfluidic chip for real-time screening of the combined effect of antibiotic concentration and fluid shear stress (FSS) on biofilms. Biofilms of Escherichia coli LF82 and Pseudomonas aeruginosa were tested against gentamicin and streptomycin to examine the time dependent effects of concentration and FSS on the integrity of the biofilm. A MatLab image analysis method was developed to measure the bacterial surface coverage and total fluorescent intensity of the biofilms before and after each treatment. The chip consists of two layers. The top layer contains the concentration gradient generator (CGG) capable of diluting the input drug linearly into four concentrations. The bottom layer contains four expanding FSS chambers imposing three different FSSs on cultured biofilms. As a result, 12 combinatorial states of concentration and FSS can be investigated on the biofilm simultaneously. Our proof-of-concept study revealed that the reduction of E. coli biofilms was directly dependent upon both antibacterial dose and shear intensity, whereas the P. aeruginosa biofilms were not impacted as significantly. This confirmed that the effectiveness of biofilm removal is dependent on bacterial species and the environment. Our experimental system could be used to investigate the physicochemical responses of other biofilms or to assess the effectiveness of biofilm removal methods.

Hydrodynamic Effects on Biofilm Development and Recombinant Protein Expression

Hydrodynamics play an important role in the rate of cell attachment and nutrient and oxygen transfer, which can affect biofilm development and the level of recombinant protein production. In the present study, the effects of different flow conditions on the development of Escherichia coli biofilms and the expression of a model recombinant protein (enhanced green fluorescent protein, eGFP) were examined. Planktonic and biofilm cells were grown at two different flow rates in a recirculating flow cell system for 7 days: 255 and 128 L h−1 (corresponding to a Reynolds number of 4600 and 2300, respectively). The fluorometric analysis showed that the specific eGFP production was higher in biofilms than in planktonic cells under both hydrodynamic conditions (3-fold higher for 255 L h−1 and 2-fold higher for 128 L h−1). In the biofilm cells, the percentage of eGFP-expressing cells was on average 52% higher at a flow rate of 255 L h−1. Furthermore, a higher plasmid copy number (PCN) was obtained for the highest flow rate for both planktonic (244 PCN/cell versus 118 PCN/cell) and biofilm cells (43 PCN/cell versus 29 PCN/cell). The results suggested that higher flow velocities promoted eGFP expression in E. coli biofilms.

A Selection of Platforms to Evaluate Surface Adhesion and Biofilm Formation in Controlled Hydrodynamic Conditions

The early colonization of surfaces and subsequent biofilm development have severe impacts in environmental, industrial, and biomedical settings since they entail high costs and health risks. To develop more effective biofilm control strategies, there is a need to obtain laboratory biofilms that resemble those found in natural or man-made settings. Since microbial adhesion and biofilm formation are strongly affected by hydrodynamics, the knowledge of flow characteristics in different marine, food processing, and medical device locations is essential. Once the hydrodynamic conditions are known, platforms for cell adhesion and biofilm formation should be selected and operated, in order to obtain reproducible biofilms that mimic those found in target scenarios. This review focuses on the most widely used platforms that enable the study of initial microbial adhesion and biofilm formation under controlled hydrodynamic conditions—modified Robbins devices, flow chambers, rotating biofilm devices, microplates, and microfluidic devices—and where numerical simulations have been used to define relevant flow characteristics, namely the shear stress and shear rate.

Mechanisms and Impact of Biofilms and Targeting of Biofilms Using Bioactive Compounds-A Review

Biofilms comprising aggregates of microorganisms or multicellular communities have been a major issue as they cause resistance against antimicrobial agents and biofouling. To date, numerous biofilm-forming microorganisms have been identified, which have been shown to result in major effects including biofouling and biofilm-related infections. Quorum sensing (which describes the cell communication within biofilms) plays a vital role in the regulation of biofilm formation and its virulence. As such, elucidating the various mechanisms responsible for biofilm resistance (including quorum sensing) will assist in developing strategies to inhibit and control the formation of biofilms in nature. Employing biological control measures (such as the use of bioactive compounds) in targeting biofilms is of great interest since they naturally possess antimicrobial activity among other favorable attributes and can also possibly act as potent antibiofilm agents. As an effort to re-establish the current notion and understanding of biofilms, the present review discuss the stages involved in biofilm formation, the factors contributing to its development, the effects of biofilms in various industries, and the use of various bioactive compounds and their strategies in biofilm inhibition.

Effects of Hydrophilicity, Adhesion Work, and Fluid Flow on Biofilm Formation of PDMS in Microfluidic Systems

Polydimethylsiloxane (PDMS) has been the most widely used material in microfluidic systems, especially for cell biology applications. However, the antibacterial performance of PDMS in flow conditions has never been reported in the literature. In this paper, we analyzed the effects of contact angle (CA), adhesion force (work), and surface free energy on the antibacterial activities of PDMS by varying the ratio of curing agents (crosslinking degree) and surface modification with oxygen plasma. The results show that the Young's modulus has no particular effects on bacterial adhesion compared to the CAs of samples. For the first time, we analyzed the adhesion work (AW) effect on biofilm formation, and we found that biofilms tend to form on the surface with less AW. Furthermore, we analyzed the dual effect of hydrophilicity and shear force induced by fluid flow on the bacterial adhesion in PDMS microfluidic systems. We found that at low flow rates in microfluidic conditions, the adhesion of the bacteria on the PDMS surface is inhibited when the fluid flow exceeds a certain value. It required higher shear force to inhibit bacterial adhesion on the hydrophilic surface than on the hydrophobic surface. Therefore, hydrophilicity might be the dominant factor affecting bacterial adhesion.

Characterization of planktonic and biofilm cells from two filamentous cyanobacteria using a shotgun proteomic approach

Cyanobacteria promote marine biofouling with significant impacts. A qualitative proteomic analysis, by LC-MS/MS, of planktonic and biofilm cells from two cyanobacteria was performed. Biofilms were formed on glass and perspex at two relevant hydrodynamic conditions for marine environments (average shear rates of 4 s^-1 and 40 s^-1). For both strains and surfaces, biofilm development was higher at 4 s^-1. Biofilm development of Nodosilinea sp. LEGE 06145 was substantially higher than Nodosilinea sp. LEGE 06119, but no significant differences were found between surfaces. Overall, 377 and 301 different proteins were identified for Nodosilinea sp. LEGE 06145 and Nodosilinea sp. LEGE 06119. Differences in protein composition were more noticeable in biofilms formed under different hydrodynamic conditions than in those formed on different surfaces. Ribosomal and photosynthetic proteins were identified in most conditions. The characterization performed gives new insights into how shear rate and surface affect the planktonic to biofilm transition, from a structural and proteomics perspective.

Interactions between spoilage bacteria in tri-species biofilms developed under simulated meat processing conditions

The formation of biofilms in the food industry is a major issue, as they are a frequent source of contamination of products, which can result in significant economic losses for processors through spoilage of foods or pose serious health concerns for consumers when foodborne pathogens are present. In this study, experiments were carried out using CDC Biofilm Reactors to produce biofilms on two test surfaces (polystyrene and stainless steel coupons) under a regimen for simulated meat processing conditions (SMPC). This entailed a 12 day regimen of daily cycles of filling the reactors with a meat slurry and letting stand for 16 h, followed by draining and refilling with water for an 8 h period in order to mimic a possible scenario of fluctuating periods of nutrient availability and starvation in a meat processing facility. Strains of Pseudomonas fluorescens, Lactobacillus plantarum and Leuconostoc pseudomesenteroides were used for mono and mixed cultures biofilms as they are relevant spoilage bacteria in the meat processing industry. In monoculture, the viable cell densities (CFU/cm²) of the two lactic acid bacteria species tested were higher for biofilms grown on polystyrene as compared to those obtained on stainless steel, whereas viable cell numbers in P. fluorescens monoculture were surface-independent. Synergistic interactions were demonstrated during growth of multi-species biofilms. Results from experiments where one of the 3 strains was inoculated 24 h before introduction of the other two strains showed increased levels of L. plantarum within biofilms grown on both test surfaces. The model developed here serves as a baseline to study the interactions between potential spoilage bacteria during biofilm development.

Impact of modified diamond-like carbon coatings on the spatial organization and disinfection of mixed-biofilms composed of Escherichia coli and Pantoea agglomerans industrial isolates

This work investigated the effects of diamond-like carbon (DLC) coatings on the architecture and biocide reactivity of dual-species biofilms mimicking food processing contaminants. Biofilms were grown using industrial isolates of Escherichia coli and Pantoea agglomerans on bare stainless steel (SST) and on two DLC surface coatings (a-C:H:Si:O designated by SICON® and a-C:H:Si designated by SICAN) in order to evaluate their antifouling activities. Quantification and spatial organization in single- and dual-species biofilms were examined by confocal laser scanning microscopy (CLSM) using a strain specific labelling procedure. Those assays revealed that the E. coli isolate exhibited a higher adhesion to the modified surfaces and a decreased susceptibility to disinfectant in presence of P. agglomerans than alone in axenic culture. While SICON® reduced the short-term growth of E. coli in axenic conditions, both DLC surfaces increased the E. coli colonization in presence of P. agglomerans. However, both modified surfaces triggered a significantly higher log reduction of E. coli cells within mixed-species biofilms, thus the use of SICON® and SICAN surfaces may be a good approach to facilitate the disinfection process in critical areas of food processing plants. This study presents a new illustration of the importance of interspecies interactions in surface-associated community functions, and of the need to evaluate the effectiveness of hygienic strategies with relevant multi-species consortia.

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.

Surface conditioning with Escherichia coli cell wall components can reduce biofilm formation by decreasing initial adhesion

Bacterial adhesion and biofilm formation on food processing surfaces pose major risks to human health. Non-efficient cleaning of equipment surfaces and piping can act as a conditioning layer that affects the development of a new biofilm post-disinfection. We have previously shown that surface conditioning with cell extracts could reduce biofilm formation. In the present work, we hypothesized that E. coli cell wall components could be implicated in this phenomena and therefore mannose, myristic acid and palmitic acid were tested as conditioning agents. To evaluate the effect of surface conditioning and flow topology on biofilm formation, assays were performed in agitated 96-well microtiter plates and in a parallel plate flow chamber (PPFC), both operated at the same average wall shear stress (0.07 Pa) as determined by computational fluid dynamics (CFD). It was observed that when the 96-well microtiter plate and the PPFC were used to form biofilms at the same shear stress, similar results were obtained. This shows that the referred hydrodynamic feature may be a good scale-up parameter from high-throughput platforms to larger scale flow cell systems as the PPFC used in this study. Mannose did not have any effect on E. coli biofilm formation, but myristic and palmitic acid inhibited biofilm development by decreasing cell adhesion (in about 50%). These results support the idea that in food processing equipment where biofilm formation is not critical below a certain threshold, bacterial lysis and adsorption of cell components to the surface may reduce biofilm buildup and extend the operational time.

Using micro-patterned surfaces to inhibit settlement and biofilm formation by Bacillus subtilis

Biofilm is a biological complex caused by bacteria attachment to the substrates and their subsequent reproduction and secretion. This phenomenon reduces heat transfer efficiency and causes significant losses in treated sewage heat-recovering systems. This paper describes a physical approach to inhibit bacteria settlement and biofilm formation by Bacillus subtilis, which is the dominant species in treated sewage. Here, micro-patterned surfaces with different characteristics (stripe and cube) and dimensions (1-100 μm) were fabricated as surfaces of interest. Model sewage was prepared and a rotating coupon device was used to form the biofilms. Precision balance, scanning electron microscopy, and confocal laser scanning microscopy (CLSM) were employed to investigate the inhibitory effects and the mechanisms of the biofilm-surface interactions. The results have shown that surfaces with small pattern sizes (1 and 2 μm) all reduced biofilm formation significantly. Interestingly, the CLSM images showed that the surfaces do not play a role in "killing" the bacteria. These findings are useful for future development of new process surfaces on which bacteria settlement and biofilm formation can be inhibited or minimized.