Determination of spatial distributions of zinc and active biomass in microbial biofilms by two-photon laser scanning microscopy

Active-bromide and surfactant synergy for enhanced microfouling control

Biofilms are structured microbial communities encased in a matrix of self-produced extracellular polymeric substance (EPS) and pose significant challenges in various industrial cooling systems. A nuclear power plant uses a biocide active-bromide for control of biological growth in its condenser cooling system. This study is aimed at evaluating the anti-bacterial and anti-biofilm efficacy of active-bromide against planktonic and biofilm-forming bacteria that are commonly encountered in seawater cooling systems. The results demonstrated that active-bromide at the concentration used at the power plant (1 ppm) exhibited minimal killing activity against Pseudomonas aeruginosa planktonic cells. The bacterial cell surface hydrophobicity assay using Staphylococcus aureus and P. aeruginosa indicated that Triton-X 100 significantly decreased the hydrophobicity of planktonic cells, enhancing the susceptibility of the cells to active-bromide. Biofilm inhibition assays revealed limited efficacy of active-bromide at 1 ppm concentration, but significant inhibition at 5 ppm and 10 ppm. However, the addition of a surfactant, Triton-X 100, in combination with 1 ppm active-bromide displayed a synergistic effect, leading to significant biofilm dispersal of pre-formed P. aeruginosa biofilms. This observation was substantiated by epifluorescence microscopy using a live/dead bacterial assay that showed the combination treatment resulted in extensive cell death within the biofilm, as indicated by a marked increase in red fluorescence, compared to treatments with either agent alone. These findings suggest that active bromide alone may be insufficient for microfouling control in the seawater-based condenser cooling system of the power plant. Including a biocompatible surfactant that disrupts established biofilms (microfouling) can significantly improve the efficacy of active bromide treatment.

Simultaneous Two- and Three-Photon Deep Imaging of Autofluorescence in Bacterial Communities

The intrinsic fluorescence of bacterial samples has a proven potential for label-free bacterial characterization, monitoring bacterial metabolic functions, and as a mechanism for tracking the transport of relevant components through vesicles. The reduced scattering and axial confinement of the excitation offered by multiphoton imaging can be used to overcome some of the limitations of single-photon excitation (e.g., scattering and out-of-plane photobleaching) to the imaging of bacterial communities. In this work, we demonstrate in vivo multi-photon microscopy imaging of Streptomyces bacterial communities, based on the excitation of blue endogenous fluorophores, using an ultrafast Yb-fiber laser amplifier. Its parameters, such as the pulse energy, duration, wavelength, and repetition rate, enable in vivo multicolor imaging with a single source through the simultaneous two- and three-photon excitation of different fluorophores. Three-photon excitation at 1040 nm allows fluorophores with blue and green emission spectra to be addressed (and their corresponding ultraviolet and blue single-photon excitation wavelengths, respectively), and two-photon excitation at the same wavelength allows fluorophores with yellow, orange, or red emission spectra to be addressed (and their corresponding green, yellow, and orange single-photon excitation wavelengths). We demonstrate that three-photon excitation allows imaging over a depth range of more than 6 effective attenuation lengths to take place, corresponding to an 800 micrometer depth of imaging, in samples with a high density of fluorescent structures.

Opportunities for visual techniques to determine characteristics and limitations of electro-active biofilms

Optimization of bio-electrochemical systems (BESs) relies on a better understanding of electro-active biofilms (EABfs). These microbial communities are studied with a range of techniques, including electrochemical, visual and chemical techniques. Even though each of these techniques provides very valuable and wide-ranging information about EABfs, such as performance, morphology and biofilm composition, they are often destructive. Therefore, the information obtained from EABfs development and characterization studies are limited to a single characterization of EABfs and often limited to one time point that determines the end of the experiment. Despite being scarcer and not as commonly reported as destructive techniques, non-destructive visual techniques can be used to supplement EABfs characterization by adding in-situ information of EABfs functioning and its development throughout time. This opens the door to EABfs monitoring studies that can complement the information obtained with destructive techniques. In this review, we provide an overview of visual techniques and discuss the opportunities for combination with the established electrochemical techniques to study EABfs. By providing an overview of suitable visual techniques and discussing practical examples of combination of visual with electrochemical methods, this review aims at serving as a source of inspiration for future studies in the field of BESs.

Treatment of high-load organic wastewater by novel basalt fiber carrier media

The carrier medium plays a key role in improving existing remediation potential of conventional biological contact oxidation reactors. In this study, a biological contact oxidation reactor was constructed using basalt fiber (R-BF) as a biological carrier. The bioreactor performance was investigated in terms of reduction in chemical oxygen demand (COD), ammonium nitrogen (NH₄⁺-N), and total nitrogen (TN) at organic loadings rate of 15.243 kg/m³·d and nitrogen loading rate of 1.068 kg/m³·d. We found that COD, NH₄⁺-N, and TN were reduced to 99.1%, 97.9%, and 97.8%, respectively. Within the R-BF, a bio-nest was developed which had abundant pores and channels and supported successful movement of nutrients, resulting in high biological activity (55.78%). The microbial communities within the bio-nest were diverse and rich and sludge production during operation was minimal. This makes BF a promising application for wastewater treatment. This research might be useful in the construction of integrated bioreactors that can operate under high organic and nitrogen loadings rates with reduced energy consumption, i.e. 75% in this study.

Biological-based methods for the removal of volatile organic compounds (VOCs) and heavy metals

The current scenario of increased population and industrial advancement leads to the spoliation of freshwater and tapper of the quality of water. These results decrease in freshwater bodies near all of the areas. Besides, organic and inorganic compounds discharged from different sources into the available natural water bodies are the cause of pollution. The occurrence of heavy metals in water and volatile organic compounds (VOCs) in the air is responsible for a vast range of negative impacts on the atmosphere and human health. Nonetheless, high uses of heavy metals for human purposes may alter the biochemical and geochemical equilibrium. The major air contaminants which are released into the surroundings known as VOCs are produced through different kinds of sources, such as petrochemical and pharmaceutical industries. VOCs are known to cause various health hazards. VOCs are a pivotal group of chemicals that evaporate readily at room temperature. To get over this problem, biofiltration technology has been evolved for the treatment of heavy metals using biological entities such as plants, algae, fungi, and bacteria. Biofiltration technology is a beneficial and sustainable method for the elimination of toxic pollutants from the aquatic environment. Various types of biological technologies ranging from biotrickling filters to biofilters have been developed and they are cost-effective, simple to fabricate, and easy to perform. A significant advantage of this process is the pollutant that is transformed into biodegradable trashes which can decompose within an average time period, thus yielding no secondary pollutants. The aim of this article is to scrutinize the role of biofiltration in the removal of heavy metals in wastewater and VOCs and also to analyze the recent bioremediation technologies and methods.

Bacillus subtilis Modulates Its Usage of Biofilm-Bound Iron in Response to Environmental Iron Availability

Iron (Fe) is one of the most important micronutrients for most life forms on earth. While abundant in soil, Fe bioavailability in oxic soil is very low. Under environmental conditions, bacteria need to acquire sufficient Fe to sustain growth while limiting the energy cost of siderophore synthesis. Biofilm formation might mitigate this Fe stress, since it was shown to accumulate Fe in certain Gram-negative bacteria and that this Fe could be mobilized for uptake. However, it is still unclear if, and to what extent, the amount of Fe accumulated in the biofilm can sustain growth and if the mobilization of this local Fe pool is modulated by the availability of environmental Fe (i.e., Fe outside the biofilm matrix). Here, we use a nondomesticated strain of the ubiquitous biofilm-forming soil bacterium Bacillus subtilis and stable Fe isotopes to precisely evaluate the origin of Fe during growth in the presence of tannic acid and hydroxides, used as proxies for different environmental conditions. We report that this B. subtilis strain can accumulate a large quantity of Fe in the biofilm, largely exceeding Fe associated with cells. We also report that only a fraction of biofilm-bound Fe is available for uptake in the absence of other sources of Fe in the vicinity of the biofilm. We observed that the availability of environmental Fe modulates the usage of this pool of biofilm-bound Fe. Finally, our data suggest that consumption of biofilm-bound Fe relates to the efficacy of B. subtilis to transport Fe from the environment to the biofilm, possibly through siderophores.IMPORTANCE Recent pieces of evidence suggest that Fe bound to the biofilm could assume at least two important functions, a local source of Fe for uptake and a support to extracellular metabolism, such as extracellular electron transfer. Our results show that B. subtilis can use biofilm-bound Fe for uptake only if it does not compromise Fe homeostasis of the biofilm, i.e., maintains a minimum Fe concentration in the biofilm for extracellular purposes. We propose a theoretical framework based on our results and recent literature to explain how B. subtilis manages biofilm-bound Fe and Fe uptake in response to environmental Fe availability. These results provide important insights into the management of biofilm-bound and environmental Fe by B. subtilis in response to Fe stress.

Feasibility of using basalt fiber as biofilm Carrier to construct bio-nest for wastewater treatment

Generally, biofilms developed for wastewater treatment readily detach from carrier medium once available thickness exceeds about 2 mm. Carrier media made of basalt fibers (BFs) could form ball-like aggregates (more than 10 cm in size, and called bio-nest). To demonstrate its feasibility for wastewater treatment, both reactors with and without BF carriers (RBF and RCO) were evaluated in terms of nutrient removal, oxygen mass transport and biological viabilities as well as biofilm adsorption characteristics. Therefore, oxygen microprofiles and confocal images for bio-nest as well as functional groups for biofilm-attached BF were performed on microsensor systems, confocal laser scanning microscopy (CLSM) and Fourier transform infrared (FTIR). Despite COD:N ratio, both reactors removed about 90% of COD, while only RBF reactor achieved high denitrification capabilities, with nitrogen removal efficiencies varying between 60.10 ± 0.45% and 82.07 ± 0.64%. Microprofile and confocal images showed that dissolved oxygen could reach the core with depth up to 50 mm, at which viable bacteria were detected. Characteristic peaks on FT-IR spectrum demonstrated that various functional groups of polysaccharide and proteins in EPS played a key role in aggregating biofilm-attached BFs into a bio-nest. Thus, BF provides a promising alternative to conventional carrier medium for wastewater treatment.

Influence of Household Water Filters on Bacteria Growth and Trace Metals in Tap Water of Doha, Qatar

Deteriorating water quality from aging infrastructure, growing threat of pollution from industrialization and urbanization, and increasing awareness about waterborne diseases are among the factors driving the surge in worldwide use of point-of-entry (POE) and point-of-use (POU) filters. Any adverse influence of such consumer point-of-use systems on quality of water at the tap remains poorly understood, however. We determined the chemical and microbiological changes in municipal water from the point of entry into the household plumbing system until it leaves from the tap in houses equipped with filters. We show that POE/POU devices can induce significant deterioration of the quality of tap water by functioning as traps and reservoirs for sludge, scale, rust, algae or slime deposits which promote microbial growth and biofilm formation in the household water distribution system. With changes in water pressure and physical or chemical disturbance of the plumbing system, the microorganisms and contaminants may be flushed into the tap water. Such changes in quality of household water carry a potential health risk which calls for some introspection in widespread deployment of POE/POU filters in water distribution systems.

When nanoparticles meet biofilms-interactions guiding the environmental fate and accumulation of nanoparticles

Bacteria are essential components of all natural and many engineered systems. The most active fractions of bacteria are now recognized to occur as biofilms, where cells are attached and surrounded by a secreted matrix of “sticky” extracellular polymeric substances. Recent investigations have established that significant accumulation of nanoparticles (NPs) occurs in aquatic biofilms. These studies point to the emerging roles of biofilms for influencing partitioning and possibly transformations of NPs in both natural and engineered systems. While attached biofilms are efficient “sponges” for NPs, efforts to elucidate the fundamental mechanisms guiding interactions between NPs and biofilms have just begun. In this mini review, special attention is focused on NP–biofilm interactions within the aquatic environment. We highlight key physical, chemical, and biological processes that affect interactions and accumulation of NPs by bacterial biofilms. We posit that these biofilm processes present the likely possibility for unique biological and chemical transformations of NPs. Ultimately, the environmental fate of NPs is influenced by biofilms, and therefore requires a more in-depth understanding of their fundamental properties.

Investigation of microbial biofilm structure by laser scanning microscopy

Microbial bioaggregates and biofilms are hydrated three-dimensional structures of cells and extracellular polymeric substances (EPS). Microbial communities associated with interfaces and the samples thereof may come from natural, technical, and medical habitats. For imaging such complex microbial communities confocal laser scanning microscopy (CLSM) is the method of choice. CLSM allows flexible mounting and noninvasive three-dimensional sectioning of hydrated, living, as well as fixed samples. For this purpose a broad range of objective lenses is available having different working distance and resolution. By means of CLSM the signals detected may originate from reflection, autofluorescence, reporter genes/fluorescence proteins, fluorochromes binding to specific targets, or other probes conjugated with fluorochromes. Recorded datasets can be used not only for visualization but also for semiquantitative analysis. As a result CLSM represents a very useful tool for imaging of microbiological samples in combination with other analytical techniques.

Reducing the risk of infection associated with vascular access devices through nanotechnology: a perspective

Intravascular catheter-related infections are still a major problem in health care and are associated with significant morbidity, mortality, and additional cost. The formation of microbial biofilm on catheters makes these infections particularly complicated, as microbial cells that detach from the biofilm can lead to infection, and because these microorganisms are highly resistant to many antimicrobial agents; thus, catheter removal is often required to successfully treat infection. To reduce the risks of catheter-related infections, many strategies have been applied, such as improvements in aseptic insertion and post-insertion care practices, implantation techniques, and antibiotic coated or impregnated materials. However, despite significant advances in using these methods, it has not been possible to completely eradicate biofilm infections. Currently, nanotechnology approaches seem to be among the most promising for preventing biofilm formation and resultant catheter-related bloodstream infection (especially with multi-resistant bacterial strains). In this review, current knowledge about catheter technology and design, the mechanisms of catheter-related bloodstream infection, and the insertion and care practices performed by medical staff, are discussed, along with novel, achievable approaches to infection prevention, based on nanotechnology.

The interaction of CuS and Halothiobacillus HT1 biofilm in microscale using synchrotron radiation-based techniques

In order to investigate the microbe-mineral interaction in the micro scale, spatial distribution and speciation of Cu and S in Halothiobacillus HT1 biofilm formed on a CuS surface was examined using synchrotron-based X-ray techniques. Confocal laser scanning microscope (CLSM) results indicated that Halothiobacillus HT1 biofilm formation gave rise to distinct chemical and redox gradients, leading to diverse niches in the biofilm. Live cells were distributed at the air-biofilm and membrane-biofilm interface. CuS was oxidized by Halothiobacillus HT1 biofilm, and copper penetrated into the biofilm. Sulfide was oxidized to cysteine (77.3%), sulfite (3.8%) and sulfonate (18.9%). Cu-cysteine-like species were involved in the copper homeostasis. These results significantly improve our understanding of the interfacial properties of the biofilm-mineral interface.

Enhancing the utility of existing antibiotics by targeting bacterial behaviour?

The discovery of novel classes of antibiotics has slowed dramatically. This has occurred during a time when the appearance of resistant strains of bacteria has shown a substantial increase. Concern is therefore mounting over our ability to continue to treat infections in an effective manner using the antibiotics that are currently available. While ongoing efforts to discover new antibiotics are important, these must be coupled with strategies that aim to maintain as far as possible the spectrum of activity of existing antibiotics. In many instances, the resistance to antibiotics exhibited by bacteria in chronic infections is mediated not by direct resistance mechanisms, but by the adoption of modes of growth that confer reduced susceptibility. These include the formation of biofilms and the occurrence of subpopulations of 'persister' cells. As our understanding of these processes has increased, a number of new potential drug targets have been revealed. Here, advances in our ability to disrupt these systems that confer reduced susceptibility, and in turn increase the efficacy of antibiotic therapy, are discussed.

Diffusion and activity of antibiotics against Burkholderia pseudomallei biofilms

The diffusion and activity of ceftazidime (CAZ), imipenem (IPM) and trimethoprim/sulfamethoxazole (TMP/SMX) against Burkholderia pseudomallei biofilms were comparatively tested using the high biofilm-producing strain B. pseudomallei 377 and the biofilm-defective mutant B. pseudomallei M6. Biofilms were generated by inoculation of bacteria on polycarbonate membranes placed on the surface of tryptic soy agar plates. The results showed that diffusion of TMP/SMX through B. pseudomallei biofilms was similar for both strains. However, diffusion of CAZ and IPM was significantly faster through strain M6 biofilm in comparison with strain 377 biofilm. The viabilities of strain 377 biofilm were significantly higher than those observed with strain M6 for all antibiotics challenged at 4 h, suggesting that the biofilm-forming capacity may be involved in antibiotic susceptibilities in B. pseudomallei. These results re-emphasise the importance of biofilm for antibiotic resistance in B. pseudomallei.

Manganese oxidation induced by water table fluctuations in a sand column

On-off cycles of production wells, especially in bank filtration settings, cause oscillations in the local water table, which can deliver significant amounts of dissolved oxygen (DO) to the shallow groundwater. The potential for DO introduced in this manner to oxidize manganese(II) (Mn(II)), mediated by the obligate aerobe Pseudomonas putida GB-1, was tested in a column of quartz sand fed with anoxic influent solution and subject to 1.3 m water table changes every 30-50 h. After a period of filter ripening, 100 μM Mn was rapidly removed during periods of low water table and high dissolved oxygen concentrations. The accumulation of Mn in the column was confirmed by XRF analysis of the sand at the conclusion of the study, and both measured net oxidation rates and XAS analysis suggest microbial oxidation as the dominant process. The addition of Zn, which inhibited GB-1 Mn oxidation but not its growth, interrupted the Mn removal process, but Mn oxidation recovered within one water table fluctuation. Thus transient DO conditions could support microbially mediated Mn oxidation, and this process could be more relevant in shallow groundwater than previously thought.

Sorption and distribution of copper in unsaturated Pseudomonas putida CZ1 biofilms as determined by X-ray fluorescence microscopy

The spatial and temporal distribution of metals in unsaturated Pseudomonas putida CZ1 biofilms was determined using synchrotron-based X-ray fluorescence microscopy (XRF). It was found that Fe, Mn, and Ca were mainly distributed near the air-biofilm interface of a biofilm grown on 40 mM citrate, while there were two Fe-, Mn-, and Ca-rich layers within a biofilm grown on 10 mM citrate. The sorption of copper by biofilm grown in medium containing 10 mM citrate was rapid, with copper being found throughout the biofilm after only 1 h of exposure. Copper initially colocalized with Fe and Mn element layers in the biofilm and then precipitated in a 40-μm-thick layer near the air-biofilm interface when exposed for 12 h. Cu K-edge X-ray absorption near edge structure (XANES) analysis revealed that Cu was primarily bound with citrate within the biofilm, and the precipitate formed in the biofilm exposed to copper for 12 h was most similar to copper phosphate. LIVE/DEAD staining revealed that cells at the biofilm-membrane interface were mostly alive even when the copper concentration reached 80.5 mg copper g(-1) biomass. This suggests that the biofilm matrix provided significant protection for cells in this area. These results significantly improve our understanding of metal acquisition, transportation, and immobilization in unsaturated biofilm systems.

Confocal Laser Microscopic Analysis of Biofilm on Newer Feldspar Ceramic

When the glazed surface of glass ceramics is submitted to chairside preparation, polishing with rubber points followed by felt disks impregnated with a fine-aluminum oxide particle appears be a good option to polish the surface of the glass ceramic, since this finishing-polishing method results in biofilm formation similar to the glazed ceramic surface.

Interactions of nanosilver with Escherichia coli cells in planktonic and biofilm cultures

Biofilms are often more resistant to toxic chemicals such as heavy metals and antimicrobial agents than planktonic cells. Nanosilver has a broad range of applications with strong antimicrobial activity. However, biofilm susceptibility to nanosilver toxicity is not well understood. We studied the bacterial activity in planktonic or biofilm cultures after nanosilver exposure using oxygen quenching fluorescence-based microrespirometry. We also determined the aggregation behavior and the spatial distribution of nanosilver having red fluorescence in biofilms of Escherichia coli expressing green fluorescent protein. At the same bacterial concentrations (3 × 10(8) CFU/mL), biofilms were about four times more resistant to nanosilver inhibition than planktonic cells. The minimum bactericidal concentrations (MBCs) of nanosilver (size from 15 to 21 nm), defined as the lowest concentration that kills at least 99.9% of a planktonic or biofilm bacterial population, were 38 and 10 mg/L Ag, respectively. For comparison, silver ions were more toxic to E. coli than nanosilver with MBCs of 2.4 and 1.2 mg/L Ag for planktonic and biofilm cultures, respectively. Nanosilver was aggregated in the presence of planktonic or biofilm-forming cells resulting in an increase of average particle size by a factor of 15 and 40, respectively. The nanosilver particles were able to penetrate to approximately 40 μm in a thick biofilm after 1-h exposure. These findings suggested that biofilm resistance to nanosilver could be at least partially due to nanoparticle aggregation and retarded silver ion/particle diffusion.

Three common metal contaminants of urban runoff (Zn, Cu & Pb) accumulate in freshwater biofilm and modify embedded bacterial communities

We investigated the absorption rates of zinc, copper and lead in freshwater biofilm and assessed whether biofilm bacterial populations are affected by exposure to environmentally relevant concentrations of these metals in flow chamber microcosms. Metals were rapidly accumulated by the biofilm and then retained for at least 14 days after transfer to uncontaminated water. Changes in bacterial populations were assessed by Automated Ribosomal Intergenic Spacer Analysis (ARISA) and 16S rRNA gene clone libraries. Significant differences in bacterial community structure occurred within only three days of exposure to metals and remained detectable at least 14 days after transfer to uncontaminated water. The rapid uptake of stormwater-associated metals and their retention in the biofilm highlight the potential role of biofilms in the transfer of metals to organisms at higher trophic levels. The sensitivity of stream biofilm bacterial populations to metal exposure supports their use as an indicator of stream ecological health.

Advanced imaging techniques for assessment of structure, composition and function in biofilm systems

Scientific imaging represents an important and accepted research tool for the analysis and understanding of complex natural systems. Apart from traditional microscopic techniques such as light and electron microscopy, new advanced techniques have been established including laser scanning microscopy (LSM), magnetic resonance imaging (MRI) and scanning transmission X-ray microscopy (STXM). These new techniques allow in situ analysis of the structure, composition, processes and dynamics of microbial communities. The three techniques open up quantitative analytical imaging possibilities that were, until a few years ago, impossible. The microscopic techniques represent powerful tools for examination of mixed environmental microbial communities usually encountered in the form of aggregates and films. As a consequence, LSM, MRI and STXM are being used in order to study complex microbial biofilm systems. This mini review provides a short outline of the more recent applications with the intention to stimulate new research and imaging approaches in microbiology.

Chapter 4: In vitro biofilm models: an overview

Observing naturally occurring biofilms in situ or ex situ has revealed the wide distribution of sessile microbial communities. The ubiquity, variety and complexity of biofilms is now widely accepted by microbiologists. While they are associated with many beneficial functions such as nutrient cycling, bioremediation and colonization resistance, adverse effects including recalcitrance, their involvement in industrial fouling, contamination and infection have made biofilms a priority research topic. We know that most biofilms, other than within certain infections and laboratory flasks, are composed of multiple species and that there is arguably no unifying biofilm architecture. Biofilms do however share certain properties including the presence of gradients of nutrients, gasses and metabolic products, relatively increased cell density, deposition of extracellular polymeric substances and marked recalcitrance towards antimicrobial treatments. Much of our understanding of biofilm physiology and micro-ecology originates from experiments using in vitro biofilm models. Broadly speaking, such models may be used to replicate environmental conditions within the laboratory or to focus on selected variables such a growth rate or fluid flow, etc. This chapter provides an overview of some commonly used biofilm models including microtitre plate systems, flow cells, the constant depth film fermenter, annular reactors and the perfused biofilm fermenter. While perfused biofilm fermenters, in particular, enable growth rate to be controlled within thin, relatively homogenous, quasi steady-state biofilms through modulation of flow rate nutrient availability, other models provide representative modelling of in situ conditions where steady states may be uncommon.

Exploiting advances in imaging technology to study biofilms by applying multiphoton laser scanning microscopy as an imaging and manipulation tool

Biofilms are an important element of the natural ecosystems but can be detrimental in health care and industrial settings. To improve our ability to combat biofilms, we need to understand the processes that facilitate their formation and dispersal. One approach that has proven to be invaluable is to image biofilms as they grow. Here we describe tools and protocols to visualize biofilms with multiphoton laser scanning microscopy, compare this with single photon laser scanning confocal microscopy and highlight best working procedures. Furthermore, we describe how with multiphoton laser scanning microscopy the laser can be used to manipulate the biofilm, specifically to achieve localized bleaching, killing or ablation within the biofilm biomass. These applications open novel ways to study the dynamics of biofilm formation, regeneration and dispersal.

Nickel promotes biofilm formation by Escherichia coli K-12 strains that produce curli

The survival of bacteria exposed to toxic compounds is a multifactorial phenomenon, involving well-known molecular mechanisms of resistance but also less-well-understood mechanisms of tolerance that need to be clarified. In particular, the contribution of biofilm formation to survival in the presence of toxic compounds, such as nickel, was investigated in this study. We found that a subinhibitory concentration of nickel leads Escherichia coli bacteria to change their lifestyle, developing biofilm structures rather than growing as free-floating cells. Interestingly, whereas nickel and magnesium both alter the global cell surface charge, only nickel promotes biofilm formation in our system. Genetic evidence indicates that biofilm formation induced by nickel is mediated by the transcriptional induction of the adhesive curli-encoding genes. Biofilm formation induced by nickel does not rely on efflux mechanisms using the RcnA pump, as these require a higher concentration of nickel to be activated. Our results demonstrate that the nickel-induced biofilm formation in E. coli is an adaptational process, occurring through a transcriptional effect on genes coding for adherence structures. The biofilm lifestyle is obviously a selective advantage in the presence of nickel, but the means by which it improves bacterial survival needs to be investigated.

Microbial extracellular polymeric substances: central elements in heavy metal bioremediation

Extracellular polymeric substances (EPS) of microbial origin are a complex mixture of biopolymers comprising polysaccharides, proteins, nucleic acids, uronic acids, humic substances, lipids, etc. Bacterial secretions, shedding of cell surface materials, cell lysates and adsorption of organic constituents from the environment result in EPS formation in a wide variety of free-living bacteria as well as microbial aggregates like biofilms, bioflocs and biogranules. Irrespective of origin, EPS may be loosely attached to the cell surface or bacteria may be embedded in EPS. Compositional variation exists amongst EPS extracted from pure bacterial cultures and heterogeneous microbial communities which are regulated by the organic and inorganic constituents of the microenvironment. Functionally, EPS aid in cell-to-cell aggregation, adhesion to substratum, formation of flocs, protection from dessication and resistance to harmful exogenous materials. In addition, exopolymers serve as biosorbing agents by accumulating nutrients from the surrounding environment and also play a crucial role in biosorption of heavy metals. Being polyanionic in nature, EPS forms complexes with metal cations resulting in metal immobilization within the exopolymeric matrix. These complexes generally result from electrostatic interactions between the metal ligands and negatively charged components of biopolymers. Moreover, enzymatic activities in EPS also assist detoxification of heavy metals by transformation and subsequent precipitation in the polymeric mass. Although the core mechanism for metal binding and / or transformation using microbial exopolymer remains identical, the existence and complexity of EPS from pure bacterial cultures, biofilms, biogranules and activated sludge systems differ significantly, which in turn affects the EPS-metal interactions. This paper presents the features of EPS from various sources with a view to establish their role as central elements in bioremediation of heavy metals.

Multimetal resistance and tolerance in microbial biofilms

Geochemical cycling and industrial pollution have made toxic metal ions a pervasive environmental pressure throughout the world. Biofilm formation is a strategy that microorganisms might use to survive a toxic flux in these inorganic compounds. Evidence in the literature suggests that biofilm populations are protected from toxic metals by the combined action of chemical, physical and physiological phenomena that are, in some instances, linked to phenotypic variation among the constituent biofilm cells. Here, we propose a multifactorial model by which biofilm populations can withstand metal toxicity by a process of cellular diversification.

In situ activity of suspended and immobilized microbial communities as measured by fluorescence lifetime imaging

In this study, the feasibility of fluorescence lifetime imaging (FLIM) for measurement of RNA:DNA ratios in microorganisms was assessed. The fluorescence lifetime of a nucleic acid-specific probe (SYTO 13) was used to directly measure the RNA:DNA ratio inside living bacterial cells. In vitro, SYTO 13 showed shorter fluorescence lifetimes in DNA solutions than in RNA solutions. Growth experiments with bacterial monocultures were performed in liquid media. The results demonstrated the suitability of SYTO 13 for measuring the growth-phase-dependent RNA:DNA ratio in Escherichia coli cells. The fluorescence lifetime of SYTO 13 reflected the known changes of the RNA:DNA ratio in microbial cells during different growth phases. As a result, the growth rate of E. coli cells strongly correlated with the fluorescence lifetime. Finally, the fluorescence lifetimes of SYTO 13 in slow- and fast-growing biofilms were compared. For this purpose, biofilms developed from activated sludge were grown as autotrophic and heterotrophic communities. The FLIM data clearly showed a longer fluorescence lifetime for the fast-growing heterotrophic biofilms and a shorter fluorescence lifetime for the slow-growing autotrophic biofilms. Furthermore, starved biofilms showed shorter lifetimes than biofilms supplied with glucose, indicating a lower RNA:DNA ratio in starved biofilms. It is suggested that FLIM in combination with SYTO 13 represents a useful tool for the in situ differentiation of active and inactive bacteria. The technique does not require radioactive chemicals and may be applied to a broad range of sample types, including suspended and immobilized microorganisms.

Low-load compression testing: a novel way of measuring biofilm thickness

Biofilms are complex and dynamic communities of microorganisms that are studied in many fields due to their abundance and economic impact. Biofilm thickness is an important parameter in biofilm characterization. Current methods of measuring biofilm thicknesses have several limitations, including application, availability, and costs. Here, we present low-load compression testing (LLCT) as a new method for measuring biofilm thickness. With LLCT, biofilm thicknesses are measured during compression by inducing small loads, up to 5 Pa, corresponding to 0.1% deformation, making LLCT essentially a nondestructive technique. Comparison of the thicknesses of various bacterial and yeasts biofilms obtained by LLCT and by using confocal laser scanning microscopy (CLSM) resulted in the conclusion that CLSM underestimates the biofilm thickness due to poor penetration of different fluorescent dyes, especially through the thicker biofilms, whereas LLCT does not suffer from this thickness limitation.

Zinc in wound healing: theoretical, experimental, and clinical aspects

Zinc is an essential trace element in the human body and its importance in health and disease is appreciated. It serves as a cofactor in numerous transcription factors and enzyme systems including zinc-dependent matrix metalloproteinases that augment autodebridement and keratinocyte migration during wound repair. Zinc confers resistance to epithelial apoptosis through cytoprotection against reactive oxygen species and bacterial toxins possibly through antioxidant activity of the cysteine-rich metallothioneins. Zinc deficiency of hereditary or dietary cause can lead to pathological changes and delayed wound healing. Oral zinc supplementation may be beneficial in treating zinc-deficient leg ulcer patients, but its therapeutic place in surgical patients needs further clarification. Topical administration of zinc appears to be superior to oral therapy due to its action in reducing superinfections and necrotic material via enhanced local defense systems and collagenolytic activity, and the sustained release of zinc ions that stimulates epithelialization of wounds in normozincemic individuals. Zinc oxide in paste bandages (Unna boot) protects and soothes inflamed peri-ulcer skin. Zinc is transported through the skin from these formulations, although the systemic effects seem insignificant. We present here the first comprehensive account of zinc in wound management in relation to current concepts of wound bed preparation and the wound-healing cascade. This review article suggests that topical zinc therapy is underappreciated even though clinical evidence emphasizes its importance in autodebridement, anti-infective action, and promotion of epithelialization.

Toward automated analysis of biofilm architecture: bias caused by extraneous confocal laser scanning microscopy images

An increasing number of studies utilize confocal laser scanning microscopy (CLSM) for in situ visualization of biofilms and rely on the use of image analysis programs to extract quantitative descriptors of architecture. Recently, designed programs have begun incorporating procedures to automatically determine threshold values for three-dimensional CLSM image stacks. We have found that the automated threshold calculation is biased when a stack contains images lacking pixels of biological significance. Consequently, we have created the novel program Auto PHLIP-ML to resolve this bias by iteratively excluding extraneous images based on their area coverage of biomass. A procedure was developed to identify the optimal percent area coverage value used for extraneous image removal (PACVEIR). The optimal PACVEIR was defined to occur when the standard deviation of mean thickness, determined from replicate image stacks, was at a maximum, because it more accurately reflected inherent structural variation. Ten monoculture biofilms of either Ralstonia eutropha JMP228n::gfp or Acinetobacter sp. strain BD413 were tested to verify the routine. All biofilms exhibited an optimal PACVEIR between 0 and 1%. Prior to the exclusion of extraneous images, JMP228n::gfp appeared to develop more homogeneous biofilms than BD413. However, after the removal of extraneous images, JMP228n::gfp biofilms were found to form more heterogeneous biofilms. Similarly, JMP228n::gfp biofilms grown on glass surfaces vis-à-vis polyethylene membranes produced significantly different architectures after extraneous images had been removed but not when such images were included in threshold calculations. This study shows that the failure to remove extraneous images skewed a seemingly objective analysis of biofilm architecture and significantly altered statistically derived conclusions.