Membrane-aerated biofilm proton and oxygen flux during chemical toxin exposure

Effect of Cd(II) shock loading on performance, microbial enzymatic activity and microbial community in a sequencing batch reactor

The performance, microbial enzymatic activity and microbial community of a sequencing batch reactor (SBR) were explored under instantaneous Cd(II) shock loading. After a 24-h Cd(II) shock loading of 100 mg/L, the chemical oxygen demand and NH₄⁺-N removal efficiencies decreased significantly from 92.73% and 99.56% on day 22 to 32.73% and 43% on day 24, respectively, and then recovered to the normal values gradually. The specific oxygen utilization rate (SOUR), specific ammonia oxidation rate (SAOR), specific nitrite oxidation rate (SNOR), specific nitrite reduction rate (SNIRR) and specific nitrate reduction rate (SNRR) decreased by 64.81%, 73.28%, 77.77%, 56.84% and 52.46% on day 23 in comparison with the absence of Cd(II) shock loading, respectively, and they gradually returned to the normal levels. The changing trends of their associated microbial enzymatic activities including dehydrogenase, ammonia monooxygenase, nitrite oxidoreductase, nitrite reductase and nitrate reductase were in accordance with SOUR, SAOR, SNOR, SNIRR and SNRR, respectively. Cd(II) shock loading promoted the microbial reactive oxygen species production and lactate dehydrogenase release, indicating that instantaneous shock caused oxidative stress and damaged to cell membranes of the activated sludge. The microbial richness and diversity, and the relative abundance of Nitrosomonas and Thauera obviously decreased under the stress of Cd(II) shock loading. PICRUSt prediction showed that Cd (II) shock loading significantly affected Amino acid biosynthesis, Nucleoside and nucleotide biosynthesis. The present results are conducive to take adequate precautions to reduce the adverse effect on bioreactor performance in wastewater treatment systems.

Structural characteristics and microbial function of biofilm in membrane-aerated biofilm reactor for the biodegradation of volatile pyridine

In order to avoid the serious air pollution caused by the volatilization of high recalcitrant pyridine, membrane-aerated biofilm reactor (MABR) with bubble-free aeration was used in this study, with the structural characteristics and microbial function of biofilm emphasized. The results showed that as high as 0.6 kg·m^-3·d^-1 pyridine could be completely removed in MABR. High pyridine loading thickened the biofilm, but without obvious detachment observed. The distinct stratification of microbes and extracellular polymeric substances were shaped by elevated pyridine load, enhancing the structural heterogeneity of biofilm. The increased tryptophan-like substances as well as α-helix and β-sheet proportion in proteins stabilized the biofilm structure against high influent loading. Based on the identified intermediates, possible pyridine biodegradation pathways were proposed. Multi-omics analyses revealed that the metabolic pathways with initial hydroxylation and reduction reaction was enhanced at high pyridine loading. The functional genes were mainly associated with Pseudomonas and Delftia, might responsible for pyridine biodegradation. The results shed light on the effective treatment of wastewater containing recalcitrant pollutants such as pyridine via MABR.

Recent progress using membrane aerated biofilm reactors for wastewater treatment

The membrane biofilm reactor (MBfR), which is based on the counter diffusion of the electron donors and acceptors into the biofilm, represents a novel technology for wastewater treatment. When process air or oxygen is supplied, the MBfR is known as the membrane aerated biofilm reactor (MABR), which has high oxygen transfer rate and efficiency, promoting microbial growth and activity within the biofilm. Over the past few decades, laboratory-scale studies have helped researchers and practitioners understand the relevance of influencing factors and biological transformations in MABRs. In recent years, pilot- to full-scale installations are increasing along with process modeling. The resulting accumulated knowledge has greatly improved understanding of the counter-diffusional biological process, with new challenges and opportunities arising. Therefore, it is crucial to provide new insights by conducting this review. This paper reviews wastewater treatment advancements using MABR technology, including design and operational considerations, microbial community ecology, and process modeling. Treatment performance of pilot- to full-scale MABRs for process intensification in existing facilities is assessed. This paper also reviews other emerging applications of MABRs, including sulfur recovery, industrial wastewater, and xenobiotics bioremediation, space-based wastewater treatment, and autotrophic nitrogen removal. In conclusion, commercial applications demonstrate that MABR technology is beneficial for pollutants (COD, N, P, xenobiotics) removal, resource recovery (e.g., sulfur), and N₂O mitigation. Further research is needed to increase packing density while retaining efficient external mass transfer, understand the microbial interactions occurring, address existing assumptions to improve process modeling and control, and optimize the operational conditions with site-specific considerations.

Characterization of Anaerobic Biofilms Growing on Carbon Felt Bioanodes Exposed to Air

The role of oxygen in anodic biofilms is still a matter of debate. In this study, we tried to elucidate the structure and performance of an electrogenic biofilm that develops on air-exposed, carbon felt electrodes, commonly used in bioelectrochemical systems. By simultaneously recording the current density produced by the bioanode and dissolved oxygen concentration, both inside and in the vicinity of the biofilm, it was possible to demonstrate the influence of a protective aerobic layer present in the biofilm (mainly formed by Pseudomonas genus bacteria) that prevents electrogenic bacteria (such as Geobacter sp.) from hazardous exposure to oxygen during its normal operation. Once this protective barrier was deactivated for a long period of time, the catalytic capacity of the biofilm was severely affected. In addition, our results highlighted the importance of the material’s porous structure for oxygen penetration in the electrode.

Digital Proxy of a Bio-Reactor (DIYBOT) combines sensor data and data analytics to improve greywater treatment and wastewater management systems

Technologies to treat wastewater in decentralized systems are critical for sustainable development. Bioreactors are suitable for low-energy removal of inorganic and organic compounds, particularly for non-potable applications where a small footprint is required. One of the main problems associated with bioreactor use is sporadic spikes of chemical toxins, including nanoparticles. Here, we describe the development of DIYBOT (Digital Proxy of a Bio-Reactor), which enables remote monitoring of bioreactors and uses the data to inform decisions related to systems management. To test DIYBOT, a household-scale membrane aerated bioreactor with real-time water quality sensors was used to treat household greywater simulant. After reaching steady-state, silver nanoparticles (AgNP) representative of the mixture found in laundry wastewater were injected into the system to represent a chemical contamination. Measurements of carbon metabolism, effluent water quality, biofilm sloughing rate, and microbial diversity were characterized after nanoparticle exposure. Real-time sensor data were analyzed to reconstruct phase-space dynamics and extrapolate a phenomenological digital proxy to evaluate system performance. The management implication of the stable-focus dynamics, reconstructed from observed data, is that the bioreactor self-corrects in response to contamination spikes at AgNP levels below 2.0 mg/L. DIYBOT may help reduce the frequency of human-in-the-loop corrective management actions for wastewater processing.

Simultaneous p-nitrophenol and nitrogen removal in PNP wastewater treatment: Comparison of two integrated membrane-aerated bioreactor systems

The chemical p-nitrophenol (PNP) is a priority pollutant, and PNP wastewater is highly toxic and resistant to biodegradation. The traditional physical and chemical methods (adsorption, extraction, and oxidation) for treating PNP wastewater have the disadvantages of complicated processes, high costs and secondary pollution generation. In this study, two integrated membrane-aerated bioreactor systems (RA and RB) with anoxic and aerated zones were constructed to enhance PNP biodegradation. The results showed that a helical silicone rubber membrane module displayed a high oxygen supply rate under a low membrane aeration pressure, and the hydraulic flow state of the reactor approached ideal mixing. At an influent PNP concentration of 500 mg/L, the average removal rates of PNP, chemical oxygen demand (COD) and total nitrogen (TN) reached 95.86%, 89.77%, and 94.81%, respectively, for RA and 89.48%, 74.26% and 64.78%, respectively, for RB, indicating efficient simultaneous PNP and nitrogen removal. Compared with that of RB, the pre-anoxic zone in RA not only performed detoxification pretreatment but also enhanced PNP degradation and denitrification effects, which relieved the biological treatment burden of the subsequent aerated zone. Based on these comprehensive analyses of reactor performance, the hydroquinone pathway might be the main route in the aerobic degradation of PNP.

Effects of cadmium on calcium homeostasis in the white-rot fungus Phanerochaete chrysosporium

Due to the widespread application of white-rot fungi for the treatment of pollutants, it's crucial to exploit the special effects of pollutants on the microbes. Here, we studied the effects of cadmium on calcium homeostasis in the most studied white-rot fungus Phanerochaete chrysosporium. The response of P. chrysosporium to cadmium stress is concentration-dependent. A high concentration of cadmium caused the release of calcium from P. chrysosporium, while a hormesis effect was observed at a lower cadmium concentration (10 μM), which resulted in a significant increase in calcium uptake and reversed the decrease in cell viability. Calcium (50 μM) promoted cell viability (127.2% of control), which reflects that calcium can protect P. chrysosporium from environmental stress. Real-time changes in the Ca²⁺ and Cd²⁺ fluxes of P. chrysosporium were quantified using the noninvasive microtest technique. Ca²⁺ influx decreased significantly under cadmium exposure, and the Ca²⁺ channel was involved in Ca²⁺ and Cd²⁺ influx. The cadmium and/or calcium uptake results coupled with the real-time Ca²⁺ and Cd²⁺ influxes microscale signatures can enhance our knowledge of the homeostasis of P. chrysosporium with respect to cadmium stress, which may provide useful information for improving the bioremediation process.

Measuring Spatial and Temporal Oxygen Flux Near Plant Tissues Using a Self-Referencing Optrode

Self-referencing optrodic microsensing is a noninvasive method for measuring oxygen transport into/from tissues. The sensing mechanism is based on fluorescence quenching by molecular oxygen at the tip of a fiber-optic probe, and facilitates microscale spatial mapping and continuous monitoring at 100-350 mHz sampling frequency. Over the last decade, this technique has been applied for plant tissues, including roots, seeds, leaves, and flowers in both liquid and air. Here, we describe the operating principle of self-referencing optrodic microsensing for the study of plant tissues with a specific focus on juvenile roots.

Extracellular polymeric substances, microbial activity and microbial community of biofilm and suspended sludge at different divalent cadmium concentrations

The differences between biofilm and suspended sludge (S-sludge) in extracellular polymeric substances (EPS), microbial activity, and microbial community in an anoxic-aerobic sequencing batch biofilm reactor (SBBR) at different concentrations of divalent cadmium (Cd(II)) were investigated. As the increase of Cd(II) concentration from 0 to 50mgL(-1), the specific ammonium oxidation rate (SAOR), specific nitrite oxidation rate (SNOR), and specific nitrate reduction rate (SNRR) of biofilm decreased from 4.85, 5.22 and 45mgNg(-1) VSSh(-1) to 1.54, 2.38 and 26mgNg(-1)VSSh(-1), respectively, and the SAOR, SNOR and SNRR of S-sludge decreased from 4.80, 5.02 and 34mgNg(-1)VSSh(-1) to 1.46, 2.20 and 17mgNg(-1)VSSh(-1), respectively. Biofilm had higher protein (PN) content in EPS than S-sludge. Contrast to S-sludge, biofilm could provide Nitrobacter vulgaris, beta proteobacterium INBAF015, and Pseudoxanthomonas mexicana with the favorable conditions of growth and reproduction.

Monitoring sulfide-oxidizing biofilm activity on cement surfaces using non-invasive self-referencing microsensors

Microbially influenced corrosion (MIC) in concrete results in significant cost for infrastructure maintenance. Prior studies have employed molecular techniques to identify microbial community species in corroded concrete, but failed to explore bacterial activity and functionality during deterioration. In this study, biofilms of different sulfur-oxidizing bacteria compositions were developed on the surface of cement paste samples to simulate the natural ecological succession of microbial communities during MIC processes. Noninvasive, self-referencing (SR) microsensors were used to quantify real time changes of oxygen, hydrogen ion and calcium ion flux for the biofilm to provide more information about bacterial behavior during deterioration. Results showed higher transport rates in oxygen consumption, and hydrogen ion at 4 weeks than 2 weeks, indicating increased bacterial activity over time. Samples with five species biofilm had the highest hydrogen ion and calcium ion transport rates, confirming attribution of acidophilic sulfur-oxidizing microorganisms (ASOM). Differences in transport rates between three species samples and two species samples confirmed the diversity between Thiomonas intermedia and Starkeya novella. The limitations of SR sensors in corrosion application could be improved in future studies when combined with molecular techniques to identify the roles of major bacterial species in the deterioration process.

Physiological fluxes and antioxidative enzymes activities of immobilized Phanerochaete chrysosporium loaded with TiO2 nanoparticles after exposure to toxic pollutants in solution

Immobilized Phanerochaete chrysosporium loaded with TiO2 nanoparticles (PTNs) are novel high-value bioremediation materials for adsorbing cadmium and for degrading 2,4-dichlorophenol (2,4-DCP). The real-time changes in H(+) and O2 fluxes were measured using the noninvasive microtest technique (NMT). The H(+) influx increased after the addition of 2,4-DCP, and shifted to efflux following the addition of Cd(2+). The O2 flux decreased after the addition of both 2,4-DCP and Cd(2+). A larger Cd(2+) flux was immediately observed after exposure to 0.5mM Cd(2+) (-351.25 pmol cm(-2) s(-1)) than to 0.1 mM Cd(2+) (-107.47 pmol cm(-2) s(-1)). The removal of Cd(2+) by the PTNs increased more after treatment with the 0.5 mM exposure solution (27.6 mg g(-1)) than with the 0.1 mM exposure solution (3.49 mg g(-1)). The enzyme activities were analyzed to review the antioxidative defense system of PTNs in a solution containing various concentrations of Cd(2+). The activities of the coenzyme nicotinamide adenine dinucleotide (NADH) oxidase as well as the enzyme catalase (CAT) plateaued at 6.5 U g(-1) FW and 9.7 U g(-1) FW, respectively, after exposure to 0.25 mM Cd(2+). The activity of superoxide dismutase (SOD) increased gradually in solutions containing 0.1-0.6 mM Cd(2+), and eventually reached a maximum (68.86 U g(-1) FW). These results illustrate how the antioxidative defense system and the physiological fluxes of PTNs respond to the stress caused by toxic pollutants.

Stress responses as determinants of antimicrobial resistance in Pseudomonas aeruginosa: multidrug efflux and more

Pseudomonas aeruginosa is a notoriously antimicrobial-resistant organism that is increasingly refractory to antimicrobial chemotherapy. While the usual array of acquired resistance mechanisms contribute to resistance development in this organism a multitude of endogenous genes also play a role. These include a variety of multidrug efflux loci that contribute to both intrinsic and acquired antimicrobial resistance. Despite their roles in resistance, however, it is clear that these efflux systems function in more than just antimicrobial efflux. Indeed, recent data indicate that they are recruited in response to environmental stress and, therefore, function as components of the organism's stress responses. In fact, a number of endogenous resistance-promoting genes are linked to environmental stress, functioning as part of known stress responses or recruited in response to a variety of environmental stress stimuli. Stress responses are, thus, important determinants of antimicrobial resistance in P. aeruginosa. As such, they represent possible therapeutic targets in countering antimicrobial resistance in this organism.

Aggregation behavior of engineered nanoparticles and their impact on activated sludge in wastewater treatment

The ever-increasing daily use of engineered nanoparticles will lead to heightened levels of these materials in the environment. These nanomaterials will eventually go into the wastewater treatment plant (WWTP), therefore, resulting into a pressing need for information on their aggregation behavior and kinetics in the wastewater aqueous matrix. In this work, we dispersed two different metal oxide nanoparticles (ZnO and TiO2) into the influent of two different WWTPs. Through the time-resolved dynamic light scattering analysis and transmission electron microscopy, the metal oxide nanoparticles (NPs) were quite stably existed in the wastewater matrix with aggregates of diameter 300-400 nm after 4.5h or more suspension. We confirmed that the dissolved organic matters (DOMs) attributed to the stability of nanoparticles. No propensity of NPs to aggregate were observed in the presence of both monovalent and divalent electrolytes even at high concentrations up to 0.15 M in NaCl or 0.025 M in CaCl2, indicating that the destabilization of nanoparticles in the complicated wastewater matrix was not achieved by the compression of electrical double layer, therefore, their aggregation kinetics cannot be simply predicted by the classic Derjaguin-Landau-Verwey-Overbeek theory of colloidal stability. However, obvious aggregation of nanoparticles in the Al2(SO4)3 solution system was observed with the likely mechanism of bridging of the metal oxide nanoparticles and aggregates due to the formation of hydrous alumina (Al(OH)3·H2O) in the Al2(SO4)3 solution. In the wastewater matrix, we used the noninvasive measurement technology to detect the O2 flux of activated sludge before and after treatment with 1, 10 and 100 mg L(-1) NPs. The results confirmed that both ZnO and TiO2 NPs showed an adverse impact on the O2 uptake of activated sludge when the exposure time extended to 4.5 h.

The fluxes of H2O2 and O2 can be used to evaluate seed germination and vigor of Caragana korshinskii

Seed deterioration is detrimental to plant germplasm conservation, and predicting seed germination and vigor with reliability and sensitivity means is urgently needed for practical problems. We investigated the link between hydrogen peroxide (H2O2) flux, oxygen influx and seed vigor of Caragana korshinskii by the non-invasive micro-test technique (NMT). Some related physiological and biochemical changes in seeds were also determined to further explain the changes in the molecular fluxes. The results showed that there was a good linear relationship between germination and H2O2 flux, and that O2 influx was more suitable for assessing seed vigor. H2O2 flux changed relatively little initially, mainly affected by antioxidants (APX, CAT and GSH) and H2O2 content; afterward, the efflux increased more and more rapidly due to high membrane permeability. With the damage of mitochondrial respiration and membrane integrity, O2 influx was gradually reduced. We propose that monitoring H2O2 and O2 fluxes by NMT may be a reliable and sensitive method to evaluate seed germination and vigor.

New insight into molecular interaction of heavy metal pollutant--cadmium(II) with human serum albumin

Cadmium (Cd) is an extremely toxic metal commonly found as an environmental contaminant from industrial and agricultural sources, posing severe risks to human health. In this study, the binding mechanism of Cd(II)-human serum albumin (HSA) complex and the effect of Cd(II) on the conformational stability and structural state of HSA were comprehensively investigated through a series of efficient and appropriate methods. X-ray photoelectron spectroscopy accurately described the microenvironmental changes around protein C, N, and O atoms in the presence of Cd(II). Fluorescence results indicated that the probable mechanism of Cd(II)-HSA interaction is a static quenching process. Fourier transform infrared spectroscopy and dynamic light scattering showed Cd(II) complexation altered HSA conformation and the microenvironments of Trp and Tyr residues, accompanied by the size increases of HSA aggregates. This research will be helpful for understanding the toxic effects of Cd(II) on protein function in vivo.

A real-time, non-invasive, micro-optrode technique for detecting seed viability by using oxygen influx

Quantifying seed viability is required for seed bank maintenance. The classical methods for detecting seed viability are time consuming and frequently cause seed damage and unwanted germination. We have established a novel micro-optrode technique (MOT) to measure seed viability in a quick and non-invasive manner by measuring the oxygen influxes of intact seeds, approximately 10 seconds to screen one seed. Here, we used soybean, wheat, and oilseed rape as models to test our method. After 3-hour imbibition, oxygen influxes were recorded in real-time with the total measurement taking less than 5 minutes. The results indicated a significantly positive correlation between oxygen influxes and viability in all 3 seed types. We also established a linear equation between oxygen influxes and seed viability for each seed type. For measurements, seeds were kept in the early imbibition stage without germination. Thus, MOT is a reliable, quick, and low-cost seed viability detecting technique.

Emerging technologies for non-invasive quantification of physiological oxygen transport in plants

Oxygen plays a critical role in plant metabolism, stress response/signaling, and adaptation to environmental changes (Lambers and Colmer, Plant Soil 274:7-15, 2005; Pitzschke et al., Antioxid Redox Signal 8:1757-1764, 2006; Van Breusegem et al., Plant Sci 161:405-414, 2001). Reactive oxygen species (ROS), by-products of various metabolic pathways in which oxygen is a key molecule, are produced during adaptation responses to environmental stress. While much is known about plant adaptation to stress (e.g., detoxifying enzymes, antioxidant production), the link between ROS metabolism, O2 transport, and stress response mechanisms is unknown. Thus, non-invasive technologies for measuring O2 are critical for understanding the link between physiological O2 transport and ROS signaling. New non-invasive technologies allow real-time measurement of O2 at the single cell and even organelle levels. This review briefly summarizes currently available (i.e., mainstream) technologies for measuring O2 and then introduces emerging technologies for measuring O2. Advanced techniques that provide the ability to non-invasively (i.e., non-destructively) measure O2 are highlighted. In the near future, these non-invasive sensors will facilitate novel experimentation that will allow plant physiologists to ask new hypothesis-driven research questions aimed at improving our understanding of physiological O2 transport.

Toxic effects of cadmium (Cd²⁺) on anaerobic biomass: kinetic and metabolic implications

Cadmium ion (Cd(2+)) toxicity on anaerobic systems, used for organic matter removal, was assessed by studying its effect on kinetic parameters and metabolic changes. This fundamental study was performed in a continuous fixed bed anaerobic bioreactor that treated synthetic wastewater simulating domestic sewage. The biomass was immobilized on a fixed bed made of polyurethane foam. Under influent cadmium concentrations of 0.0, 0.4, 4.4 and 6.2 mg Cd(2+) L(-1) the organic matter removal efficiencies were 84%, 82%, 72% and 52%, respectively. At influent concentration of 6.2 mg Cd(2+) L(-1) the reactor had reached its limit for cadmium toxicity. In the removal of dissolved organic matter, the first-order apparent kinetic coefficients (k(1)(app)) were 0.84, 0.67 and 0.10 h(-1) for the operations with 0.0, 0.4 and 4.4 mg Cd(2+) L(-1), respectively. The apparent inhibition coefficient for cadmium (k(i)(app)) was 1.69 mg L(-1). Despite the toxic effects of cadmium on anaerobic organic matter removal at large Cd(2+) concentrations, the results demonstrated that the anaerobic process was suitable for cadmium concentrations below 29.8 mg Cd(2+) L(-1), considering the bioavailable fraction of adsorbed cadmium in the support when the cadmium influent concentration was 6.2 mg Cd(2+) L(-1).

Shear-induced detachment of biofilms from hollow fiber silicone membranes

A suite of techniques was utilized to evaluate the correlation between biofilm physiology, fluid-induced shear stress, and detachment in hollow fiber membrane aerated bioreactors. Two monoculture species biofilms were grown on silicone fibers in a hollow fiber membrane aerated bioreactors (HfMBR) to assess detachment under laminar fluid flow conditions. Both physiology (biofilm thickness and roughness) and nutrient mass transport data indicated the presence of a steady state mature biofilm after 3 weeks of development. Surface shear stress proved to be an important parameter for predicting passive detachment for the two biofilms. The average shear stress at the surface of Nitrosomonas europaea biofilms (54.5 ± 3.2 mPa) was approximately 20% higher than for Pseudomonas aeruginosa biofilms (45.8 ± 7.7 mPa), resulting in higher biomass detachment. No significant difference in shear stress was measured between immature and mature biofilms of the same species. There was a significant difference in detached biomass for immature vs. mature biofilms in both species. However, there was no difference in detachment rate between the two species.

Responses of Phanerochaete chrysosporium to toxic pollutants: physiological flux, oxidative stress, and detoxification

The white-rot fungus Phanerochaete chrysosporium has been widely used for the treatment of waste streams containing heavy metals and toxic organic pollutants. The development of fungal-based treatment technologies requires detailed knowledge of the relationship between bulk water quality and the physiological responses of fungi. A noninvasive microtest technique was used to quantify real-time changes in proton, oxygen, and cadmium ion fluxes following the exposure of P. chrysosporium to environmental toxic (2,4-dichlorophenol and cadmium). Significant changes in H(+) and O(2) flux occurred after exposure to 10 mg/L 2,4-dichlorophenol and 0.1 mM cadmium. Cd(2+) flux decreased with time. Reactive oxygen species formation and antioxidant levels increased after cadmium treatment. Superoxide dismutase activity correlated well with malondialdehyde levels (r(2) = 0.964) at low cadmium concentrations. However, this correlation diminished and malondialdehyde levels significantly increased at the highest cadmium concentration tested. Real-time microscale signatures of H(+), O(2), and Cd(2+) fluxes coupled with oxidative stress analysis can improve our understanding of the physiological responses of P. chrysosporium to toxic pollutants and provide useful information for the development of fungal-based technologies to improve the treatment of wastes cocontaminated with heavy metals and organic pollutants.

Bacterial stress responses as determinants of antimicrobial resistance

Bacteria encounter a myriad of stresses in their natural environments, including, for pathogens, their hosts. These stresses elicit a variety of specific and highly regulated adaptive responses that not only protect bacteria from the offending stress, but also manifest changes in the cell that impact innate antimicrobial susceptibility. Thus exposure to nutrient starvation/limitation (nutrient stress), reactive oxygen and nitrogen species (oxidative/nitrosative stress), membrane damage (envelope stress), elevated temperature (heat stress) and ribosome disruption (ribosomal stress) all impact bacterial susceptibility to a variety of antimicrobials through their initiation of stress responses that positively impact recruitment of resistance determinants or promote physiological changes that compromise antimicrobial activity. As de facto determinants of antimicrobial, even multidrug, resistance, stress responses may be worthy of consideration as therapeutic targets.

Pentachlorophenol induction of the Pseudomonas aeruginosa mexAB-oprM efflux operon: involvement of repressors NalC and MexR and the antirepressor ArmR

Pentachlorophenol (PCP) induced expression of the NalC repressor-regulated PA3720-armR operon and the MexR repressor-controlled mexAB-oprM multidrug efflux operon of Pseudomonas aeruginosa. PCP's induction of PA3720-armR resulted from its direct modulation of NalC, the repressor's binding to PA3720-armR promoter-containing DNA as seen in electromobility shift assays (EMSAs) being obviated in the presence of this agent. The NalC binding site was localized to an inverted repeat (IR) sequence upstream of PA3720-armR and overlapping a promoter region whose transcription start site was mapped. While modulation of MexR by the ArmR anti-repressor explains the upregulation of mexAB-oprM in nalC mutants hyperexpressing PA3720-armR, the induction of mexAB-oprM expression by PCP is not wholly explainable by PCP induction of PA3720-armR and subsequent ArmR modulation of MexR, inasmuch as armR deletion mutants still showed PCP-inducible mexAB-oprM expression. PCP failed, however, to induce mexAB-oprM in a mexR deletion strain, indicating that MexR was required for this, although PCP did not modulate MexR binding to mexAB-oprM promoter-containing DNA in vitro. One possibility is that MexR responds to PCP-generated in vivo effector molecules in controlling mexAB-oprM expression in response to PCP. PCP is an unlikely effector and substrate for NalC and MexAB-OprM - its impact on NalC binding to the PA3720-armR promoter DNA occurred only at high µM levels - suggesting that it mimics an intended phenolic effector/substrate(s). In this regard, plants are an abundant source of phenolic antimicrobial compounds and, so, MexAB-OprM may function to protect P. aeruginosa from plant antimicrobials that it encounters in nature.

Toxicological studies on silver nanoparticles: challenges and opportunities in assessment, monitoring and imaging

Silver nanoparticles (Ag NPs) are becoming increasingly prevalent in consumer products as antibacterial agents. The increased use of Ag NP-enhanced products may lead to an increase in toxic levels of environmental silver, but regulatory control over the use or disposal of such products is lagging due to insufficient assessment on the toxicology of Ag NPs and their rate of release into the environment. In this article we discuss recent research on the transport, activity and fate of Ag NPs at the cellular and organismic level, in conjunction with traditional and recently established methods of nanoparticle characterization. We include several proposed mechanisms of cytotoxicity based on such studies, as well as new opportunities for investigating the uptake and fate of Ag NPs in living systems.

A self referencing platinum nanoparticle decorated enzyme-based microbiosensor for real time measurement of physiological glucose transport

Glucose is the central molecule in many biochemical pathways, and numerous approaches have been developed for fabricating micro biosensors designed to measure glucose concentration in/near cells and/or tissues. An inherent problem for microsensors used in physiological studies is a low signal-to-noise ratio, which is further complicated by concentration drift due to the metabolic activity of cells. A microsensor technique designed to filter extraneous electrical noise and provide direct quantification of active membrane transport is known as self-referencing. Self-referencing involves oscillation of a single microsensor via computer-controlled stepper motors within a stable gradient formed near cells/tissues (i.e., within the concentration boundary layer). The non-invasive technique provides direct measurement of trans-membrane (or trans-tissue) analyte flux. A glucose micro biosensor was fabricated using deposition of nanomaterials (platinum black, multiwalled carbon nanotubes, Nafion) and glucose oxidase on a platinum/iridium microelectrode. The highly sensitive/selective biosensor was used in the self-referencing modality for cell/tissue physiological transport studies. Detailed analysis of signal drift/noise filtering via phase sensitive detection (including a post-measurement analytical technique) are provided. Using this highly sensitive technique, physiological glucose uptake is demonstrated in a wide range of metabolic and pharmacological studies. Use of this technique is demonstrated for cancer cell physiology, bioenergetics, diabetes, and microbial biofilm physiology. This robust and versatile biosensor technique will provide much insight into biological transport in biomedical, environmental, and agricultural research applications.