A Thermoplastic Microsystem to Perform Antibiotic Susceptibility Testing by Monitoring Oxygen Consumption.

Antibiotic susceptibility testing (AST) is a routine procedure in diagnostic laboratories to determine pathogen resistance profiles toward antibiotics. The need for fast and accurate resistance results is rapidly increasing with a global rise in pathogen antibiotic resistance over the past years. Microfluidic technologies can enable AST with lower volumes, lower cell numbers, and a reduction in the sample-to-result time compared to state-of-the-art systems. We present a protocol to perform AST on a miniaturized nanoliter chamber array platform. The chambers are filled with antibiotic compounds and oxygen-sensing nanoprobes that serve as a viability indicator. The growth of bacterial cells in the presence of different concentrations of antibiotics is monitored; living cells consume oxygen, which can be observed as an increase of a luminesce signal within the growth chambers. Here, we demonstrate the technique using a quality control Escherichia coli strain, ATCC 35218. The AST requires 20 µL of a diluted bacterial suspension (OD600 = 0.02) and provides resistance profiles about 2-3 h after the inoculation. The microfluidic method can be adapted to other aerobic pathogens and is of particular interest for slow-growing strains.

Parallel study of transient dosing of antibiotics in a microfluidic device.

Microfluidic tools are well suited for studying bacteria as they enable the analysis of small colonies or single cells. However, current techniques for studying bacterial response to antibiotics are largely limited to static dosing. Here, we describe a microfluidic device and a method for entrapping and cultivating bacteria in hydrogel plugs. Ring-shaped isolation valves are used to define the shape of the plugs and also to control exposure of the plugs to the surrounding medium. We demonstrate bacterial cultivation, determination of the minimum inhibitory concentration of an antibiotic, and transient dosing of an antibiotic at sub-1-h doses. The transient dosing experiments reveal that at dose durations on the order of minutes, ampicillin's bactericidal effect has both a time and concentration dependency.

Synchronized Reagent Delivery in Double Emulsions for Triggering Chemical Reactions and Gene Expression.

Microfluidic methods for the formation of single and double emulsion (DE) droplets allow for the encapsulation and isolation of reactants inside nanoliter compartments. Such methods have greatly enhanced the toolbox for high-throughput screening for cell or enzyme engineering and drug discovery. However, remaining challenges in the supply of reagents into these enclosed compartments limit the applicability of droplet microfluidics. Here, a strategy is introduced for on-demand delivery of reactants in DEs. Lipid vesicles are used as reactant carriers, which are co-encapsulated in double emulsions and release their cargo upon addition of an external trigger, here the anionic surfactant sodium dodecyl sulfate (SDS). The reagent present inside the lipid vesicles stays isolated from the remaining content of the DE vessel until SDS enters the DE lumen and solubilizes the vesicles' lipid bilayer. The versatility of the method is demonstrated with two critical applications chosen as representative assays for high-throughput screening: the induction of gene expression in bacteria and the initiation of an enzymatic reaction. This method not only allows for the release of the lipid vesicle content inside DEs to be synchronized for all DEs but also for the release to be triggered at any desired time.

Microbial factories: monitoring vitamin B2 production by Escherichia coli in microfluidic cultivation chambers.

Microbial cells represent a standard production host for various important biotechnological products. Production yields can be increased by optimising strains and growth conditions and understanding deviations in production rates over time or within the microbial population. We introduce here microfluidic cultivation chambers for highly parallel studies on microbial cultures, enabling continuous biosynthesis monitoring of the industrially relevant product by Escherichia coli cells. The growth chambers are defined by ring-valves that encapsulate a volume of 200 pL when activated. Bacterial cells, labelled with magnetic beads, are inoculated in a small magnetic trap, positioned in the centre of each chamber. Afterwards, the ring-valves are partially activated, allowing for exchange reagents, such as the addition of fresh media or specific inducers of biosynthesis, while the bacterial cells and their progeny are maintained inside. On this platform, we monitor the production of riboflavin (vitamin B2). We used different variants of a riboflavin-overproducing bacterial strain with different riboflavin production levels and could distinguish them on the level of individual micro-colonies. In addition, we could also observe differences in the bacterial morphology with respect to the production. The presented platform represents a flexible microfluidic tool for further studies of microbial cell factories.

Real-Time Respiration Changes as a Viability Indicator for Rapid Antibiotic Susceptibility Testing in a Microfluidic Chamber Array.

Rapid identification of a pathogen and the measurement of its antibiotic susceptibility are key elements in the diagnostic process of bacterial infections. Microfluidic technologies offer great control over handling and manipulation of low sample volumes with the possibility to study microbial cultures on the single-cell level. Downscaling the dimensions of cultivation systems directly results in a lower number of bacteria required for antibiotic susceptibility testing (AST) and thus in a reduction of the time to result. The developed platform presented in this work allows the reading of pathogen resistance profiles within 2-3 h based on the changes of dissolved oxygen levels during bacterial cultivation. The platform contains hundreds of individual growth chambers prefilled with a hydrogel containing oxygen-sensing nanoprobes and different concentrations of antibiotic compounds. The performance of the developed platform is tested using quality control Escherichia coli strains (ATCC 25922 and ATCC 35218) in response to clinically relevant antibiotics. The results are in agreement with values given in reference guidelines and independent measurements using a clinical AST protocol. Finally, the platform is successfully used for the AST of an E. coli clinical isolate obtained from a patient blood culture.

3D deterministic lateral displacement (3D-DLD) cartridge system for high throughput particle sorting.

A new 3D architecture for the deterministic lateral displacement (DLD) microfluidic devices based on ultra-high aspect ratio arch shaped pillars is presented. The proposed system addresses the major flow rate and shear rate limitations of standard planar devices.

"Basicles": Microbial Growth and Production Monitoring in Giant Lipid Vesicles.

We present an optimized protocol to encapsulate bacteria inside giant unilamellar lipid vesicles combined with a microfluidic platform for real-time monitoring of microbial growth and production. The microfluidic device allows us to immobilize the lipid vesicles and record bacterial growth and production using automated microscopy. Moreover, the lipid vesicles retain hydrophilic molecules and therefore can be used to accumulate products of microbial biosynthesis, which we demonstrate here for a riboflavin-producing bacterial strain. We show that stimulation as well as inhibition of bacterial production can be performed through the liposomal membrane simply by passive diffusion of inducing or antibiotic compounds, respectively. The possibility to introduce as well as accumulate compounds in liposomal cultivation compartments represents great advantage over the current state of the art systems, emulsion droplets, and gel beads. Additionally, the encapsulation of bacteria and monitoring of individual lipid vesicles have been accomplished on a single microfluidic device. The presented system paves the way toward highly parallel microbial cultivation and monitoring as required in biotechnology, basic research, or drug discovery.

Simple Fabrication of Structured Magnetic Metallic Nano-Platelets for Bio-Analytical Applications.

This short communication presents a simple method of preparation of thin-metal nano-platelets utilizing metal sputtering and lift-off photolithography. The method offers complete control over size, shape and properties of nano-platelets of sub-micrometer thickness. Platelets with a thickness of 50⁻200 nm and with defined arbitrary shapes and sizes in the range of 15⁻300 µm were prepared from single or multiple metal layers by magnetron sputtering. Deposition of different metals in layers enabled fabrication of bi- or tri-metallic platelets with a magnetic core and differently composed surfaces. Highly reflective nano-platelets with a magnetic core allowed manipulation by magnetic fields, while different metallic surfaces served for functionalization by selected molecules. Submicron thin nano-platelets are extremely light (e.g., ~20 ng for a 100 µm × 100 µm × 0.1 µm gold nano-platelet) so that they can be attached to surfaces by only a few chemical bonds. At the same time their area is sufficiently large for simple optical recognition of their shape which is intended to label various characteristics depending on the specific surface functionalization of the given shape.

Resolution improvement of 3D stereo-lithography through the direct laser trajectory programming: Application to microfluidic deterministic lateral displacement device.

The vast majority of current microfluidic devices are produced using soft lithography, a technique with strong limitations regarding the fabrication of three-dimensional architectures. Additive manufacturing holds great promises to overcome these limitations, but conventional machines still lack the resolution required by most microfluidic applications. 3D printing machines based on two-photon lasers, in contrast, have the needed resolution but are too limited in speed and size of the global device. Here we demonstrate how the resolution of conventional stereolithographic machines can be improved by a direct programming of the laser path and can contribute to bridge the gap between the two above technologies, allowing the direct printing of features between 10 and 100 µm, corresponding to a large fraction of microfluidic applications. This strategy allows to achieve resolutions limited only by the physical size of the laser beam, decreasing by a factor at least 2× the size of the smallest features printable, and increasing their reproducibility by a factor 5. The approach was applied to produce an open microfluidic device with the reversible seal, integrating periodical patterns using the simple motifs, and validated by the fabrication of a deterministic lateral displacement particles sorting device. The sorting of polystyrene beads (diameter: 20 µm and 45 µm) was achieved with a specificity >95%, comparable with that achieved with arrays prepared by microlithography.

Detection of electrochemiluminescence from floating metal platelets in suspension.

We present a generation of electrochemiluminescence (ECL) signal, based on square shaped gold electrodes with a size of 50 µm positioned inside a fused silica capillary. The ECL was generated using electric pulses with duration in the range from 100 ms to 5 s and an electrical field strength from 300 V cm(-1) to 500 V cm(-1). We have demonstrated that the electrochemical reaction with detectable optical output can be produced using freely moving and thus disposable electrodes.

Application of thin metal film elements in bioanalysis.

Advanced metal deposition and microfabrication techniques enable preparation of metal surfaces with high precision and excellent control over their size and shape with subnanometer resolution. Thin metal films of different types and functions can be found in many analytical instruments. Surfaces with high optical quality serve as mirrors, beam splitters, antireflective coatings etc. Smooth metal coating is crucial in electron microscopy. Unique properties of the thin metal films are widely used in optical systems, as tools for sample manipulation but also for chemical sensing and detection. While some of the applications are widespread and belong to the basic curriculum in analytical chemistry, the newer or less common uses of thin metal films are well known only to the experts in the field. The purpose of this critical review is to highlight the role of thin metal films in bioanalysis and summarize some of their main applications in current bioanalytical instrumentation.

Fabrication and characterization of solid mercury amalgam electrodes for protein analysis.

Gold and carbon electrodes have been largely used as transducers in protein and DNA sensors and arrays. Liquid mercury electrodes, with potential windows allowing detection of DNA and protein reduction processes at highly negative potentials, were considered as useless in such arrays. Here, we show that solid amalgam electrode (SAE) arrays can be prepared as a substitution of liquid mercury in the analysis of the above biomacromolecules. Vacuum metal sputtering on a glass substrate, photolithography, and galvanic mercury amalgam formation were used for fabrication of an inexpensive disposable electrode array. The resulting ultrathin (less than 1 microm) amalgam microelectrodes were characterized with respect to influence of the electrode composition and size on the reproducibility and stability of electrochemical signals. Further characterization was performed using electron microscopy and the well-established ruthenium electrochemistry. Final, optimized, design was applied in protein analysis employing the recently described electrocatalytic chronopotentiometric peak H.