DropSOAC: Stabilizing Microfluidic Drops for Time-Lapse Quantification of Single-Cell Bacterial Physiology

Synthetic gene circuit evolution: Insights and opportunities at the mid-scale

Directed evolution focuses on optimizing single genetic components for predefined engineering goals by artificial mutagenesis and selection. In contrast, experimental evolution studies the adaptation of entire genomes in serially propagated cell populations, to provide an experimental basis for evolutionary theory. There is a relatively unexplored gap at the middle ground between these two techniques, to evolve in vivo entire synthetic gene circuits with nontrivial dynamic function instead of single parts or whole genomes. We discuss the requirements for such mid-scale evolution, with hypothetical examples for evolving synthetic gene circuits by appropriate selection and targeted shuffling of a seed set of genetic components in vivo. Implementing similar methods should aid the rapid generation, functionalization, and optimization of synthetic gene circuits in various organisms and environments, accelerating both the development of biomedical and technological applications and the understanding of principles guiding regulatory network evolution.

Single-cell herpes simplex virus type 1 infection of neurons using drop-based microfluidics reveals heterogeneous replication kinetics

Single-cell analyses of viral infections reveal heterogeneity that is not detected by traditional population-level studies. This study applies drop-based microfluidics to investigate the dynamics of herpes simplex virus type 1 (HSV-1) infection of neurons at the single-cell level. We used micrometer-scale Matrigel beads, termed microgels, to culture individual murine superior cervical ganglia (SCG) neurons or epithelial cells. Microgel-cultured cells are encapsulated in individual media-in-oil droplets with a dual-fluorescent reporter HSV-1, enabling real-time observation of viral gene expression and replication. Infection within drops revealed that the kinetics of initial viral gene expression and replication were dependent on the inoculating dose. Notably, increasing inoculating doses led to earlier onset of viral gene expression and more frequent productive viral replication. These observations provide crucial insights into the complexity of HSV-1 infection in neurons and emphasize the importance of studying single-cell outcomes of viral infection. These techniques for cell culture and infection in drops provide a foundation for future virology and neurobiology investigations.

Droplet-based methodology for investigating bacterial population dynamics in response to phage exposure

An alarming rise in antimicrobial resistance worldwide has spurred efforts into the search for alternatives to antibiotic treatments. The use of bacteriophages, bacterial viruses harmless to humans, represents a promising approach with potential to treat bacterial infections (phage therapy). Recent advances in microscopy-based single-cell techniques have allowed researchers to develop new quantitative methodologies for assessing the interactions between bacteria and phages, especially the ability of phages to eradicate bacterial pathogen populations and to modulate growth of both commensal and pathogen populations. Here we combine droplet microfluidics with fluorescence time-lapse microscopy to characterize the growth and lysis dynamics of the bacterium Escherichia coli confined in droplets when challenged with phage. We investigated phages that promote lysis of infected E. coli cells, specifically, a phage species with DNA genome, T7 (Escherichia virus T7) and two phage species with RNA genomes, MS2 (Emesvirus zinderi) and Qβ (Qubevirus durum). Our microfluidic trapping device generated and immobilized picoliter-sized droplets, enabling stable imaging of bacterial growth and lysis in a temperature-controlled setup. Temporal information on bacterial population size was recorded for up to 25 h, allowing us to determine growth rates of bacterial populations and helping us uncover the extent and speed of phage infection. In the long-term, the development of novel microfluidic single-cell and population-level approaches will expedite research towards fundamental understanding of the genetic and molecular basis of rapid phage-induced lysis and eco-evolutionary aspects of bacteria-phage dynamics, and ultimately help identify key factors influencing the success of phage therapy.

Panoramic quantitative phase imaging of adherent live cells in a microfluidic environment

Understanding how cells respond to external stimuli is crucial. However, there are a lack of inspection systems capable of simultaneously stimulating and imaging cells, especially in their natural states. This study presents a novel microfluidic stimulation and observation system equipped with flat-fielding quantitative phase contrast microscopy (FF-QPCM). This system allowed us to track the behavior of organelles in live cells experiencing controlled microfluidic stimulation. Using this innovative imaging platform, we successfully quantified the cellular response to shear stress including directional cellular shrinkage and mitochondrial distribution change in a label-free manner. Additionally, we detected and characterized the cellular response, particularly mitochondrial behavior, under varying fluidic conditions such as temperature and drug induction time. The proposed imaging platform is highly suitable for various microfluidic applications at the organelle level. We advocate that this platform will significantly facilitate life science research in microfluidic environments.

Single-cell Herpes Simplex Virus type-1 infection of neurons using drop-based microfluidics reveals heterogeneous replication kinetics

Single-cell analyses of viral infections often reveal heterogeneity that is not detected by traditional population-level studies. This study applies drop-based microfluidics to investigate the dynamics of HSV-1 infection of neurons at the single-cell level. We used micron-scale Matrigel beads, termed microgels, to culture individual murine Superior Cervical ganglia (SCG) neurons or epithelial cells. Microgel-cultured cells are subsequently enclosed in individual media-in-oil droplets with a dual fluorescent-reporter HSV-1, enabling real-time observation of viral gene expression and replication. Infection within drops revealed that the kinetics of initial viral gene expression and replication were dependent on the inoculating dose. Notably, increasing inoculating doses led to earlier onset of viral gene expression and more frequent productive viral replication. These observations provide crucial insights into the complexity of HSV-1 infection in neurons and emphasize the importance of studying single-cell outcomes of viral infection. The innovative techniques presented here for cell culture and infection in drops provide a foundation for future virology and neurobiology investigations.

Droplet Array-Based Platform for Parallel Optical Analysis of Dynamic Extracellular Vesicle Secretion from Single Cells

Extracellular vesicles (EVs) are essential intercellular communication tools, but the regulatory mechanisms governing heterogeneous EV secretion are still unclear due to the lack of methods for precise analysis. Monitoring the dynamics of secretion from individually isolated cells is crucial because in bulk analysis, secretion activity can be perturbed by cell-cell interactions, and a cell population rarely performs secretion in a magnitude- or duration-synchronized manner. Although various microfluidic techniques have been adopted to evaluate the abundance of single-cell-derived EVs, none can track their secretion dynamics continually for extended periods. Here, we have developed a droplet array-based method that allowed us to optically quantify the EV secretion dynamics of >300 single cells every 2 h for 36 h, which covers the cell doubling time of many cell types. The experimental results clearly show the highly heterogeneous nature of single-cell EV secretion and suggest that cell division facilitates EV secretion, showing the usefulness of this platform for discovering EV regulation machinery.

Microfluidic platform for spatially segregated experimental evolution studies with E. coli

Microdroplet emulsions allow investigators to build controllable microenvironments for applications in experimental evolution and synthetic ecology. We designed a microfluidic platform that uses highly homogenous microdroplets to enable these experiments. We also present a step-by-step protocol for the rapid production of highly homogeneous microdroplets suitable for experimental evolution. We also describe protocols for the propagation and serial passage of microbial populations across a range of selection schemes and potential spatial structures.

For complete details on the use and execution of this protocol, please refer to Seo et al. (2021).

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Microfluidics for the study of microbial evolution and biomarker discovery

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Highly homogenous microdroplets as spatially segregated microenvironments

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Platform to identify evolutionary trajectories leading to antimicrobial resistance

Microfluidic devices for studying bacterial taxis, drug testing and biofilm formation

Some bacteria have coevolved to establish symbiotic or pathogenic relationships with plants, animals or humans. With human association, the bacteria can cause a variety of diseases. Thus, understanding bacterial phenotypes at the single-cell level is essential to develop beneficial applications. Traditional microbiological techniques have provided great knowledge about these organisms; however, they have also shown limitations, such as difficulties in culturing some bacteria, the heterogeneity of bacterial populations or difficulties in recreating some physical or biological conditions. Microfluidics is an emerging technique that complements current biological assays. Since microfluidics works with micrometric volumes, it allows fine-tuning control of the test conditions. Moreover, it allows the recruitment of three-dimensional (3D) conditions, in which several processes can be integrated and gradients can be generated, thus imitating physiological 3D environments. Here, we review some key microfluidic-based studies describing the effects of different microenvironmental conditions on bacterial response, biofilm formation and antimicrobial resistance. For this aim, we present different studies classified into six groups according to the design of the microfluidic device: (i) linear channels, (ii) mixing channels, (iii) multiple floors, (iv) porous devices, (v) topographic devices and (vi) droplet microfluidics. Hence, we highlight the potential and possibilities of using microfluidic-based technology to study bacterial phenotypes in comparison with traditional methodologies.

Droplet-based microsystems as novel assessment tools for oral microbial dynamics

The human microbiome comprises thousands of microbial species that live in and on the body and play critical roles in human health and disease. Recent findings on the interplay among members of the oral microbiome, defined by a personalized set of microorganisms, have elucidated the role of bacteria and yeasts in oral health and diseases including dental caries, halitosis, and periodontal infections. However, the majority of these studies rely on traditional culturing methods which are limited in their ability of replicating the oral microenvironment, and therefore fail to evaluate key microbial interactions in microbiome dynamics. Novel culturing methods have emerged to address this shortcoming. Here, we reviewed the potential of droplet-based microfluidics as an alternative approach for culturing microorganisms and assessing the oral microbiome dynamics. We discussed the state of the art and recent progress in the field of oral microbiology. Although at its infancy, droplet-based microtechnology presents an interesting potential for elucidating oral microbial dynamics and pathophysiology. We highlight how new findings provided by current microfluidic-based methodologies could advance the investigation of the oral microbiome. We anticipate that our work involving the droplet-based microfluidic technique with a semipermeable membrane will lay the foundations for future microbial dynamics studies and further expand the knowledge of the oral microbiome and its implication in oral health.

Single-Cell Technologies to Study Phenotypic Heterogeneity and Bacterial Persisters

Antibiotic persistence is a phenomenon in which rare cells of a clonal bacterial population can survive antibiotic doses that kill their kin, even though the entire population is genetically susceptible. With antibiotic treatment failure on the rise, there is growing interest in understanding the molecular mechanisms underlying bacterial phenotypic heterogeneity and antibiotic persistence. However, elucidating these rare cell states can be technically challenging. The advent of single-cell techniques has enabled us to observe and quantitatively investigate individual cells in complex, phenotypically heterogeneous populations. In this review, we will discuss current technologies for studying persister phenotypes, including fluorescent tags and biosensors used to elucidate cellular processes; advances in flow cytometry, mass spectrometry, Raman spectroscopy, and microfluidics that contribute high-throughput and high-content information; and next-generation sequencing for powerful insights into genetic and transcriptomic programs. We will further discuss existing knowledge gaps, cutting-edge technologies that can address them, and how advances in single-cell microbiology can potentially improve infectious disease treatment outcomes.

One cell at a time: droplet-based microbial cultivation, screening and sequencing

Microbes thrive and, in turn, influence the earth's environment, but most are poorly understood because of our limited capacity to reveal their natural diversity and function. Developing novel tools and effective strategies are critical to ease this dilemma and will help to understand their roles in ecology and human health. Recently, droplet microfluidics is emerging as a promising technology for microbial studies with value in microbial cultivating, screening, and sequencing. This review aims to provide an overview of droplet microfluidics techniques for microbial research. First, some critical points or steps in the microfluidic system are introduced, such as droplet stabilization, manipulation, and detection. We then highlight the recent progress of droplet-based methods for microbiological applications, from high-throughput single-cell cultivation, screening to the targeted or whole-genome sequencing of single cells.

Screening of Additive Formulations Enables Off-Chip Drop Reverse Transcription Quantitative Polymerase Chain Reaction of Single Influenza A Virus Genomes

The miniaturization of polymerase chain reaction (PCR) using drop-based microfluidics allows for amplification of single nucleic acids in aqueous picoliter-sized drops. Accurate data collection during PCR requires that drops remain stable to coalescence during thermocycling and drop contents are retained. Following systematic testing of known PCR additives, we identified an optimized formulation of 1% w/v Tween-20, 0.8 μg/μL bovine serum albumin, 1 M betaine in the aqueous phase, and 3 wt % (w/w) of the polyethylene glycol-perfluoropolyether2 surfactant in the oil phase of 50 μm diameter drops that maintains drop stability and prevents dye transport. This formulation enables a method we call off-chip drop reverse transcription quantitative PCR (OCD RT-qPCR) in which drops are thermocycled in a qPCR machine and sampled at various cycle numbers "off-chip", or outside of a microfluidic chip. qPCR amplification curves constructed from hundreds of individual drops using OCD RT-qPCR and imaged using epifluorescence microscopy correlate with amplification curves of ≈300,000 drops thermocycled using a qPCR machine. To demonstrate the utility of OCD RT-qPCR, influenza A virus (IAV) RNA was detected down to a single viral genome copy per drop, or 0.320 cpd. This work was extended to perform multiplexed detection of IAV M gene RNA and cellular β-actin DNA in drops, and direct amplification of IAV genomes from infected cells without a separate RNA extraction step. The optimized additive formulation and the OCD-qPCR method allow for drop-based RT-qPCR without complex devices and demonstrate the ability to quantify individual or rare nucleic acid species within drops with minimal processing.

Mineralogy of microbially induced calcium carbonate precipitates formed using single cell drop-based microfluidics

Microbe-mineral interactions are ubiquitous and can facilitate major biogeochemical reactions that drive dynamic Earth processes such as rock formation. One example is microbially induced calcium carbonate precipitation (MICP) in which microbial activity leads to the formation of calcium carbonate precipitates. A majority of MICP studies have been conducted at the mesoscale but fundamental questions persist regarding the mechanisms of cell encapsulation and mineral polymorphism. Here, we are the first to investigate and characterize precipitates on the microscale formed by MICP starting from single ureolytic E. coli MJK2 cells in 25 µm diameter drops. Mineral precipitation was observed over time and cells surrounded by calcium carbonate precipitates were observed under hydrated conditions. Using Raman microspectroscopy, amorphous calcium carbonate (ACC) was observed first in the drops, followed by vaterite formation. ACC and vaterite remained stable for up to 4 days, possibly due to the presence of organics. The vaterite precipitates exhibited a dense interior structure with a grainy exterior when examined using electron microscopy. Autofluorescence of these precipitates was observed possibly indicating the development of a calcite phase. The developed approach provides an avenue for future investigations surrounding fundamental processes such as precipitate nucleation on bacteria, microbe-mineral interactions, and polymorph transitions.

Photodegradable Hydrogels for Rapid Screening, Isolation, and Genetic Characterization of Bacteria with Rare Phenotypes

Screening mutant libraries (MLs) of bacteria for strains with specific phenotypes is often a slow and laborious process that requires assessment of tens of thousands of individual cell colonies after plating and culturing on solid media. In this report, we develop a three-dimensional, photodegradable hydrogel interface designed to dramatically improve the throughput of ML screening by combining high-density cell culture with precision extraction and the recovery of individual, microscale colonies for follow-up genetic and phenotypic characterization. ML populations are first added to a hydrogel precursor solution consisting of polyethylene glycol (PEG) o-nitrobenzyl diacrylate and PEG-tetrathiol macromers, where they become encapsulated into 13 μm thick hydrogel layers at a density of 90 cells/mm², enabling parallel monitoring of 2.8 × 10⁴ mutants per hydrogel. Encapsulated cells remain confined within the elastic matrix during culture, allowing one to track individual cells that grow into small, stable microcolonies (45 ± 4 μm in diameter) over the course of 72 h. Colonies with rare growth profiles can then be identified, extracted, and recovered from the hydrogel in a sequential manner and with minimal damage using a high-resolution, 365 nm patterned light source. The light pattern can be varied to release motile cells, cellular aggregates, or microcolonies encapsulated in protective PEG coatings. To access the benefits of this approach for ML screening, an Agrobacterium tumefaciens C58 transposon ML was screened for rare, resistant mutants able to grow in the presence of cell free culture media from Rhizobium rhizogenes K84, a well-known inhibitor of C58 cell growth. Subsequent genomic analysis of rare cells (9/28,000) that developed into microcolonies identified that seven of the resistant strains had mutations in the acc locus of the Ti plasmid. These observations are consistent with past research demonstrating that the disruption of this locus confers resistance to agrocin 84, an inhibitory molecule produced by K84. The high-throughput nature of the screen allows the A. tumefaciens genome (approximately 5.6 Mbps) to be screened to saturation in a single experimental trial, compared to hundreds of platings required by conventional plating approaches. As a miniaturized version of the gold-standard plating assay, this materials-based approach offers a simple, inexpensive, and highly translational screening technique that does not require microfluidic devices or complex liquid handling steps. The approach is readily adaptable to other applications that require isolation and study of rare or phenotypically pure cell populations.

Quantitation and Comparison of Phenotypic Heterogeneity Among Single Cells of Monoclonal Microbial Populations

Phenotypic heterogeneity within microbial populations arises even when the cells are exposed to putatively constant and homogeneous conditions. The outcome of this phenomenon can affect the whole function of the population, resulting in, for example, new "adapted" metabolic strategies and impacting its fitness at given environmental conditions. Accounting for phenotypic heterogeneity becomes thus necessary, due to its relevance in medical and applied microbiology as well as in environmental processes. Still, a comprehensive evaluation of this phenomenon requires a common and unique method of quantitation, which allows for the comparison between different studies carried out with different approaches. Consequently, in this study, two widely applicable indices for quantitation of heterogeneity were developed. The heterogeneity coefficient (HC) is valid when the population follows unimodal activity, while the differentiation tendency index (DTI) accounts for heterogeneity implying outbreak of subpopulations and multimodal activity. We demonstrated the applicability of HC and DTI for heterogeneity quantitation on stable isotope probing with nanoscale secondary ion mass spectrometry (SIP-nanoSIMS), flow cytometry, and optical microscopy datasets. The HC was found to provide a more accurate and precise measure of heterogeneity, being at the same time consistent with the coefficient of variation (CV) applied so far. The DTI is able to describe the differentiation in single-cell activity within monoclonal populations resolving subpopulations with low cell abundance, individual cells with similar phenotypic features (e.g., isotopic content close to natural abundance, as detected with nanoSIMS). The developed quantitation approach allows for a better understanding on the impact and the implications of phenotypic heterogeneity in environmental, medical and applied microbiology, microbial ecology, cell biology, and biotechnology.