Digital electrical impedance analysis for single bacterium sensing and antimicrobial susceptibility testing

Microfluidic technologies for advanced antimicrobial susceptibility testing

Antimicrobial resistance is getting serious and becoming a threat to public health worldwide. The improper and excessive use of antibiotics is responsible for this situation. The standard methods used in clinical laboratories, to diagnose bacterial infections, identify pathogens, and determine susceptibility profiles, are time-consuming and labor-intensive, leaving the empirical antimicrobial therapy as the only option for the first treatment. To prevent the situation from getting worse, evidence-based therapy should be given. The choosing of effective drugs requires powerful diagnostic tools to provide comprehensive information on infections. Recent progress in microfluidics is pushing infection diagnosis and antimicrobial susceptibility testing (AST) to be faster and easier. This review summarizes the recent development in microfluidic assays for rapid identification and AST in bacterial infections. Finally, we discuss the perspective of microfluidic-AST to develop the next-generation infection diagnosis technologies.

Automated and miniaturized screening of antibiotic combinations via robotic-printed combinatorial droplet platform

Antimicrobial resistance (AMR) has become a global health crisis in need of novel solutions. To this end, antibiotic combination therapies, which combine multiple antibiotics for treatment, have attracted significant attention as a potential approach for combating AMR. To facilitate advances in antibiotic combination therapies, most notably in investigating antibiotic interactions and identifying synergistic antibiotic combinations however, there remains a need for automated high-throughput platforms that can create and examine antibiotic combinations on-demand, at scale, and with minimal reagent consumption. To address these challenges, we have developed a Robotic-Printed Combinatorial Droplet (RoboDrop) platform by integrating a programmable droplet microfluidic device that generates antibiotic combinations in nanoliter droplets in automation, a robotic arm that arranges the droplets in an array, and a camera that images the array of thousands of droplets in parallel. We further implement a resazurin-based bacterial viability assay to accelerate our antibiotic combination testing. As a demonstration, we use RoboDrop to corroborate two pairs of antibiotics with known interactions and subsequently identify a new synergistic combination of cefsulodin, penicillin, and oxacillin against a model E. coli strain. We therefore envision RoboDrop becoming a useful tool to efficiently identify new synergistic antibiotic combinations toward combating AMR.

Rapid and Accurate Antimicrobial Susceptibility Testing Using Label-Free Electrical Impedance-Based Microfluidic Platform

Antimicrobial resistance has become a serious threat to the global public health. Accurate and rapid antimicrobial susceptibility testing (AST) allows evidence-based prescribing of antibiotics to improve patient care and clinical outcomes. Current culture-based AST assays are inherently limited by the doubling time of bacterial reproduction, which require at least 24 h to have a decisive result. Herein, a label-free electrical impedance-based microfluidic platform designed to expedite and streamline AST procedure for clinical practice is presented. Following a 30-min exposure of bacterial samples to antibiotics, the presented high-throughput, single-bacterium level impedance characterization platform enables a rapid 2-min AST assay. The platform facilitates accurate analysis of individual bacterial viability, as indicated by changes in electrical characteristics, thereby enabling the determination of antimicrobial resistance. Moreover, the potential clinical applicability of this platform is demonstrated by testing different E. coli strains against five antibiotics, yielding 100% categorical agreements compared to standard culture methods.

Inoculum size-insensitive susceptibility determination of urine sample based on in-situ measurement of inducible enzyme activity after 20 min of antibiotic exposure

BACKGROUND

The empirical antibiotic therapies for bacterial infections cause the emergence and propagation of multi-drug resistant bacteria, which not only impair the effectiveness of existing antibiotics but also raise healthcare costs. To reduce the empirical treatments, rapid antimicrobial susceptibility testing (AST) of causative microorganisms in clinical samples should be conducted for prescribing evidence-based antibiotics. However, most of culture-based ASTs suffer from inoculum effect and lack differentiation of target pathogen and commensals, hampering their adoption for evidence-based antibiotic prescription. Therefore, rapid ASTs which can specifically determine pathogens' susceptibilities, regardless of the bacterial load in clinical samples, are in urgent need.

RESULTS

We present a pathogen-specific and inoculum size-insensitive AST to achieve the reliable susceptibility determination on Escherichia coli (E. coli) in urine samples. The developed AST is featured with an 1 h sample-to-result workflow in a filter, termed on-filter AST. The AST results can be obtained by using an inducible enzymatic assay to in-situ measure the cell response of E. coli collected from urine after 20 min of antibiotic exposure. The calculated detection limit of our AST (1.95 × 104 CFU/mL) is much lower than the diagnosis threshold of urinary tract infections. The specific expression of the inducible enzyme enables on-filter AST to correctly profile the susceptibilities of target pathogen to multi-type antibiotics without the interference from commensals. We performed the on-filter AST on 1 mL urine samples with bacterial loads varying from 105 CFU/mL to 107 CFU/mL and compared the results to that of standard method, demonstrating its insensitivity to inoculum size.

SIGNIFICANCE

The developed AST is demonstrated to be of high sensitivity, specificity, and insensitive to inoculum size. With further developments for additional bacteria and clinical validation, on-filter AST is promising as a rapid and reliable surrogate of culture-based AST to promote the evidence-based prescription at the first visit and minimize the emergency of new multi-drug resistant microorganisms.

Rapid Minimum Inhibitory Concentration (MIC) Analysis Using Lyophilized Reagent Beads in a Novel Multiphase, Single-Vessel Assay

Antimicrobial resistance (AMR) is a global threat fueled by incorrect (and overuse) of antibiotic drugs, giving rise to the evolution of multi- and extreme drug-resistant bacterial strains. The longer time to antibiotic administration (TTA) associated with the gold standard bacterial culture method has been responsible for the empirical usage of antibiotics and is a key factor in the rise of AMR. While polymerase chain reaction (PCR) and other nucleic acid amplification methods are rapidly replacing traditional culture methods, their scope has been restricted mainly to detect genotypic determinants of resistance and provide little to no information on phenotypic susceptibility to antibiotics. The work presented here aims to provide phenotypic antimicrobial susceptibility testing (AST) information by pairing short growth periods (~3-4 h) with downstream PCR assays to ultimately predict minimum inhibitory concentration (MIC) values of antibiotic treatment. To further simplify the dual workflows of the AST and PCR assays, these reactions are carried out in a single-vessel format (PCR tube) using novel lyophilized reagent beads (LRBs), which store dried PCR reagents along with primers and enzymes, and antibiotic drugs separately. The two reactions are separated in space and time using a melting paraffin wax seal, thus eliminating the need to transfer reagents across different consumables and minimizing user interactions. Finally, these two-step single-vessel reactions are multiplexed by using a microfluidic manifold that allows simultaneous testing of an unknown bacterial sample against different antibiotics at varying concentrations. The LRBs used in the microfluidic system showed no interference with the bacterial growth and PCR assays and provided an innovative platform for rapid point-of-care diagnostics (POC-Dx).

A novel inductively coupled capacitor wireless sensor system for rapid antibiotic susceptibility testing

BACKGROUND

The increasing prevalence and severity of antimicrobial resistance (AMR) present a major challenge to our healthcare system. Rapid detection of AMR is essential for lifesaving under emergent conditions such as sepsis. The current gold standard phenotypic antibiotic susceptibility testing (AST) takes more than a day to obtain results. Genotypic ASTs are faster (hours) in detecting the presence of resistance genes but require specific probes/knowledge of each AMR gene and do not provide specific information at the phenotype level. To address this unmet challenge, we developed a new rapid phenotypic AST.

RESULT

We designed a new electrochemical biosensor based on the concept of magnetically coupled LC sensors. The engineered LC sensors can be placed in 96-well plates and communicate the reading remotely with a receiver coil for signal analysis. The sensors were validated by monitoring the growth of Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa in the presence and absence of different antibiotics. Drug-resistant strains were used as controls. Bacterial growth was detected within 30 min after inoculation, allowing rapid determination of antibiotic susceptibility at the phenotype level. The sensor also functions in the presence of host proteins when tested with 2% FBS in growth media.

CONCLUSIONS

With the compatibility with 96-well plates, this label-free rapid 30-min AST has the potential for low-cost applications with simple integration into the existing workflow in clinical settings.

Direct single-cell antimicrobial susceptibility testing of Escherichia coli in urine using a ready-to-use 3D microwell array chip

Empirical antibiotic therapies are prescribed for treating uncomplicated urinary tract infections (UTIs) due to the long turnaround time of conventional antimicrobial susceptibility testing (AST), leading to the prevalence of multi-drug resistant pathogens. We present a ready-to-use 3D microwell array chip to directly conduct comprehensive AST of pathogenic agents in urine at the single-cell level. The developed device features a highly integrated 3D microwell array, offering a dynamic range from 10² to 10⁷ CFU mL^-1, and a capillary valve-based flow distributor for flow equidistribution in dispensing channels and uniform sample distribution. The chip with pre-loaded reagents and negative pressure inside only requires the user to initiate AST by loading samples (∼3 s) and can work independently. We demonstrate an accessible sample-to-result workflow, including syringe filter-based bacteria separation and rapid single-cell AST on chip, which enables us to bypass the time-consuming bacteria isolation and pre-culture, speeding up the AST in ∼3 h from 2 days of conventional methods. Moreover, the bacterial concentration and AST with minimum inhibitory concentrations can be assessed simultaneously to provide comprehensive information on infections. With further development for multiple antibiotic conditions, the Dsc-AST assay could contribute to timely prescription of targeted drugs for better patient outcomes and mitigation of the threat of drug-resistant bacteria.

Rapid Phenotypic Antimicrobial Susceptibility Testing Using a Coulter Counter and Proliferation Rate Discrepancy

The rapid determination of antimicrobial susceptibility and evidence-based antimicrobial prescription is necessary to combat widespread antimicrobial resistance and promote effectively treatment for bacterial infections. This study developed a rapid phenotypic antimicrobial susceptibility determination method competent for seamless clinical implementation. A laboratory-friendly Coulter counter-based antimicrobial susceptibility testing (CAST) was developed and integrated with bacterial incubation, population growth monitoring, and result analysis to quantitatively detect differences in bacterial growth between resistant and susceptible strains following a 2 h exposure to antimicrobial agents. The distinct proliferation rates of the different strains enabled the rapid determination of their antimicrobial susceptibility phenotypes. We evaluated the performance efficacy of CAST for 74 clinically isolated Enterobacteriaceae subjected to 15 antimicrobials. The results were consistent with those obtained via the 24 h broth microdilution method, showing 90.18% absolute categorical agreement.

Electrical Broth Micro-Dilution for Rapid Antibiotic Resistance Testing

Rapid tests to assess the susceptibility of bacteria to antibiotics are required to inform antibiotic stewardship. We have developed a novel test, which measures changes in the impedance of a 100 nanoliter volume of bacterial suspension to determine an "electrical" minimum inhibitory concentration (eMIC). Two representative strains of Klebsiella pneumoniae, Acinetobacter baumannii, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus were tested against a panel of frontline antibiotics with different modes of action (ciprofloxacin, doxycycline, colistin and imipenem, gentamicin, and ceftazidime). The eMIC measured at 1 h correlated strongly with a standard 24 h microbroth dilution MIC for all combinations of antibiotics and bacteria, allowing strains to be correctly assigned as sensitive or resistant measured in a fraction of the time.

Electrokinetics in antimicrobial resistance analysis: A review

Infections caused by antimicrobial resistance are a serious problem in the world. Currently, commercial devices for antimicrobial susceptibility testing and resistant bacteria identification are time-consuming. There is an urgent need to develop fast and accurate methods, especially in the process of sample pretreatment. Electrokinetic (EK) is a family of electric-field-based kinetic phenomena of fluid or embedded objects, and EK applications have been found in various fields. In this paper, EK bacteria manipulation, including enrichment and separation, is reviewed. Focus is given to the rapid electric-based minimum inhibitory concentration measurement. The future directions and major challenges in this field are also outlined.

Combinatorial screening SlipChip for rapid phenotypic antimicrobial susceptibility testing

Antimicrobial resistance (AMR) by bacteria is a serious global threat, and a rapid, high-throughput, and easy-to-use phenotypic antimicrobial susceptibility testing (AST) method is essential for making timely treatment decisions and controlling the spread of antibiotic resistant micro-organisms. Traditional culture-based methods are time-consuming, and their capability to screen against a large number of different conditions is limited; meanwhile genotypic based methods, including sequencing and PCR based methods, are constrained by rarely identified resistance genes and complicated resistance mechanisms. Here, a combinatorial-screening SlipChip (cs-SlipChip) containing 192 nanoliter-sized compartments is developed which can perform high-throughput phenotypic AST within three hours by monitoring the bacterial growth within nanoliter-sized droplets with bright-field imaging and analyzing the changes in bacterial number and morphology. The minimum inhibitory concentration (MIC) of Escherichia coli ATCC 25922 against four antibiotics (ampicillin, ciprofloxacin, ceftazidime, and nitrofurantoin) can be measured in one chip within 3 hours. Furthermore, five antibiotic-resistant E. coli strains were isolated from patients diagnosed with urinary tract infections (UTIs), and an individual isolate was tested using four antibiotics and eleven antibiotic combinations simultaneously with three different concentrations of each. The results from the cs-SlipChip agree with those of a VITEK 2 automated system. This cs-SlipChip provides a practical high-throughput and rapid phenotypic method for AST and can also be used to screen different chemicals and antibiotic combinations for the treatment of multiple antibiotic-resistant bacteria.

Rapid Detection of Urinary Tract Infection in 10 min by Tracking Multiple Phenotypic Features in a 30 s Large-Volume Scattering Video of Urine Microscopy

Rapid point-of-care (POC) diagnosis of bacterial infection diseases provides clinical benefits of prompt initiation of antimicrobial therapy and reduction of the overuse/misuse of unnecessary antibiotics for nonbacterial infections. We present here a POC compatible method for rapid bacterial infection detection in 10 min. We use a large-volume solution scattering imaging (LVSi) system with low magnifications (1-2×) to visualize bacteria in clinical samples, thus eliminating the need for culture-based isolation and enrichment. We tracked multiple intrinsic phenotypic features of individual cells in a short video. By clustering these features with a simple machine learning algorithm, we can differentiate Escherichia coli from similar-sized polystyrene beads, distinguish bacteria with different shapes, and distinguish E. coli from urine particles. We applied the method to detect urinary tract infections in 104 patient urine samples with a 30 s LVSi video, and the results showed 92.3% accuracy compared with the clinical culture results. This technology provides opportunities for rapid bacterial infection diagnosis at POC settings.

Rapid antimicrobial susceptibility profiling using impedance spectroscopy

The present antibiotic susceptibility testing (AST) techniques based on bacterial culture, gene amplification and mass spectrometry are highly time consuming, labour intensive or expensive. Impedance spectroscopy is an emerging tool for rapid bacterial analysis as it is label-free, real-time, affordable and high-throughput. The over-reliance of this technique on complex chip designs and cell enrichment strategies has, however, slowed its foray into clinical AST. We demonstrate a label-free approach in which a low conductivity zwitterionic buffer is used for boosting impedance sensitivity in simple interdigitated electrodes (IDEs) allowing rapid AST in just 20 min without any liquid flow, biofunctionalization or cell enrichment steps. The detection principle relies on measuring changes in solution resistance due to antibiotic-induced bacterial cell death or growth. While the death-based approach is faster (20 min), it's restricted to surface-acting bactericidal antibiotics. The cell growth approach is longer (60-80 min) but more versatile as it applies to all drug types. Results for antibiotic sensitivity analysis and minimum inhibitory concentration (MIC) determination are illustrated for Escherichia coli, Klebsiella pneumoniae and Staphylococcus aureus against a wide class of antibiotics (penicillins, cephalosporins, polymyxins, carbapenems etc.).

Copper-Electroplating-Modified Liquid Metal Microfluidic Electrodes

Here, we report a novel technology for the fabrication of copper-electroplating-modified liquid metal microelectrodes. This technology overcomes the complexity of the traditional fabrication of sidewall solid metal electrodes and successfully fabricates a pair of tiny stable solid-contact microelectrodes on both sidewalls of a microchannel. Meanwhile, this technology also addresses the instability of liquid metal electrodes when directly contacted with sample solutions. The fabrication of this microelectrode depends on controllable microelectroplating of copper onto the gallium electrode by designing a microelectrolyte cell in a microfluidic chip. Using this technology, we successfully fabricate various microelectrodes with different microspacings (from 10 μm to 40 μm), which were effectively used for capacitive sensing, including droplet detection and oil particle counting.

Combinatorial nanodroplet platform for screening antibiotic combinations

The emergence and spread of multidrug resistant bacterial strains and concomitant dwindling of effective antibiotics pose worldwide healthcare challenges. To address these challenges, advanced engineering tools are developed to personalize antibiotic treatments by speeding up the diagnostics that is critical to prevent antibiotic misuse and overuse and make full use of existing antibiotics. Meanwhile, it is necessary to investigate novel antibiotic strategies. Recently, repurposing mono antibiotics into combinatorial antibiotic therapies has shown great potential for treatment of bacterial infections. However, widespread adoption of drug combinations has been hindered by the complexity of screening techniques and the cost of reagent consumptions in practice. In this study, we developed a combinatorial nanodroplet platform for automated and high-throughput screening of antibiotic combinations while consuming orders of magnitude lower reagents than the standard microtiter-based screening method. In particular, the proposed platform is capable of creating nanoliter droplets with multiple reagents in an automatic manner, tuning concentrations of each component, performing biochemical assays with high flexibility (e.g., temperature and duration), and achieving detection with high sensitivity. A biochemical assay, based on the reduction of resazurin by the metabolism of bacteria, has been characterized and employed to evaluate the combinatorial effects of the antibiotics of interest. In a pilot study, we successfully screened pairwise combinations between 4 antibiotics for a model Escherichia coli strain.