A portable microfluidic platform for rapid determination of microbial load and somatic cell count in milk

Label-free detection of E. coli by alternating current electrokinetic capacitive sensors

BACKGROUND

Escherichia coli (E. coli) is one of the most common pathogens in water and foods, and it poses a significant threat to global public health, particularly in low- and middle-income countries. However, most of the state-of-art detection method for E. coli bacteria sensing needs fluorescence staining or pre-immobilizations or specific biomaker as labels before tests, making the detection process a time costing and labor intensitive work. Thus, it is desirable to develop a label-free and no-immobilization detection method for the sensitive detection of E. coli or other pathogens.

RESULTS

In this work, we developed a sensitive method for E. coli detection without any labels or pre-immobilizations. Here the source AC signal provides ACEK effects to trap bacteria onto the electrode surface. As the source AC signal is cut off, the enriched bacteria would release from sensor electrodes. Then, a significant interfacial capacitance change along with particle releasing is observed by applying a measurement AC signal. The DEP capture of E. coli bacteria at different AC frequencies and voltages are experimentally investigated to optimize the performance of capacitive sensors. To note, the total area of our sensitive unit can be as large as 2 mm × 2 mm with the geometry design of isomotive DEP. Capacitive sensing tests of river water collected from local river and tap water with several control groups are performed to show its reliability as a biosensing approach for the sensitive detection of E. coli.

SIGNIFICANCE

The most significant advantage of this method is that we do not need to pre-immobilize any probes (antigens, or antibodies, etc.) or take incubation onto the sensor electrode before taking capacitance measurement. Moreover, the straight forward operation is pre-immobilization free, and therefore it meets the requirements of on-site bacteria detection for pathogen detection in water quality monitoring or food safety.

Microfluidic rapid isolation and electrochemical detection of S. pneumonia via aptamer-decorated surfaces

BACKGROUND

S. pneumoniae is widely recognized as a leading cause of respiratory infections worldwide, often resulting in high mortality rates. However, the advent of microfluidic technologies has brought significant advancements, including the simplified, sensitive, cost-effective, and rapid approach to pneumococcal bacteremia detection. In this study, a microfluidic magnetic platform is presented for rapid isolation, and an electrode array is utilized for the electrochemical detection of S. pneumoniae. Aptamer-decorated surfaces were employed for both isolation and detection. For isolation, silica magnetic microparticles were synthesized and decorated with aptamer.

RESULTS

Isolation performance was assessed for phosphate-buffered saline (PBS) and blood samples for different concentrations of S. pneumoniae. Electrical impedance spectroscopy (EIS) with fabricated gold interdigitated electrodes (IDEs) decorated with aptamer was implemented for the detection of S. pneumoniae at different bacteria concentrations. The microfluidic platform performed bacteria isolation at comparable isolation efficiency with batch systems but at a much faster rate (isolation took about a minute, and the aptamer-decorated electrode array exhibited a limit of detection (LOD) at 962 CFU/mL and linear range between 104 and 107 CFU/mL.

SIGNIFICANCE

Our method represents a significant advancement compared to previous reports. Our microfluidic platform can efficiently isolate 60 μL of the bacteria sample within about one minute. The entire process takes about two minutes including the detection step. Furthermore, our method achieves a notable improvement in the detection limit for S. pneumoniae compared to conventional ELISA and magnetic microfluidics ELISA.

Aptamer decorated PDA@magnetic silica microparticles for bacteria purification

One significant constraint in the advancement of biosensors is the signal-to-noise ratio, which is adversely affected by the presence of interfering factors such as blood in the sample matrix. In the present investigation, a specific aptamer binding was chosen for its affinity, while exhibiting no binding affinity towards non-target bacterial cells. This selective binding property was leveraged to facilitate the production of magnetic microparticles decorated with aptamers. A novel assay was developed to effectively isolate S. pneumoniae from PBS or directly from blood samples using an aptamer with an affinity constant of 72.8 nM. The capture experiments demonstrated efficiencies up to 87% and 66% are achievable for isolating spiked S. pneumoniae in 1 mL PBS and blood samples, respectively.

Novel 3D-Printed Microfluidic Magnetic Platform for Rapid DNA Isolation

This study presents a novel miniaturized device as a 3D-printed microfluidic magnetic platform specifically designed to manipulate magnetic microparticles in a microfluidic chip for rapid deoxyribonucleic acid (DNA) isolation. The novel design enables the movement of the magnetic particles in the same or opposite directions with the flow or suspends them in continuous flow. A computational model was developed to assess the effectiveness of the magnetic manipulation of the particles. Superparamagnetic monodisperse silica particles synthesized in-house are utilized for the isolation of fish sperm DNA and human placenta DNA. It was demonstrated that the proposed platform can perform DNA isolation within 10 min with an isolation efficiency of 50% at optimum operating conditions.

A sample-preparation-free, point-of-care testing system for in situ detection of bovine mastitis

We present a highly integrated point-of-care testing (POCT) device capable of immediately and accurately screening bovine mastitis infection based on somatic cell counting (SCC). The system primarily consists of a homemade cell-counting chamber and a miniature fluorescent microscope. The cell-counting chamber is pre-embedded with acridine orange (AO) in advance, which is simple and practical. And then SCC is directly identified by microscopic imaging analysis to evaluate the bovine mastitis infection. Only 4 μL of raw bovine milk is required for a simple sample testing and accurate SCC. The entire assay process from sampling to result in presentation is completed quickly within 6 min, enabling instant "sample-in and answer-out." Under laboratory conditions, we mixed bovine leukocyte suspension with whole milk and achieved a detection limit as low as 2.12 × 104 cells/mL on the system, which is capable of screening various types of clinical standards of bovine milk. The fitting degrees of the proposed POCT system with manual fluorescence microscopy were generally consistent (R2 > 0.99). As a proof of concept, four fresh milk samples were used in the test. The average accuracy of somatic cell counts was 98.0%, which was able to successfully differentiate diseased cows from healthy ones. The POCT system is user-friendly and low-cost, making it a potential tool for on-site diagnosis of bovine mastitis in resource-limited areas.

A Study on the Uniform Distribution and Counting Method of Raw Cow's Milk Somatic Cells

The somatic cell count (SCC) in raw milk is an important basis for determining whether a cow is suffering from mastitis. To address the problem of an uneven distribution of somatic cells due to cell-adherent sedimentation, among other reasons, during milk sampling, which in turn results in unrepresentative somatic cell counting, a method is proposed for obtaining a uniform distribution of somatic cells and improving the counting accuracy based on a nine-cell grid microfluidic chip. Firstly, a simulation was performed to verify the uniformity of the somatic cell distribution within the chip observation cavities. Secondly, a nine-cell grid microfluidic chip was prepared and a negative-pressure injection system integrating staining and stirring was developed to ensure that the somatic cells were uniformly distributed and free from air contamination during the injection process. As well as the structure of the chip, a microscopic imaging system was developed, and the nine chip observation cavities were photographed. Finally, the somatic cells were counted and the uniformity of the somatic cell distribution was verified using image processing. The experimental results show that the standard deviation coefficient of the SCC in each group of nine images was less than 1.61%. The automatic counting accuracy of the system was between 97.07% and 99.47%. This research method lays the foundation for the detection and prevention of mastitis in cows.

Counting bacteria in microfluidic devices: Smartphone compatible 'dip-and-test' viable cell quantitation using resazurin amplified detection in microliter capillary arrays

Viable bacterial cell counting is fundamental to analytical microbiology and agar plate colony counting remains common yet laborious and slow. Here, we demonstrate two methods for counting bacteria using commercially available microfluidic devices. We show that accurate viable cell counting is possible using simple and easy 'dip and test' arrays of microcapillaries. Colorimetric and fluorescent growth detection both permit viable cell counting in microcapillaries either by limiting dilution into multiple microfluidic compartments using a single endpoint measurement, or alternatively by quantifying growth kinetics. The microcapillary devices are compatible with conventional 96 well plates and multichannel pipettes, expanding each microplate row into 120 individual 1 or 2 μL samples. At limiting dilution, counting the proportion of positive compartments permitted accurate calculation of gram-negative and gram-positive bacteria (E. coli and S. saprophyticus) at concentrations down to as low as 10 CFU/mL with almost 1:1 agreement with agar plate colony counts over four orders of magnitude. A smartphone camera was sufficient to record endpoint images of resazurin growth detection both colorimetrically and fluorescently. Viable cell counting of E. coli and S. saprophyticus was also possible through recording growth kinetics and determining the time taken to detect resazurin conversion. However, only the limiting dilution method remained consistent in the presence of urine matrix, as some interference in growth rate was observed when bacteria were spiked into higher concentrations of normal urine to simulate urinary tract infection patient samples. However, with the limiting dilution counting method endpoint growth was always detected even in the presence of 90% urine matrix, suggesting that this method might permit bacterial pathogen counting directly in clinical samples without agar plating.

Microfluidics for the rapid detection of Escherichia coli O157:H7 using antibody-coated microspheres

This study developed a novel method for the rapid detection of Escherichia coli (E. coli) O157:H7 on a microfluidic platform. First, the concentration of bacteria in a sample was determined with the adenosine triphosphate (ATP) method. Then, the specific detection of E. coli was achieved in a microfluidic chip by the immune-microsphere technique. The influences of the culture time, flow rate and capture time on the detection of the target bacteria were investigated systematically. Generally, with increasing capture time, more bacteria could be captured by the microspheres, which had a positive effect on bacterial detection. Furthermore, the sensitivity and specificity of the method were also tested. The results showed that this method could specifically detect E. coli with a sensitivity as high as 49.1 cfu/μL; the consumption of bacteria was 1 μL, and the reagent was at the microliter level. The testing time can be controlled within one and a half hours, and the cost of testing was approximately RMB 10. The method described in this article is simple and accurate and has great application value in bacterial detection for medical diagnostics.