Amperometric detection of Enterobacteriaceae in river water by measuring β-galactosidase activity at interdigitated microelectrode arrays

Functionalized-Graphene Field Effect Transistor-Based Biosensor for Ultrasensitive and Label-Free Detection of β-Galactosidase Produced by Escherichia coli

The detection of β-galactosidase (β-gal) activity produced by Escherichia coli (E. coli) can quickly analyze the pollution degree of seawater bodies in bathing and fishing grounds to avoid large-scale outbreaks of water pollution. Here, a functionalized biosensor based on graphene-based field effect transistor (GFET) modified with heat-denatured casein was developed for the ultrasensitive and label-free detection of the β-gal produced by E. coli in real water samples. The heat-denatured casein coated on the graphene surface, as a probe linker and blocker, plays an important role in fabricating GEFT biosensor. The GFET biosensor response to the β-gal produced by E. coli has a wide concentration dynamic range spanning nine orders of magnitude, in a concentration range of 1 fg·mL-1-100 ng·mL-1, with a limit of detection (LOD) 0.187 fg·mL-1 (1.61 aM). In addition to its attomole sensitivity, the GFET biosensor selectively recognized the β-gal in the water sample and showed good selectivity. Importantly, the detection process of the β-gal produced by E. coli can be completed by a straightforward one-step specific immune recognition reaction. These results demonstrated the usefulness of the approach, meeting environmental monitoring requirements for future use.

An enzyme-activatable dual-readout probe for sensitive β-galactosidase sensing and Escherichia coli analysis

Rapid and accurate sensing of β-galactosidase (β-gal) activity is particularly critical for the early detection of many diseases and has become a topic of interest in recent years. However, most traditional probes for β-gal sensing often suffer from the disadvantages of narrow dynamic range, low reaction efficiency and are only employed with either colorimetric or fluorescence sensing. Furthermore, β-galactosidase sensing based assay for efficient detection and antibiotic resistance analysis of Escherichia coli (E.coli) is not available. Here, an enzyme-induced probe assay was reported for dual sensitive fluorescence and colorimetric measurement of β-gal activity, and was further employed for detection of Escherichia coli and their antibiotic resistance analysis. The DCM-βgal probe was virtually non-emissive in aqueous solution, while it could be activated by β-gal to produce bright emission. Under optimized conditions, DCM-βgal displayed high sensitivity, selectivity and rapid response to β-gal with a low detection limit of 1.5 × 10⁻³ U ml⁻¹. Importantly, this assay was successfully applied to sensitive detection of E. coli cells with a fast detection process within 5 h and a low detection concentration of 1 × 10³ CFU ml⁻¹. Furthermore, the enzyme-activatable assay was also successfully applied for high throughput E. coli antibiotic resistance analysis. The DCM-βgal strategy is applied for the first time on the detection of E. coli cells and their antibiotic resistance analysis. It is provided with the advantages of high selectively, a simple operation, low cost and rapid detection. The detection platform can also be extended to analyze the level of β-gal in other types of cells or biological samples. Overall, the simple, effective and dual-readout assay holds promise for efficient sensing of β-gal activity and provides a potential tool for E. coli detection and their antibiotic resistance analysis.

The β-galactosidase assay in perspective: Critical thoughts for biosensor development

Recently, the β-galactosidase assay has become a key component in the development of assays and biosensors for the detection of enterobacteria and E. coli in water quality monitoring. The assay has often performed below its maximum potential, mainly due to a poor choice of conditions. In this study we establish a set of optimal conditions and provide a rough estimate of how departure from optimal values reduces the output of the assay potentially decreasing its sensitivity. We have established that maximum response for detecting low cell concentrations requires an induction of the samples using IPTG at a concentration of 0.2 mM during 180 min. Permeabilization of the samples is mandatory as lack of it results in an almost 60% reduction in assay output. The choice of enzyme substrate is critical as different substrates yield products with different extinction coefficients or fluorescence yields. The concentration of substrate used must be high enough (around 3 to 4 times K_m) to ensure that the activity measured is not substrate limited. Finally, as the color/fluorescence of the reaction products is highly dependent on pH, care must be taken to ensure that pH at the time of reading is high enough to provide maximum signal.

Fieldwork-based determination of design priorities for point-of-use drinking water quality sensors for use in resource-limited environments

Improved capabilities in microfluidics, electrochemistry, and portable assays have resulted in the development of a wide range of point-of-use sensors intended for environmental, medical, and agricultural applications in resource-limited environments of developing countries. However, these devices are frequently developed without direct interaction with their often-remote intended user base, creating the potential for a disconnect between users' actual needs and those perceived by sensor developers. As different analytical techniques have inherent strengths and limitations, effective measurement solution development requires determination of desired sensor attributes early in the development process. In this work, we present our findings on design priorities for point-of-use microbial water sensors based on fieldwork in rural India, as well as a guide to fieldwork methodologies for determining desired sensor attributes. We utilized group design workshops for initial identification of design priorities, and then conducted choice-based conjoint analysis interviews for quantification of user preferences among these priorities. We found the highest user preference for integrated reporting of contaminant concentration and recommended actions, as well as significant preferences for mostly reusable sensor architectures, same-day results, and combined ingredients. These findings serve as a framework for future microbial sensor development and a guide for fieldwork-based understanding of user needs.

The coupling of immunomagnetic enrichment of bacteria with paper-based platform

In this study, the coupling of magnetic enrichment of bacteria from real samples with rapid surface enhanced Raman spectroscopy (SERS) detection was reported. The selective isolation and enrichment for the model bacteria Escherichia coli (E. coli) was performed using E. coli (primary) antibody bound-magnetic gold (Fe₃O₄@Au) nanoparticles. Following isolation and enrichment, the rennet enzyme was used to cleave of casein modified Fe₃O₄/Au-PEI nanoparticles from primary antibody-bound bacteria to prevent the nanoparticle aggregation and provide the movement of bacteria on nitrocellulose membrane. In the first part of the study, optimization studies were carried out namely; the amounts of gold nanoparticles (AuNPs), polyethyleneimine coated magnetic gold (Fe₃O₄/Au-PEI) nanoparticles, casein and rennet enzyme. The SERS signals of DTNB (5,5'-Dithiobis(2-nitrobenzoic acid)) molecule were collected on the test line and a calibration curve was plotted by using signal intensities. The correlation between the concentration of E. coli and SERS signal was found to be linear within the range of 10¹-10⁷ cfu/mL (R² = 0.984, LOD = 0.52 cfu/mL and LOQ = 1.57 cfu/mL). The selectivity of the paper-based lateral flow immunoassay (LFIA) was examined with Bacillus subtilis (B. subtilis), Micrococcus luteus (M. luteus), Salmonella enteritidis (S. enteritidis) which did not produce any significant response compared with E. coli measurement. Finally, the developed paper-based LFIA was tested with urine and milk samples. The obtained SERS results were compared with a plate counting method results which were in a good accordance. The developed method was found as rapid and sensitive to E. coli with a total analysis time of less than 60 min.

Novel sensor platform for rapid detection and quantification of coliforms on food contact surfaces

In this paper, a novel sensor platform based on screen printed carbon electrode coated by graphene modified polyacrylamide gel (GR/PAAGC) was developed and implemented for sampling, detection and enumeration of coliform bacteria (coliforms) on food contact surfaces. The optimized formula of polyacrylamide (PAA) and agar-agar increased the adhesive properties of the gel, being crucial for the coliforms recovery, attached to food contact surfaces. The 6-Chloro-3-indoxyl-β-D-galactopyranoside (6-CIGP) was used as a new electrochemical reporter for β-D-galactosidase activity. The released 6,6'-Dichloro-Indigo (6-DI) was directly detected by GR/PAAGC sensor. The presence of Isopropyl-β-D-thiogalactopyranoside (IPTG) and n-Octyl-β-D-thiogalactopyranoside (OBDG) in the gel contributed to reduction of the detection time. The addition of graphene enhanced the voltammetric signal and increased the conductivity of PAA gel. The anodic and cathodic peaks of the released product were directly proportional to the concentration of coliforms. Bacterial cell concentrations ranging from 1.6log₁₀CFU/mL to 6.6log₁₀CFU/mL were detected. Well-shaped, sharp voltammetric curves were generated within 3 h. Redox peaks exhibited good sensitivity with detection limits (LOD) < 0.6log₁₀CFU/mL. After series of optimization experiments, coliforms ranging from 0.6log₁₀CFU/cm² to 6.6₁₀CFU/cm² on stainless steel surfaces have been detected within 30 min with a LOD of 0.1log₁₀CFU/cm². The developed rapid, sensitive, reproducible and specific sensor successfully applied for single detection as well as for real-time monitoring of growth of coliform bacteria on stainless steel surfaces during food processing.

Lyophilized Engineered Phages for Escherichia coli Detection in Food Matrices

Ease of use, low cost, and convenient transport are the key requirements for a commercial bacteria detection kit designed for resource-limited settings. Here, we report the colorimetric detection of Escherichia coli (E. coli) in food samples using freeze-dried engineered bacteriophages (phages). In this approach, we have engineered T7 phages to carry the lacZ operon driven by T7 promoter to overexpress reporter enzymes. The engineered phages were freeze-dried in a water-soluble polymer for storage and transportation. When used for the detection of E. coli cells, the intracellular enzyme [β-galactosidase (β-gal)] was overexpressed and released into the surrounding media, providing an enzyme-amplified colorimetric signal. Using this strategy, we were able to detect E. coli cells at the concentration of 10² CFU mL^-1 in food samples without the need for sophisticated instruments or skilled operators.

Integrating recognition elements with nanomaterials for bacteria sensing

Pathogenic bacterial contamination is a major threat to human health and safety. In this review, we summarize recent strategies for the integration of recognition elements with nanomaterials for the detection and sensing of pathogenic bacteria. Nanoprobes can provide sensitive and specific detection of bacterial cells, which can be applied across multiple applications and industries.

Colorimetric Detection of Escherichia coli Based on the Enzyme-Induced Metallization of Gold Nanorods

A novel enzyme-induced metallization colorimetric assay is developed to monitor and measure beta-galactosidase (β-gal) activity, and is further employed for colorimetric bacteriophage (phage)-enabled detection of Escherichia coli (E. coli). This assay relies on enzymatic reaction-induced silver deposition on the surface of gold nanorods (AuNRs). In the presence of β-gal, the substrate p-aminophenyl β-d-galactopyranoside is hydrolyzed to produce p-aminophenol (PAP). Reduction of silver ions by PAP generates a silver shell on the surface of AuNRs, resulting in the blue shift of the longitudinal localized surface plasmon resonance peak and multicolor changes of the detection solution from light green to orange-red. Under optimized conditions, the detection limit for β-gal is 128 pM, which is lower than the conventional colorimetric assay. Additionally, the assay has a broader dynamic range for β-gal detection. The specificity of this assay for the detection of β-gal is demonstrated against several protein competitors. Additionally, this technique is successfully applied to detect E. coli bacteria cells in combination with bacteriophage infection. Due to the simplicity and short incubation time of this enzyme-induced metallization colorimetric method, the assay is well suited for the detection of bacteria in low-resource settings.

Rapid amperometric detection of Escherichia coli in wastewater by measuring β-D glucuronidase activity with disposable carbon sensors

An assay on the indirect amperometric quantification of the β-D-Glucuronidase (GLUase) activity was developed for the rapid and specific detection of Escherichia coli (E. coli) in complex environmental samples. The p-aminophenyl β-D-glucopyranoside (PAPG) was selected as an electrochemical substrate for GLUase measurement and the p-aminophenol (PAP) released during the enzymatic hydrolysis was monitored by cyclic voltammetry with disposable carbon screen-printed sensors. The intensity of the measured anodic peak current was proportional to the amount of GLUase, and therefore to the number of E. coli in the tested sample. Once the substrate concentration and pH values optimized, a GLUase detection limit of 10 ng mL(-1) was achieved. Using a procedure involving a filtration step of the bacteria followed by their incubation with the substrate solution containing both the nonionic detergent Triton X-100 as permeabilization agent and the culture media Luria broth to monitor the growth, filtered bacterial cells ranging from 5 × 10(4) to 10(8) UFC/membrane were detected within 3 h. The amperometric assay was applied to the determination of fecal contamination in raw and treated wastewater samples and it was successfully compared with conventional bacterial plating methods and uidA gene quantitative PCR. Owing to its ability to perform measurements in turbid media, the GLUase amperometric method is a reliable tool for the rapid and decentralized quantification of viable but also nonculturable E. coli in complex environmental samples.

Bacteriophages: biosensing tools for multi-drug resistant pathogens

Pathogen detection is of utmost importance in many sectors, such as in the food industry, environmental quality control, clinical diagnostics, bio-defence and counter-terrorism. Failure to appropriately, and specifically, detect pathogenic bacteria can lead to serious consequences, and may ultimately be lethal. Public safety, new legislation, recent outbreaks in food contamination, and the ever-increasing prevalence of multidrug-resistant infections have fostered a worldwide research effort targeting novel biosensing strategies. This review concerns phage-based analytical and biosensing methods targeted towards theranostic applications. We discuss and review phage-based assays, notably phage amplification, reporter phage, phage lysis, and bioluminescence assays for the detection of bacterial species, as well as phage-based biosensors, including optical (comprising SPR sensors and fiber optic assays), electrochemical (comprising amperometric, potentiometric, and impedimetric sensors), acoustic wave and magnetoelastic sensors.

Cyclic biamperometry at micro-interdigitated electrodes

Cyclic biamperometry was studied as an analytical method for use with commercially available, comb-type, coplanar microinterdigitated electrodes (μIDEs), using the ferri-/ferrocyanide redox couple as a model analyte. The μIDEs studied in this work were made of gold that had been deposited onto a Ti/W adhesion layer on borosilicate glass chips and had 5 and 10 μm bands with equal gap sizes. Close proximity of the two working electrodes, and their interdigitation, resulted in signal amplification by redox cycling. Results were compared with those obtained by cyclic voltammetry, where one of the two IDE electrodes was used as the working electrode and external reference and auxiliary electrodes were used. Amplification factors of almost 20 were achieved due to redox cycling. Attempts to apply cyclic voltammetry to the μIDEs, with one of the combs as the working and the other as the auxiliary electrode, were unsuccessful due to corrosion of the auxiliary electrode comb. Results of this study, and the electrochemically unique feature of biamperometry to probe but not change the net contents of the medium under examination, suggest the applicability of scanning biamperometry at μIDEs to the very small volumes and electrochemical cell dimensions that are now of great interest.