Performance assessment of three commercial assays for direct detection of Bacillus anthracis spores

Fabrication of a Terbium-Functionalized Cadmium Organic Framework with Proper Energy Levels as a Ratiometric Probe of an Anthrax Biomarker

Anthrax bacillus is a very dangerous zoonotic pathogen that seriously endangers public health. Rapid and accurate qualitative and quantitative detection of its biomarkers, 2,6-dipicolinic acid (DPA), is crucial for the prevention and treatment of this pathogenic bacterium. In this work, a novel Cd-based MOF (TTCA-Cd) has been synthesized from a polycarboxylate ligand, [1,1':2',1″-terphenyl]-4,4',4″,5'-tetracarboxylic acid (H4TTCA), and further doped with Tb(III), forming a dual-emission lanthanide-functionalized MOF hybrid (TTCA-Cd@Tb). TTCA-Cd@Tb can be developed as a high-performance ratiometric fluorescent sensor toward DPA with a very low detection limit of 7.14 nM and high selectivity in a wide detection range of 0-200 μM, demonstrating a big advancement and providing a new option for the detection of DPA.

Tb3+-Based Off-On Fluorescent Platform for Multicolor and Dosage-Sensitive Visualization of Bacterial Spore Marker

Developing a novel strategy for the sensitive and rapid detection of pathogenic bacterial spores in field or on-site settings will be helpful in minimizing their potential threats to human health, environmental safety, and food safety. In this study, Tb³⁺ was combined with glutathione (GSH)-modified copper nanoclusters (CuNCs), and an aggregation-induced emission (AIE) fluorescent probe based on Tb-GSH-CuNCs was fabricated for dipicolinic acid (DPA, a pathogenic bacterial spore marker) sensing. Making use of the competitive binding of Tb³⁺ between GSH-CuNCs and DPA, a multicolor sensing of DPA was facilely realized without introducing fluorescent materials as the reference. Due to an "off-on" response mechanism of the AIE fluorescent probe, this multicolor response to DPA exhibited a feature of rich color gradients and highly discriminative color change, allowing a dosage-sensitive visual quantification of DPA. The DPA with a concentration even as low as 0.5 μM can still be identified by the naked eye. Moreover, together with a smartphone app, which can extract the R (red), G (green), and B (blue) values from the probe system, a portable platform can be established for sensitive DPA quantification in the range of 0.5-70 μM, showing great potential for the practical monitoring of DPA in field or on-site settings.

Highly selective and multicolor ultrasensitive assay of dipicolinic acid: The integration of terbium(III) and gold nanocluster

A novel multicolor fluorescent nano-probe based on the hybridization of Tb³⁺ ion with gold nanoclusters (Au NCs) was synthesized to monitor and on-site visual assay of 2,6-pyridinedicarboxylic acid (DPA), a biomarker of bacterial spores. DPA can replace the water molecule in the center of Tb³⁺ and strongly coordinate with Tb³⁺ based on the analyte-triggered antenna effect. Simultaneously, the red fluorescence of Au NCs is not influenced after addition of DPA and can be used as steady inside fluorescence reference channel to measure background noise. On this basis, the multicolor fluorescence nano-probe based on Tb³⁺-doped Au NCs for fast analysis of DPA was fabricated. The linear range of this method is 0 to 12.5 μM and the limit of detection is 3.4 nM, which is well below the quantity of DPA concentration of 60 μM released by the spore transmission dose of anthrax infection. The proposed multicolor fluorescence nano-probe was successfully detecting DPA in actual sample with good sensitivity and specificity. In addition, the visual paper-based nano-probe is designed to detect DPA by using the color scanning application of smart phone. This developed platform possesses abroad application prospects with advantages of effective, convenient carrying, simple operation, good selectivity and repeatability.

Ion exchange fabrication of lanthanide functionalized layered double hydroxides microcapsules for rapid and visual detection of anthrax biomarker

Lanthanide ion probes have recently been considered as promising sensing materials due to their high sensitivity and good optical properties. Herein, the 3D hierarchical lanthanide functionalized layered double hydroxides microcapsules were synthesized via a facile ion exchange strategy and further developed as novel fluorescent probes for detecting trace amounts of the anthrax biomarker dipicolinicacid (DPA). Benefiting from the 3D porous superstructure and abundant unsaturated coordination sites of lanthanide ion, the ternary Ni-Fe-Ln-LDHs (Ln = Tb/Eu) not only possess a large reactive contact area to improve the sensitivity of DPA detection, but also demonstrate very fast reaction rate. The design of inexpensive fluorescent test strips can perform the on-site and real-time detection via a smartphone with a color recognition application. More prominently, the sensitivity of the system was evaluated by actual spore samples with the detection limit as low as 3.54 × 10⁴ spores/mL. The 3D lanthanide functionalized LDHs nanoprobe constructed by ion exchange exhibits a new vision for the development of a sensing platform in other research areas.

Integrated ratiometric fluorescence probe-based acoustofluidic platform for visual detection of anthrax biomarker

The sensitive detection of dipicolinic acid (DPA) as an excellent biomarker of Bacillus anthracis, especially through visual point-of-care testing, is significant for accurate and rapid diagnosis of anthrax to timely prevent anthrax disease or biological terrorist attack. Herein, an acoustofluidics-based colorimetric platform with the integrated ratiometric fluorescence probe (INT-probe) was fabricated, which improved the sensitivity of visual detection for DPA and overcame the poor reproducibility of the existing acoustofluidics-assisted colorimetric analysis. For the design of INT-probe, Eu³⁺-EDTA complex as sensing moiety was grafted onto the surface of blue organosilane-functionalized carbon dots (SiCDs)-doped SiO₂ nanoparticles (NPs). Upon exposure to DPA, Eu³⁺ was sensitized by DPA to emit red luminescence, while the SiCDs as reference inside the SiO₂ NPs still kept the blue fluorescence unchanged. Attributed to the acoustic radiation force-driven enrichment of the INT-probe, slight color changes caused by low concentration of DPA could be amplified and distinguished by naked-eyes/smartphone. With the increase of DPA concentration, obvious color variations of INT-probe/DPA aggregates from blue to pink could be observed, and the color information of the fluorescent aggregates was converted to red, green and blue values for quantitative analysis, whose lowest detectable concentration reached 100 nM that is about 2-3 orders of magnitude lower than the infectious dosage of Bacillus anthracis spores (60 μM). Importantly, benefiting from the great color signal enhancement by acoustofluidic sensing platform, the usage of Eu³⁺ reduced to as low as 0.273 μmol per gram of SiO₂ NPs, providing a meaningful way to utilize lanthanide resource efficiently.

A Smartphone Integrated Platform for Ratiometric Fluorescent Sensitive and Selective Determination of Dipicolinic Acid

A desirable lanthanide-based ratiometric fluorescence probe was designed as a multifunctional nanoplatform for the determination of dipicolinic acid (DPA), a unique bacterial endospore biomarker, with high selectivity and sensitivity. The carbon dots (CDs) with blue emission wavelengths at 470 nm are developed with europium ion (Eu³⁺) to form Eu³⁺/CDs fluorescent probes. DPA can specifically combine with Eu³⁺ and then transfer energy from DPA to Eu³⁺ sequentially through the antenna effect, resulting in a distinct increase in the red fluorescence emission peak at 615 nm. The fluorescence intensity ratio of Eu³⁺/CDs (fluorescence intensity at 615 nm/fluorescence intensity at 470 nm) showed good linearity and low detection limit. The developed ratiometric nanoplatform possesses great potential for application in complex matrices owing to its specificity for DPA. In addition, the integration of a smartphone with the Color Picker APP installed enabled point-of-care testing (POCT) with quantitative measurement capabilities, confirming the great potential of the as-prepared measurement platform for on-site testing.

Validation of a Lateral Flow Test for the Presumptive Identification of the Presence of Burkholderia mallei or Burkholderia pseudomallei in Environmental Samples

We conducted a comprehensive, multiphase laboratory evaluation of InBios Active Melioidosis Detect (AMD) rapid test, a lateral flow immunoassay designed to detect capsular polysaccharides produced by Burkholderia mallei or Burkholderia pseudomallei, used in conjunction with the Omni Array Reader (OAR) for the rapid detection of B mallei or B pseudomallei in environmental (nonclinical) samples at 2 sites. The limit of detection, using reference strains B mallei strain ATCC 23344 and B pseudomallei strain ATCC 11668, was determined to be 10³ to 10⁴ CFU/mL. In different phases of the evaluation, inclusivity strains that included geographically diverse strains of B mallei (N = 13) and B pseudomallei (N = 22), geographically diverse phylogenetic near neighbor strains (N = 66), environmental background strains (N = 64), white powder samples (N = 26), and environmental filter extracts (N = 1 pooled sample from 10 filter extracts) were also tested. A total of 1,753 tests were performed, which included positive and negative controls. Visual and OAR results showed that the AMD test detected 92.3% of B mallei and 95.5% of B pseudomallei strains. Of the 66 near-neighbor strains tested, cross-reactivity was observed with only B stabilis 2008724195 and B thailandensis 2003015869. Overall, the specificity and sensitivity were 98.8% and 98.7%, respectively. The results of this evaluation support the use of the AMD test as a rapid, qualitative assay for the presumptive detection of B mallei and B pseudomallei in suspicious environmental samples such as white powders and aerosol samples by first responders and laboratory personnel.

Construction of a Cd8Tb4 nanoring for luminescence response to 2,6-dipicolinic acid as an anthrax biomarker

One 12-metal Cd(ii)–Tb(iii) nanoring (1.2 × 2.8 × 2.8 nm) was constructed from a flexible Schiff base ligand, and it shows luminescent response to 2,6-dipicolinic acid with high sensitivity and selectivity.

Three novel metal-organic frameworks with different coordination modes for trace detection of anthrax biomarkers

Dipicolinic acid (DPA) is an anthrax biomarker. Its serious consequences make its detection a great need. In this paper, three novel metal-organic frameworks (MOFs) with different coordination modes were synthesized by a simple solvothermal method, which can be used as highly efficient fluorescence sensors for the highly selective and sensitive trace detection of DPA. MOFs 1-3 showed rapid responses to DPA (<30 s), and the limits of detection (LODs) were calculated to be 1.01 × 10^-6 M^-1 (MOF 1), 1.17 × 10^-6 M^-1 (MOF 2) and 2.07 × 10^-6 M^-1 (MOF 3). DPA detection based on MOFs 1-3 in fetal bovine serum is highly reliable based on the high recovery rates (90% to 115%). Hence, the three MOF-based sensors can be used in the real-time detection of DPA.

Ratiometric fluorescent detection of dipicolinic acid as an anthrax biomarker based on a high-nuclearity Yb18 nanoring

An 18-metal lanthanide nanoring [Yb₁₈(L¹)₈(HL²)₂(OAc)₂₀(MeOH)₈(EtOH)₆(H₂O)₄] (1), which shows a ratiometric fluorescent response to DPA, was constructed through the strategy of using two types of polydentate organic ligands. The addition of DPA increases the visible ligand-centered emission, but decreases the NIR lanthanide luminescence of 1. The limit of luminescent detection of 1 for DPA is 1.5 μM. The high fluorescence sensitivity of 1 to DPA is not affected by the existence of interferents such as aromatic carboxylates and ions.

Eu(III)-DO3A and BODIPY dyad as a chemosensor for anthrax biomarker

The sensitive and selective determination of Bacillus anthracis spores before the infection is vital for human health and safety. Dipicolinic acid (DPA) is an excellent biomarker due to its presence in the nucleus of bacterial spores at high concentrations (up to 1 M, about 15% dry weight). In the present work, a new molecular chemosensor 1, based on europium(III)-DO3A and BODIPY dyad, is developed to detect DPA in phosphate-buffered saline (PBS) buffered solution and tap water samples. Also, 1 can be used as a ratiometric optical chemosensor to track DPA.

One high-nuclearity Eu18 nanoring with rapid ratiometric fluorescence response to dipicolinic acid (an anthrax biomarker)

One 18-metal Eu(iii) nanoring (size: 1.0 × 2.7 × 2.7 nm) was constructed as a rapid ratiometric fluorescent probe for the detection of dipicolinic acid with high sensitivity and selectivity, by using two types of polydentate organic ligands.

Acoustofluidic device for acoustic capture of Bacillus anthracis spore analogues at low concentration

A portable device for the rapid concentration of Bacillus subtilis var niger spores, also known as Bacillus globigii (BG), using a thin-reflector acoustofluidic configuration is described. BG spores form an important laboratory analog for the Bacillus anthracis spores, a serious health and bioterrorism risk. Existing systems for spore detection have limitations on detection time and detection that will benefit from the combination with this technology. Thin-reflector acoustofluidic devices can be cheaply and robustly manufactured and provide a more reliable acoustic force than previously explored quarter-wave resonator systems. The system uses the acoustic forces to drive spores carried in sample flows of 30 ml/h toward an antibody functionalized surface, which captures and immobilizes them. In this implementation, spores were fluorescently labeled and imaged. Detection at concentrations of 100 CFU/ml were demonstrated in an assay time of 10 min with 60% capture. We envisage future systems to incorporate more advanced detection of the concentrated spores, leading to rapid, sensitive detection in the presence of significant noise.

Eu3+@metal-organic frameworks encapsulating carbon dots as ratiometric fluorescent probes for rapid recognition of anthrax spore biomarker

Anthrax spores pose a serious threat to human well-being and life, so it is highly desirable to develop a rapid, sensitive, and selective quantitative assay of calcium pyridine dicarboxylate (CaDPA) as a biomarker of anthrax spores. Herein, carbon dots (CDs) chelated Eu³⁺@metal-organic framework (Eu-MOFs) as dual-emissive ratiometric fluorescent (RF) probe was successfully fabricated by a simple one-pot in situ selective self-assembly synthetic strategy. The developed RF probe has an effective self-calibration function, which performs a highly sensitive and selective recognition of CaDPA in water and human serum sample. The blue-emitting CDs was employed as an effective fluorescence reference, while the Eu-MOFs exhibited enhanced red fluorescence signal through the coordination interaction with CaDPA chromophore. The sensing mechanism is attributed to that CaDPA can sensitize Eu³⁺ intrinsic luminescence due to the energy transfer from CaDPA to Eu³⁺. What's more interesting is that with the continuous drop of CaDPA, the emission color of CDs@Eu-MOF changes from blue to red. The results revealed that CDs@Eu-MOFs RF probe can detect CaDPA effectively in the range of 8-170 μg/L with good linear relationship, and exhibited a remarkable selectivity for CaDPA. More interestingly, a paper-based probe has also been devised for on-site detection of CaDPA. In addition, CaDPA is used as input signal to construct an IHIBITION logic gate device which performs the "off-on" mode. The constructed CDs@Eu-MOF probe can achieve exceptionally rapid, highly sensitive and selective detection of CaDPA, which can further expand the application prospects in environmental and biological analysis.

Construction of lanthanide-containing ratiometric probe for facile anthrax biomarker detection

Since the outbreak of anthrax attack in the United States in 2001, much effort has been done to detect anthrax spores. In this paper, a simple and facile lanthanide-containing ratiometric luminescence "turn on" probe is constructed to achieve sensitive and selective detection of anthrax spores biomarker - pyridine-2,6-carboxylic acid (DPA) by simply mixing Tb³⁺, sodium polyacrylate as well as red internal reference which could exclude some disturbance from the luminescence of biological tissue. Our probe shows remarkable luminescence enhancement upon addition of DPA. The detection limit for DPA is measured as 35.8 nM with a wide work range of 0-66.7 μM. Besides, our probe enables distinguish DPA selectively from numerous interfering materials in aqueous solution. Further test strip experiments demonstrate the potential practical application of our probe in DPA detection.

Reevaluating limits of detection of 12 lateral flow immunoassays for the detection of Yersinia pestis, Francisella tularensis, and Bacillus anthracis spores using viable risk group-3 strains

AIM

Rapid detection of biological agents in biodefense is critical for operational, tactical and strategic levels as well as for medical countermeasures. Yersinia pestis, Francisella tularensis, and Bacillus anthracis are high priority agents of biological warfare or bioterrorism and many response forces use lateral flow assays (LFAs) for their detection. Several companies produce these assays, which offer results in short time and are easy to use. Despite their importance, only few publications on the limits of detection (LOD) for LFAs are available. Most of these studies used inactivated bacteria or risk group-2 strains. As the inactivation process in previous studies might have affected the tests' performances, it was our aim in this study to determine and compare the LOD of several commercially available LFAs using viable risk group-3 strains.

METHODS AND RESULTS

Lateral flow assays from four different companies for the detection of following bacteria were evaluated: Y. pestis, F. tularensis and B. anthracis spores. Two independent quantification methods for each target organism were applied, in order to ensure high quantification accuracy. LODs varied greatly between tests and organisms and ranged between 10⁴ for Y. pestis-tests and as high as >10⁹ for one B. anthracis-test.

CONCLUSION

This work precisely determined the LODs of LFAs from four commercial suppliers. The herein determined LODs differed from results of previous studies. This illustrates the need for using accurately quantified viable risk group 3-strains for determining such LODs.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our work bridges an important knowledge gap with regard to LFA LOD. The LODs determined in this study will facilitate better assessment of LFA-results. They illustrate that a negative LFA result is not suited to exclude the presence of the respective agent in the analyzed sample.

Rapid and Sensitive Multiplex Assay for the Detection of B. anthracis Spores from Environmental Samples

Prompt and accurate detection of Bacillus anthracis spores is crucial in the event of intentional spore dissemination in order to reduce the number of expected casualties. Specific identification of these spores from environmental samples is both challenging and time-consuming. This is due to the high homology with other Bacillus species as well as the complex composition of environmental samples, which further impedes assay sensitivity. Previously, we showed that a short incubation of B.anthracis spores in a defined growth medium results in rapid germination, bacterial growth, and secretion of toxins, including protective antigen. In this work, we tested whether coupling the incubation process to a newly developed immune-assay will enable the detection of secreted toxins as markers for the presence of spores in environmental samples. The new immune assay is a flow cytometry-based multiplex that simultaneously detects a protective antigen, lethal factor, and edema factor. Our combined assay detects 1 × 10³–1 × 10⁴/mL spores after a 2 h incubation followed by the ~80 min immune-multiplex detection. Extending the incubation step to 5 h increased assay sensitivity to 1 × 10²/mL spore. The protocol was validated in various environmental samples using attenuated or fully virulent B. anthracis spores. There was no substantial influence of contaminants derived from real environmental samples on the performance of the assay compared to clean samples, which allow the unequivocal detection of 3 × 10³/mL and 3 × 10²/mL spores following 2 and 5 hour’s incubation, respectively. Overall, we propose this method as a rapid, sensitive, and specific procedure for the identification of B. anthracis spores in environmental samples.

Multiporous Terbium Phosphonate Coordination Polymer Microspheres as Fluorescent Probes for Trace Anthrax Biomarker Detection

Lanthanide coordination polymers have been recently regarded as attractive sensing materials because of their selectivity, high sensitivity, and rapid response ability. In this research, the multiporous terbium phosphonate coordination polymer microspheres (TbP-CPs) were prepared as a novel fluorescent probe, which showed a fluorescence turn-on response capability for the detection of the trace anthrax biomarker dipicolinate acid (DPA). The morphology and chemical composition of as-prepared TbP-CPs were characterized in detail. The TbP-CPs have the vegetable-flower-like structure and microporous surface. In addition, the as-prepared TbP-CPs not only possess the merits of convenience and simple preparation with high yield but also have the excellent characters as fluorescent probes, such as high stability, good selectivity, and rapid detection ability within 30 s. This proposed sensor could detect DPA with a linear relationship in concentrations ranging from 0 to 8.0 μM and a high detection sensitivity of 5.0 nM. Furthermore, the successful applications of DPA detection in urine and bovine serum were demonstrated. As a result, the recovery ranged from 93.93-101.6%, and the relative standard deviations (RSD) were less than 5%.

A lanthanide-based magnetic nanosensor as an erasable and visible platform for multi-color point-of-care detection of multiple targets and the potential application by smartphone

A magnetic Fe₃O₄@CePO₄:Tb-EDTA-Eu nanosensor as an erasable and visible platform for the multi-color detection of multiple targets was designed.

A non-luminescent Eu-MOF-based “turn-on” sensor towards an anthrax biomarker through single-crystal to single-crystal phase transition

A non-luminescent NH₂-MOF-76(Eu) exhibits selective fluorescence recovery for sensing anthrax biomarker DPA through a single-crystal to single-crystal phase transition.

A highly selective lanthanide-containing probe for ratiometric luminescence detection of an anthrax biomarker

We report a ratiometric luminescent sensor that can identify the Bacillus anthrax biomarker with outstanding selectivity, even including all of the 5 DPA isomers.

For the optical detection of anthrax biomarker using a luminescent rare earth-organic framework modified by rhodamine molecules: Synthesis, characterization and two sensing channels

This work was devoted to the synthesis and performance report of an optical sensing platform for dipicolinate (DPA), which was known as a biomarker for Bacillus anthracis spores. This DPA optical sensing platform (denoted as EuBTC@RB) was composed of luminescent rare earth MOF and rhodamine-derived sensing probe. Its structure was discussed and confirmed by means of XRD, IR, TGA, absorption, emission and excitation spectra. EuBTC@RB showed two sensing channels for DPA, including colorimetric sensing and ratiometric fluorescent sensing. Linear response was found for both sensing channels, with LOD value of 3.4 μM. Its sensing mechanism was discussed and confirmed as the combination of an emission turn off effect caused by energy transfer process (EuBTC → DPA pyridine) and an emission turn on effect of rhodamine molecules triggered by DPA-released protons. EuBTC@RB showed its advantage over traditional DPA optical sensing systems due to its dual sensing skills and the possibility of naked eye detection.

Naphthalimide-Based DNA-Coupled Hybrid Assembly for Sensing Dipicolinic Acid: A Biomarker for Bacillus anthracis Spores

We have designed and synthesized a novel, water-soluble naphthalimide-histidine receptor (1) with excellent fluorescent properties. Functioning of the synthesized receptor was performed through developing their DNA-receptor hybrid assembly (DRHA), which has shown significant changes in the emission profile upon interactions with dipicolinic acid (DPA), a biomarker for Bacillus anthracis spores. DRHA showed fluorescence enhancement upon binding with DPA with the characteristic of internal charge transfer. It is notable that this assembly exhibited a significant limit of detection (12 nM) toward DPA. The mechanism of sensing was fully defined using ethidium bromide (EtBr) interaction studies as well as Fourier transform infrared spectroscopic analysis, which describes the binding mode of DRHA with DPA. This assembly selectively interacts with DPA over other anions, common cellular cations, and aromatic acids in aqueous media.

A lanthanide functionalized MOF hybrid for ratiometric luminescence detection of an anthrax biomarker

A dual-emitting hybrid platform based on a lanthanide functionalized MOF was demonstrated as a fluorescent indicator for ratiometric detection of an anthrax biomarker—dipicolinic acid (DPA).

Evaluation of PCR Systems for Field Screening of Bacillus anthracis

There is little published data on the performance of hand-portable polymerase chain reaction (PCR) systems that can be used by first responders to determine if a suspicious powder contains a potential biothreat agent. We evaluated 5 commercially available hand-portable PCR instruments for detection of Bacillus anthracis. We used a cost-effective, statistically based test plan to evaluate systems at performance levels ranging from 0.85-0.95 lower confidence bound (LCB) of the probability of detection (POD) at confidence levels of 80% to 95%. We assessed specificity using purified genomic DNA from 13 B. anthracis strains and 18 Bacillus near neighbors, potential interference with 22 suspicious powders that are commonly encountered in the field by first responders during suspected biothreat incidents, and the potential for PCR inhibition when B. anthracis spores were spiked into these powders. Our results indicate that 3 of the 5 systems achieved 0.95 LCB of the probability of detection with 95% confidence levels at test concentrations of 2,000 genome equivalents/mL (GE/mL), which is comparable to 2,000 spores/mL. This is more than sufficient sensitivity for screening visible suspicious powders. These systems exhibited no false-positive results or PCR inhibition with common suspicious powders and reliably detected B. anthracis spores spiked into these powders, though some issues with assay controls were observed. Our testing approach enables efficient performance testing using a statistically rigorous and cost-effective test plan to generate performance data that allow users to make informed decisions regarding the purchase and use of field biodetection equipment.

The authors evaluated 5 commercially available hand-portable PCR instruments for detecting Bacillus anthracis. These systems exhibited no false-positive results or PCR inhibition with common suspicious powders and reliably detected B. anthracis spores spiked into these powders, though some issues with assay controls were observed.

Evaluation of Immunoassays and General Biological Indicator Tests for Field Screening of Bacillus anthracis and Ricin

There is little published data on the performance of biological indicator tests and immunoassays that could be used by first responders to determine if a suspicious powder contains a potential biothreat agent. We evaluated a range of biological indicator tests, including 3 protein tests, 2 ATP tests, 1 DNA test, and 1 FTIR spectroscopy instrument for their ability to screen suspicious powders for Bacillus anthracis (B. anthracis) spores and ricin. We also evaluated 12 immunoassays (mostly lateral flow immunoassays) for their ability to screen for B. anthracis and ricin. We used a cost-effective, statistically based test plan that allows instruments to be evaluated at performance levels ranging from 0.85 to 0.95 lower confidence bound of the probability of detection at confidence levels of 80% to 95%. We also assessed interference with 22 common suspicious powders encountered in the field. The detection reproducibility for the biological indicators was evaluated at 10⁸B. anthracis spores and 62.5 μg ricin, and the immunoassay detection reproducibility was evaluated at 10⁷ spores/mL (B. anthracis) and 0.1 μg/mL (ricin). Seven out of 12 immunoassays met our most stringent criteria for B. anthracis detection, while 9 out of 12 met our most stringent test criteria for ricin detection. Most of the immunoassays also detected ricin in 3 different crude castor seed preparations. Our testing results varied across products and sample preparations, indicating the importance of reviewing performance data for specific instruments and sample types of interest for the application in order to make informed decisions regarding the selection of biodetection equipment for field use.

The authors evaluated a range of biological indicator tests, including 3 protein tests, 2 ATP tests, 1 DNA test, and 1 FTIR spectroscopy instrument for their ability to screen suspicious powders for Bacillus anthracis spores and ricin.

Holographic deep learning for rapid optical screening of anthrax spores

Establishing early warning systems for anthrax attacks is crucial in biodefense. Despite numerous studies for decades, the limited sensitivity of conventional biochemical methods essentially requires preprocessing steps and thus has limitations to be used in realistic settings of biological warfare. We present an optical method for rapid and label-free screening of Bacillus anthracis spores through the synergistic application of holographic microscopy and deep learning. A deep convolutional neural network is designed to classify holographic images of unlabeled living cells. After training, the network outperforms previous techniques in all accuracy measures, achieving single-spore sensitivity and subgenus specificity. The unique "representation learning" capability of deep learning enables direct training from raw images instead of manually extracted features. The method automatically recognizes key biological traits encoded in the images and exploits them as fingerprints. This remarkable learning ability makes the proposed method readily applicable to classifying various single cells in addition to B. anthracis, as demonstrated for the diagnosis of Listeria monocytogenes, without any modification. We believe that our strategy will make holographic microscopy more accessible to medical doctors and biomedical scientists for easy, rapid, and accurate point-of-care diagnosis of pathogens.

Holographic deep learning for rapid optical screening of anthrax spores

Establishing early warning systems for anthrax attacks is crucial in biodefense. Despite numerous studies for decades, the limited sensitivity of conventional biochemical methods essentially requires preprocessing steps and thus has limitations to be used in realistic settings of biological warfare. We present an optical method for rapid and label-free screening of Bacillus anthracis spores through the synergistic application of holographic microscopy and deep learning. A deep convolutional neural network is designed to classify holographic images of unlabeled living cells. After training, the network outperforms previous techniques in all accuracy measures, achieving single-spore sensitivity and subgenus specificity. The unique "representation learning" capability of deep learning enables direct training from raw images instead of manually extracted features. The method automatically recognizes key biological traits encoded in the images and exploits them as fingerprints. This remarkable learning ability makes the proposed method readily applicable to classifying various single cells in addition to B. anthracis, as demonstrated for the diagnosis of Listeria monocytogenes, without any modification. We believe that our strategy will make holographic microscopy more accessible to medical doctors and biomedical scientists for easy, rapid, and accurate point-of-care diagnosis of pathogens.

Comprehensive Laboratory Evaluation of a Highly Specific Lateral Flow Assay for the Presumptive Identification of Bacillus anthracis Spores in Suspicious White Powders and Environmental Samples

We conducted a comprehensive, multiphase laboratory evaluation of the Anthrax BioThreat Alert(®) test strip, a lateral flow immunoassay (LFA) for the rapid detection of Bacillus anthracis spores. The study, conducted at 2 sites, evaluated this assay for the detection of spores from the Ames and Sterne strains of B. anthracis, as well as those from an additional 22 strains. Phylogenetic near neighbors, environmental background organisms, white powders, and environmental samples were also tested. The Anthrax LFA demonstrated a limit of detection of about 10(6) spores/mL (ca. 1.5 × 10(5) spores/assay). In this study, overall sensitivity of the LFA was 99.3%, and the specificity was 98.6%. The results indicated that the specificity, sensitivity, limit of detection, dynamic range, and repeatability of the assay support its use in the field for the purpose of qualitatively evaluating suspicious white powders and environmental samples for the presumptive presence of B. anthracis spores.

Fluorescent detection of dipicolinic acid as a biomarker of bacterial spores using lanthanide-chelated gold nanoparticles

Gold nanoparticles (GNPs) functionalized with ethylenediamine-lanthanide complexes (Eu-GNPs and Tb-GNPs) were used for the selective fluorescent detection of dipicolinic acid (DPA), a unique biomarker of bacterial spores, in water. Particles were characterized by transmission electron microscopy and zeta potential measurements. The coordination of DPA to the lanthanides resulted in the enhancement of the fluorescence. A selective response to DPA was observed over the nonselective binding of aromatic ligands. The ligand displacement strategy were also employed for the ratiometric fluorescent detection of DPA. 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedion (TFNB) was chosen as an antenna to synthesize ternary complexes. The addition of DPA on EuGNP:TFNB ternary complex quenched the initial emission of the complex at 615nm and increased the TFNB emission at 450nm when excited at 350nm. The results demonstrated that the ratiometric fluorescent detection of DPA was achieved by ligand displacement strategy.

Rapid Detection of Bacillus anthracis Spores Using Immunomagnetic Separation and Amperometry

Portable detection and quantitation methods for Bacillus anthracis (anthrax) spores in pure culture or in environmental samples are lacking. Here, an amperometric immunoassay has been developed utilizing immunomagnetic separation to capture the spores and remove potential interferents from test samples followed by amperometric measurement on a field-portable instrument. Antibody-conjugated magnetic beads and antibody-conjugated glucose oxidase were used in a sandwich format for the capture and detection of target spores. Glucose oxidase activity of spore pellets was measured indirectly via amperometry by applying a bias voltage after incubation with glucose, horseradish peroxidase, and the electron mediator 2,2′-azino-bis (3-ethylbenzthiazoline-6-sulphonic acid). Target capture was mediated by polyclonal antisera, whereas monoclonal antibodies were used for signal generation. This strategy maximized sensitivity (500 target spores, 5000 cfu/mL), while also providing a good specificity for Bacillus anthracis spores. Minimal signal deviation occurs in the presence of environmental interferents including soil and modified pH conditions, demonstrating the strengths of immunomagnetic separation. The simultaneous incubation of capture and detection antibodies and rapid substrate development (5 min) result in short sample-to-signal times (less than an hour). With attributes comparable or exceeding that of ELISA and LFDs, amperometry is a low-cost, low-weight, and practical method for detecting anthrax spores in the field.

Nanomaterial-based sensors for the detection of biological threat agents

The danger posed by biological threat agents and the limitations of modern detection methods to rapidly identify them underpins the need for continued development of novel sensors. The application of nanomaterials to this problem in recent years has proven especially advantageous. By capitalizing on large surface/volume ratios, dispersability, beneficial physical and chemical properties, and unique nanoscale interactions, nanomaterial-based biosensors are being developed with sensitivity and accuracy that are starting to surpass traditional biothreat detection methods, yet do so with reduced sample volume, preparation time, and assay cost. In this review, we start with an overview of bioagents and then highlight the breadth of nanoscale sensors that have recently emerged for their detection.

A ratiometric fluorescent nanoprobe based on terbium functionalized carbon dots for highly sensitive detection of an anthrax biomarker

By preparing biocompatible carbon dots (CDs) that act as the scaffold and fluorescence reference, a ratiometric fluorescent nanoprobe based on Tb³⁺ ions functionalized CDs was designed and fabricated to detect an anthrax biomarker with high selectivity and sensitivity.

Development of a simple method for the rapid identification of organisms causing anthrax by coagglutination test

A protective antigen (PA) based coagglutination test was optimized in the present study for the specific and sensitive identification of bacteria causing anthrax in a cost effective and less risky manner. The test showed 100% specificity and sensitivity up to 9 × 10(3) formalinized vegetative cells or 11 ng of PA. The optimized test also detected anthrax toxin directly from the serum as well as blood of anthrax infected animals indicating the potential application for direct diagnosis of anthrax under field conditions.