Dual Recognition Strategy for Specific and Sensitive Detection of Bacteria Using Aptamer-Coated Magnetic Beads and Antibiotic-Capped Gold Nanoclusters

Sensitive Detection of Staphylococcus aureus with Vancomycin-Conjugated Magnetic Beads as Enrichment Carriers Combined with Flow Cytometry

A novel sandwich strategy was designed to detect Staphylococcus aureus. The strategy is based on an antibacterial agent that captures bacterial cells and a fluorescein-labeled antibody that acts as the signal-output probe. Vancomycin (Van), which exerts a strong antibacterial effect on Gram-positive bacteria, was utilized as a molecular recognition agent to detect pathogenic bacteria. To effectively concentrate S. aureus, we used bovine serum albumin (BSA) as the amplification carrier to modify magnetic beads (MBs), which were then functionalized with Van. To improve the specificity of the method for S. aureus detection, we adopted fluorescein isothiocyanate (FITC)-tagged pig immunoglobulin G (FITC-pig IgG) as the signal probe and the second recognition agent that bound between the Fc fragment of pig IgG and protein A in the surface of S. aureus. To quantify S. aureus, we measured the fluorescence signal by flow cytometry (FCM). The use of multivalent magnetic nanoprobes (Van-BSA-MBs) showed a high concentration efficiency (>98%) at bacterial concentrations of only 33 colony-forming units (CFU)/mL. Furthermore, the sandwich mode (FITC-pig IgG/SA/Van-BSA-MBs) also showed ideal specificity because Van and IgG bound with S. aureus at two distinct sites. The detection limit for S. aureus was 3.3 × 10¹ CFU/mL and the total detection process could be completed within 120 min. Other Gram-positive bacteria and Gram-negative bacteria, including Listeria monocytogenes, Bacillus cereus, Cronobacter sakazakii, Escherichia coli O157:H7, and Salmonella Enteritidis, negligibly interfered with S. aureus detection. The proposed detection strategy for S. aureus possesses attractive characteristics, such as high sensitivity, simple operation, short testing time, and low cost.

Novel label-free and high-throughput microchip electrophoresis platform for multiplex antibiotic residues detection based on aptamer probes and target catalyzed hairpin assembly for signal amplification

Novel label-free and multiplex aptasensors have been developed for simultaneous detection of several antibiotics based on a microchip electrophoresis (MCE) platform and target catalyzed hairpin assembly (CHA) for signal amplification. Kanamycin (Kana) and oxytetracycline (OTC) were employed as models for testing the system. These aptasensors contained six DNA strands termed as Kana aptamer-catalysis strand (Kana apt-C), Kana inhibit strand (Kana inh), OTC aptamer-catalysis strand (OTC apt-C), OTC inhibit strand (OTC inh), hairpin structures H1 and H2 which were partially complementary. Upon the addition of Kana or OTC, the binding event of aptamer and target triggered the self-assembly between H1 and H2, resulting in the formation of many H1-H2 complexes. They could show strong signals which represented the concentration of Kana or OTC respectively in the MCE system. With the help of the well-designed and high-quality CHA amplification, the assay could yield 300-fold amplified signal comparing that from non-amplified system. Under optimal conditions, this assay exhibited a linear correlation in the ranges from 0.001ngmL^-1 to 10ngmL^-1, with the detection limits of 0.7pgmL^-1 and 0.9pgmL^-1 (S/N=3) toward Kana and OTC, respectively. The platform has the following advantages: firstly, the aptamer probes can be fabricated easily without labeling signal tags for MCE detection; Secondly, the targets can just react with probes and produce the amplified signal in one-pot. Finally, the targets can be simultaneously detected within 10min in different channels, thus high-throughput measurement can be achieved. Based on this work, it is estimated that this detection platform will be universally served as a simple, sensitive and portable platform for antibiotic contaminants detection in biological and environmental samples.

Dual-Recognition Förster Resonance Energy Transfer Based Platform for One-Step Sensitive Detection of Pathogenic Bacteria Using Fluorescent Vancomycin-Gold Nanoclusters and Aptamer-Gold Nanoparticles

The effective monitoring, identification, and quantification of pathogenic bacteria is essential for addressing serious public health issues. In this study, we present a universal and facile one-step strategy for sensitive and selective detection of pathogenic bacteria using a dual-molecular affinity-based Förster (fluorescence) resonance energy transfer (FRET) platform based on the recognition of bacterial cell walls by antibiotic and aptamer molecules, respectively. As a proof of concept, Vancomycin (Van) and a nucleic acid aptamer were employed in a model dual-recognition scheme for detecting Staphylococcus aureus (Staph. aureus). Within 30 min, by using Van-functionalized gold nanoclusters and aptamer-modified gold nanoparticles as the energy donor and acceptor, respectively, the FRET signal shows a linear variation with the concentration of Staph. aureus in the range from 20 to 10⁸ cfu/mL with a detection limit of 10 cfu/mL. Other nontarget bacteria showed negative results, demonstrating the good specificity of the approach. When employed to assay Staph. aureus in real samples, the dual-recognition FRET strategy showed recoveries from 99.00% to the 109.75% with relative standard derivations (RSDs) less than 4%. This establishes a universal detection platform for sensitive, specific, and simple pathogenic bacteria detection, which could have great impact in the fields of food/public safety monitoring and infectious disease diagnosis.

Dual Functional Core-Shell Fluorescent Ag2S@Carbon Nanostructure for Selective Assay of E. coli O157:H7 and Bactericidal Treatment

A dual functional fluorescent core-shell Ag₂S@Carbon nanostructure is prepared by a hydrothermally assisted multi-amino synthesis approach with folic acid (FA), polyethylenimine (PEI), and mannoses (Mans) as carbon and nitrogen sources (FA-PEI-Mans-Ag₂S nanocomposite shortly as Ag₂S@C). The nanostructure exhibits strong fluorescent emission at λ_ex/λ_em = 340/450 nm with a quantum yield of 12.57 ± 0.52%. Ag₂S@C is bound to E. coli O157:H7 via strong interaction with the Mans moiety in Ag₂S@C with FimH proteins on the fimbriae tip in E. coli O157:H7. Fluorescence emission from Ag₂S@C/E. coli conjugate is closely related to the content of E. coli O157:H7. Thus, a novel procedure for fluorescence assay of E. coli O157:H7 is developed, offering a detection limit of 330 cfu mL^-1. Meanwhile, the Ag₂S@C nanostructure exhibits excellent antibacterial performance against E. coli O157:H7. A 99.9% sterilization rate can be readily achieved for E. coli O157:H7 at a concentration of 10⁶-10⁷ cfu mL^-1 with 3.3 or 10 μg mL^-1 of Ag₂S@C with an interaction time of 5 or 0.5 min, respectively.

Efficient Enrichment and Analyses of Bacteria at Ultralow Concentration with Quick-Response Magnetic Nanospheres

Enrichment and purification of bacteria from complex matrices are crucial for their detection and investigation, in which magnetic separation techniques have recently show great application advantages. However, currently used magnetic particles all have their own limitations: Magnetic microparticles exhibit poor binding capacity with targets, while magnetic nanoparticles suffer slow magnetic response and high loss rate during treatment process. Herein, we used a highly controllable layer-by-layer assembly method to fabricate quick-response magnetic nanospheres (MNs), and with Salmonella typhimurium as a model, we successfully achieve their rapid and efficient enrichment. The MNs combined the advantages of magnetic microparticles and nanoparticles. On the one hand, the MNs had a fast magnetic response, and almost 100% of the MNs could be recovered by 1 min attraction with a simple magnetic scaffold. Hence, using antibody conjugated MNs (immunomagnetic nanospheres, IMNs) to capture bacteria hardly generated loss and did not need complex separation tools or techniques. On the other hand, the IMNs showed much excellent capture capacity. With 20 min interaction, almost all of the target bacteria could be captured, and even only one bacterium existing in the samples was not missed, comparing with the immunomagnetic microparticles which could only capture less than 50% of the bacteria. Besides, the IMNs could achieve the same efficient enrichment in complex matrices, such as milk, fetal bovine serum, and urine, demonstrating their good stability, strong anti-interference ability, and low nonspecific adsorption. In addition, the isolated bacteria could be directly used for culture, polymerase chain reaction (PCR) analyses, and fluorescence immunoassay without a release process, which suggested our IMNs-based enrichment strategy could be conveniently coupled with the downstream identification and analysis techniques. Thus, the MNs provided by this work showed great superiority in bacteria enrichment, which would be a promising tool for bacteria detection and investigation.

Aptamer-modified nanomaterials: principles and applications

Aptamers are promising alternative binders that can substitute antibodies in various applications. Due to the advantages of aptamers, namely their high affinity, specificity and stability, along with the benefits originating from the chemical synthesis of aptamers, they have attracted attention in various applications including their use on nanostructured material. This necessitates the immobilization of aptamers on a solid support. Since aptamer immobilization may interfere with its binding properties, the immobilization of aptamers has to be investigated and optimized. Within this review, we give general insights into the principles and factors controlling the binding affinity of immobilized aptamers. Specific features of aptamer immobilization on nanostructured surfaces and nanoparticles are highlighted and a brief overview of applications of aptamer-modified nanostructured materials is given.

Selection of affinity peptides for the purification potential of porcine circovirus type 2 (PCV2) Cap virus-like particles (VLPs)

In this study, we describe the use of a M13 phage-displayed random peptide library for screening novel peptide motifs that specifically recognize recombinant PCV2 Cap protein for the first time.

Sensitive detection of Listeria monocytogenes based on highly efficient enrichment with vancomycin-conjugated brush-like magnetic nano-platforms

Pathogens pose a significant threat to public health worldwide. Despite many technological advances in the rapid diagnosis of pathogens, sensitive pathogen detection remains challenging because target pathogenic bacteria usually exist in complex samples at very low concentrations. Here, the construction of multivalent brush-like magnetic nanoprobes and their application for the efficient enriching of pathogens are demonstrated. Brush-like magnetic nanoprobes were constructed by modification with poly-L-lysine (PLL) onto amino-modified magnetic beads, followed by coupling of PEG (amine-PEG₅₀₀₀-COOH) to the amine sites of PLL. Subsequently, vancomycin (Van), a small-molecule antibiotic with affinity to the terminal peptide (D-alanyl-D-alanine) on the cell wall of Gram-positive bacteria, was conjugated to the carboxyl of the PEG. The use of multivalent brush-like magnetic nanoprobes (Van-PEG-PLL-MNPs) results in a high enrichment efficiency (>94%) and satisfactory purity for Listeria monocytogenes (employed as a model) within 20min, even at bacterial concentrations of only 10²cfumL^-1. Integrated with the enrichment of the Van-PEG-PLL-MNP nano-platform and electrochemiluminescence (ECL) detection, Listeria monocytogenes can be rapidly and accurately detected at levels as low as 10cfumL^-1. The approach described herein holds great potential for realizing rapid and sensitive pathogen detection in clinical samples.

Rapid and Selective Detection of Pathogenic Bacteria in Bloodstream Infections with Aptamer-Based Recognition

Sepsis and bacteremia are life-threatening clinical syndromes associated with significant patient morbidity and mortality. Rapid and sensitive detection of pathogenic bacteria is the key to improve patient survival rates. Herein, we have rationally constructed a simple aptamer-based capture platform to shorten the time needed for confirmation of bacterial bloodstream infection in clinical blood samples. This capture platform is made of a mesoporous TiO2-coated magnetic nanoparticle and is modified with target aptamer. It features excellent bacterial enrichment efficiency of about 80% even at low bacterial concentrations (10-2000 CFU mL(-1)). More importantly, the bacteria can be enriched within 2 h, and the time for bacterial identification is effectively shortened in comparison to the "gold standard" in clinical diagnosis of bloodstream infection. The aptamer-based capture platform may pave a way for the detection of biomarkers and find potential applications in disease diagnosis.

A rapid SERS method for label-free bacteria detection using polyethylenimine-modified Au-coated magnetic microspheres and Au@Ag nanoparticles

A rapid and efficient method for label-free SERS detection of bacteria in solution.

Magnetic solid-phase extraction coupled with HPLC-Q-TOF-MS for rapid analysis of tyrosinase binders from San-Bai decoction by Box–Behnken statistical design

Tyrosinase is the rate-limiting enzyme for controlling the production of melanin.

Preparation of amino-functionalized magnetic nanoparticles for enhancement of bacterial capture efficiency

PEI-MNPs were successfully fabricated, which showed higher bacterial capture ability than the triaminopropylalkoxysilane directly modified NH-MNPs at low concentration.